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Questions to ask before choosing mobile app technology to ask before choosing mobile app technology<p>Embarking on a new project is exciting. So many possibilities, so many choices! But you better get them right from the start, otherwise, your project might suffer in the long run.</p><p>Choosing a platform to build your mobile app can be a daunting task. For some apps, a simple responsive web or PWA will suffice, whereas for others only native solutions will do. And there’s of course a range of popular cross-platform or hybrid technologies like Xamarin, React Native, Flutter, or Kotlin Multiplatform, to name a few.</p><p>Evaluating all these alternatives is difficult. There are no universally right or wrong answers, but to make the choice easier, we offer you a list of questions that, when answered, will help you make the right choice.</p><h2>Lifespan</h2><ol><li><strong>What is the planned lifetime period of your app?</strong> Short-lived marketing or event apps have different requirements than apps that need to live happily for years. </li><li><strong>What is more important: Time to market, or sustainable development over time?</strong> Sometimes quick’n’dirty solutions make perfect business sense, sometimes they are poison. </li><li><strong>Will the chosen technology still exist when your app approaches the end of its life?</strong> Obsolete or abandoned technology will severely hinder your ability to support and expand your app. </li><li><strong>Will the technology be supported by its authors? Will it be supported on target platforms?</strong> Open source technology can be theoretically maintained by anybody, however, in practice, the majority of work often rests on a surprisingly small number of individuals. </li><li><strong>How will the technology evolve over time?</strong> There is a significant difference between a technology that the authors primarily develop to serve their own needs (even if it’s open-sourced), and a technology that is truly meant as a general-purpose tool. </li><li><strong>Is there a risk of vendor lock-in?</strong> If the technology is currently free to use, will it still be free in the future? What is the cost of moving to an alternative solution? </li></ol><h2>Runtime</h2><ol start="7"><li><strong>What runtime environment does the app need?</strong> The app may be compiled to native code, it may need bridges, wrappers, interpreters, etc. Those can differ wildly in various regards, sometimes by an order of magnitude. </li><li><strong>How is the performance?</strong> Nobody wants sluggish, janky apps.</li><li><strong>Is it stable?</strong> Frequent crashes destroy an app's reputation quickly.</li><li><strong>How big are deployed artifacts? Do they need to be installed?</strong> A complicated or slow installation process lowers the chances that users will even <em>launch</em> your app, while every extra megabyte increases churn. </li></ol><h2>UI</h2><ol start="11"><li><strong>Does the technology use native components, or does it draw its own? Can the user tell the difference?</strong> Non-native components may look similar, but users are surprisingly sensitive to even small inconsistencies. </li><li><strong>Does it respect the look’n’feel of each platform?</strong> You don’t want your app to look unintentionally alien on the target platform. </li><li><strong>Are all platform-specific components available?</strong> Custom UI components often demand a lot of work and if many are not available, your app can get very expensive, very quickly. </li><li><strong>How difficult is it to create custom components?</strong> Even if all platform components are available, there will be times when you’ll need to create your own—and it needs to be reasonably effective to do so. </li><li><strong>How difficult is it to create animations?</strong> When done right, animations are a crucial part of the UX, but implementing animations can sometimes be exceedingly difficult. </li><li><strong>How are the components integrated with the target system?</strong> Appearances are not everything—you also need to consider things like gestures, accessibility, support for autocomplete, password managers, etc. </li></ol><h2>Compatibility and interoperability</h2><ol start="17"><li><strong>What level of abstraction does the technology bring?</strong> Some try to completely hide or unify the target platforms, some are very low-level. Both can be good, or bad. </li><li><strong>Which system functionalities does it support directly?</strong> UI is not everything—chances are your app will need to support at least some of the following things: biometry, cryptography, navigation, animations, camera, maps, access to user’s contacts or calendar, OCR, launcher widgets, mobile payment systems, AR/VR, 3D rendering, sensors, various displays, wearables, car, TV, … </li><li><strong>How difficult is it to access native APIs?</strong> Every abstraction is leaky. There will come a time when you’ll need to interact with the underlying platform directly. The difficulty to do so can vary greatly. </li><li><strong>Are cutting-edge platform features available right away?</strong> Especially when using bridges or wrappers, support for the latest features can be delayed. </li><li><strong>What other platforms does the technology support?</strong> The ability to run your app on other platforms can sometimes be very advantageous, just keep in mind that the extra investment required can vary. </li></ol><h2>Paradigm and architecture</h2><ol start="22"><li><strong>How steep is the learning curve?</strong> Your team needs to be up-and-running in a reasonable amount of time. </li><li><strong>How rigid is the technology?</strong> Some frameworks try to manage everything—painting by the numbers can be simple and effective, but at the same time, it may limit your ability to implement things for which the framework doesn’t have first-class support. On the other hand, libraries may be more difficult to wire together, but they grant you greater freedom. </li><li><strong>How distant is the given paradigm from the default way of doing things?</strong> Nonstandard or exotic approaches can steepen the learning curve significantly. </li><li><strong>Is the technology modular? On what levels?</strong> Usually, you need the ability to slice the app across various boundaries (e.g., features, layers), and at various levels (e.g., code, compilation, deployment, etc.). </li><li><strong>How does it scale?</strong> Nowadays, even mobile apps can easily grow to hundreds of screens, and the app mustn’t crumble under that weight for both its developers and users. </li></ol><h2>Tooling</h2><ol start="27"><li><strong>Is there an official IDE? What does it cost? Can it be extended with plugins?</strong> Developer productivity is paramount, and the best tools pay for themselves quickly. </li><li><strong>Which build system does the technology use?</strong> There are many of them, but they’re not all equally simple to use, fast, or extendable. </li><li><strong>How is the CI/CD support?</strong> It needs to integrate smoothly with your CI/CD system of choice. </li><li><strong>What about testing, debugging, instrumentation, or profiling?</strong> Your developers and QA people need to be able to quickly dissect your app to identify and fix potential problems. </li><li><strong>How mature and effective are the tools?</strong> Your developers should focus on your app, they shouldn’t be fighting the tools. </li><li><strong>Does the technology support hot reload, or dynamic feature modules?</strong> These features usually greatly enhance developer productivity. </li></ol><h2>Ecosystem</h2><ol start="33"><li><strong>Is the technology open source?</strong> There are countless advantages when it is. </li><li><strong>What is the availability, quality, and scope of 3rd party libraries?</strong> The ability to reuse existing, well-tested code can make or break projects. </li><li><strong>Is the official documentation up-to-date, complete, and comprehensive?</strong> While learning about particular technology by trial and error can be fun, it certainly isn’t effective. </li><li><strong>Do best practices exist?</strong> If there are many ways to do a thing, chances are some of them will end up with your developers shooting themselves in the foot. </li><li><strong>How accessible is community help? Are there blog posts, talks, or other learning materials?</strong> Search StackOverflow, or try to find newsletters, YouTube channels, podcasts, or conferences dedicated to the technology in question. </li><li><strong>Are consultants available if needed?</strong> Some of them are even helpful.</li><li><strong>What is the overall community sentiment towards the technology?</strong> Dedicated fans are a good sign, but be careful not to fall for marketing tricks. </li><li><strong>Do other similar organizations have experience with the technology?</strong> Learn from the successes and mistakes of others. </li></ol><h2>Human resources</h2><ol start="41"><li><strong>What primary programming language does the technology rely on?</strong> It isn’t enough that developers are able to <em>edit</em> source files to make the machine do something—they need to be able to write idiomatic and expressive code that can be read by human beings. </li><li><strong>Do you already have suitable developers?</strong> Why change a whole team, when you might already have a stable, well-coordinated one? </li><li><strong>Will mobile developers be effective using the language?</strong> There could be great friction when switching developers from one language to another, especially when the new language is significantly different (e.g., statically vs. dynamically typed, compiled vs. interpreted, etc.). </li><li><strong>Will non-mobile developers be effective on mobile platforms?</strong> For example, some technologies try to port web frameworks to mobile platforms, so it might look like a good idea to assign web developers to the project—but the reality is not that simple. </li><li><strong>What is the current market situation? What is the market profile of available developers?</strong> You usually need a suitable mix of junior and senior developers, but they might not be easy to find, or their cost might not be economically feasible. </li></ol><h2>Existing codebase</h2><ol start="46"><li><strong>Do you already have some existing code?</strong> Rewriting from scratch is tempting, but it isn’t always a good idea. </li><li><strong>What have you invested in it so far?</strong> It may be very cheap to throw away, or it may represent a major asset of your organization. </li><li><strong>What is its value to your organization?</strong> It may earn or save you a ton of money, or it may be a giant liability. </li><li><strong>How big is the technical debt?</strong> The value of unmaintainable code is not great, to put it mildly. </li><li><strong>Can it be maintained and evolved?</strong> The software must be, well, soft. If yours is rigid, again, its value is not that great. </li><li><strong>Can it be transformed piece-by-piece?</strong> Some technologies allow gradual migration, some are all-or-nothing propositions. </li></ol><h2>Final questions</h2><p>Each app has different needs, and there will always be tradeoffs. In the end, you’ll need to prioritize the various viewpoints implied by the aforementioned questions.</p><p>Which qualities are most important for your project? Which properties bring you opportunities? Which increase risk?</p><p>When you put the alternatives into the right perspective, you certainly have a much better chance at success. May your apps live long and prosper!</p>#project-management;#android;#iOS
Android jumps on Java release train jumps on Java release train<p>​​For many years, Android was stuck with Java 8. Finally, we got a <a href=""> big update</a>. The gap between Java 8 and Java 9 in terms of build compatibility has been overcome and more modern Java versions (up to Java 11) are officially supported on Android. On top of that, Android Gradle Plugin 7.0.0 now requires JDK 11 for running Gradle builds.</p><p>In this post, I’ll describe the technical background of this change and how it might affect your project, even if it’s written exclusively in Kotlin.</p><h2>What is the release train and why have Java 9 and 10 been skipped?</h2><p>Historically, a new major Java version has been released “every once in a while”. This has led to an irregular release schedule and the language not evolving rapidly.</p><p>Beginning with Java 9, it was decided that a new major Java version would be released every 6 months and LTS (long-term support) releases would arrive every 3 years.</p><table cellspacing="0" width="90%" class="ms-rteTable-default" style="margin-left:auto;margin-right:auto;border:1px solid black;"><tbody><tr class="ms-rteTableHeaderRow-default"><th class="ms-rteTableHeaderEvenCol-default" style="width:18%;"> <strong>​​Java version</strong> </th><th class="ms-rteTableHeaderOddCol-default" style="width:18%;"> <strong>​Release date</strong> </th><th class="ms-rteTableHeaderEvenCol-default" style="width:64%;"> <strong>​Selected language features</strong> </th></tr><tr class="ms-rteTableOddRow-default" style="background-color:#f4cccc;"><td class="ms-rteTableEvenCol-default">​Java 6</td><td class="ms-rteTableOddCol-default">​December 2006</td><td class="ms-rteTableEvenCol-default">​No language changes</td></tr><tr class="ms-rteTableEvenRow-default" style="background-color:#f4cccc;"><td class="ms-rteTableEvenCol-default">​Java 7</td><td class="ms-rteTableOddCol-default">​July 2011</td><td class="ms-rteTableEvenCol-default"> <a href=""> ​Project Coin</a>: Diamond operator, Strings in switch, etc. </td></tr><tr class="ms-rteTableOddRow-default" style="background-color:#fff2cc;"><td class="ms-rteTableEvenCol-default">​Java 8 LTS</td><td class="ms-rteTableOddCol-default">March 2014</td><td class="ms-rteTableEvenCol-default"> <a href="">​Lambdas</a><br> <a href="">Type Annotations</a><br> <a href="">Default methods in interfaces</a> </td></tr><tr class="ms-rteTableEvenRow-default" style="background-color:#fff2cc;"><td class="ms-rteTableEvenCol-default">​Java 9</td><td class="ms-rteTableOddCol-default">September 2017</td><td class="ms-rteTableEvenCol-default"> <a href="">Private methods in interfaces</a> </td></tr><tr class="ms-rteTableOddRow-default" style="background-color:#fff2cc;"><td class="ms-rteTableEvenCol-default">​Java 10</td><td class="ms-rteTableOddCol-default">March 2018</td><td class="ms-rteTableEvenCol-default"> <a href="">Local-Variable Type Inference</a> </td></tr><tr class="ms-rteTableEvenRow-default" style="background-color:#d9ead3;"><td class="ms-rteTableEvenCol-default">​Java 11 LTS</td><td class="ms-rteTableOddCol-default">September 2018</td><td class="ms-rteTableEvenCol-default"> <a href="">Local-Variable Syntax for Lambda Parameters</a> </td></tr><tr class="ms-rteTableOddRow-default" style="background-color:#fff2cc;"><td class="ms-rteTableEvenCol-default">​Java 12</td><td class="ms-rteTableOddCol-default">March 2019</td><td class="ms-rteTableEvenCol-default">No stable language features</td></tr><tr class="ms-rteTableEvenRow-default" style="background-color:#fff2cc;"><td class="ms-rteTableEvenCol-default">​Java 13</td><td class="ms-rteTableOddCol-default">September 2019</td><td class="ms-rteTableEvenCol-default">No stable language features</td></tr><tr class="ms-rteTableOddRow-default" style="background-color:#fff2cc;"><td class="ms-rteTableEvenCol-default">​Java 14</td><td class="ms-rteTableOddCol-default">March 2020</td><td class="ms-rteTableEvenCol-default"> <a href="">Switch Expressions</a> </td></tr><tr class="ms-rteTableEvenRow-default" style="background-color:#d9ead3;"><td class="ms-rteTableEvenCol-default">​Java 15</td><td class="ms-rteTableOddCol-default">September 2020</td><td class="ms-rteTableEvenCol-default"> <a href="">Text Blocks</a> </td></tr><tr class="ms-rteTableOddRow-default" style="background-color:#cfe2f3;"><td class="ms-rteTableEvenCol-default">​Java 16</td><td class="ms-rteTableOddCol-default">March 2021</td><td class="ms-rteTableEvenCol-default"> <a href="">Pattern Matching for instanceof</a><br> <a href="">Records</a> </td></tr><tr class="ms-rteTableEvenRow-default" style="background-color:#cfe2f3;"><td class="ms-rteTableEvenCol-default">​Java 17 LTS</td><td class="ms-rteTableOddCol-default">September 2021</td><td class="ms-rteTableEvenCol-default">Nothing announced yet</td></tr></tbody></table> <br> <p>Standard releases have quite a short support period and receive just 2 minor updates exactly 1 and 4 months after their initial release. The LTS releases are guaranteed to be supported till another LTS version is released, in 3 years timeframe (for details about Java release trains, I recommend reading <a href="">Stephen Colebourne's posts</a>).</p><p>Many projects have decided to follow the LTS releases only and now it seems that Google has the same plans for Android. Even though Java 15 is the latest released version, it is a non-LTS version, so Android maintains the latest LTS release, Java 11, as the required minimum.</p><h2>What complicates the update from Java 8 to Java 9 and onwards?</h2><p>Java 9 was the first version released in the new era and brought a lot of new features the community desired. The most significant of these is probably the new modular system known by the codename <a href="">“Project Jigsaw”</a>. It has a concept of dependencies that can define a public API and can keep the implementation private at the same time.</p><p>This feature is first and foremost meant to be used by libraries. As the JDK is a library itself, it has also been modularized. The main advantage is that it is possible to create a smaller runtime with only a subset of necessary modules.</p><p>During this journey, some Java APIs have been made private and others were moved to different packages. This causes trouble for some well-known annotation processors like Dagger. The generated code is usually annotated with <span class="pre-inline">@Generated</span> annotation, which has been moved to a different package in JDK 9. In the case of an Android project written in Kotlin (which has to use kapt to enable Dagger), the build fails on JDK 9 or newer due to a <a href="">missing @Generated annotation class</a>. Dagger itself has a check for the target Java level and uses <span class="pre-inline">@Generated</span> annotation from the correct package. However, there was a <a href="">bug in kapt</a> - it didn’t report configured target Java level to Java compiler, failing the build and leaving poor developers scratching their heads for hours.</p><p>The restructuralization of the JDK was actually wider than just moving Java classes around. The Java compiler needed to be changed as well in order to understand the module system and know how to handle its classpaths appropriately.</p><p>As a result, the <span class="pre-inline">-bootclasspath</span> compiler option (that was used to include the <span class="pre-inline">android.jar</span> to the build) was removed, effectively making all Android classes unavailable to the build. Projects that are written 100% in Kotlin are not affected by this until Android view binding (or similar feature) is enabled. View binding build step generates Java classes that need to be compiled by <span class="pre-inline">javac</span>. As the generated classes have dependencies on <span class="pre-inline">android.jar</span> classes, the compilation fails when the project is configured to target Java 9 or newer. This limitation has been <a href="">known and tracked </a> for quite a long time and now it has finally been resolved as part of AGP 7.0.0.</p><p>Other tools that also needed an update were D8 and R8, as they work directly with new Java versions of class files.</p><h2>What to do to upgrade to Android Gradle Plugin 7.0?</h2><p>When a project with AGP 7.0 build is executed on JDK 8, the build will fail immediately with the following error:</p><pre><code class="hljs"> An exception occurred applying plugin request [id: ''] Failed to apply plugin ''. Android Gradle plugin requires Java 11 to run. You are currently using Java 1.8. You can try some of the following options: - changing the IDE settings. - changing the JAVA_HOME environment variable. - changing `` in ``. </code></pre><p>The only requirement is to use at least JDK 11 for Gradle when building the project. This can be set through <strong>JAVA_HOME</strong> environmental variable, <strong></strong> Gradle property or in <strong>Project Structure</strong> dialog in Android Studio:</p> <img alt="Project Structure dialog in Android Studio" src="/Blog/PublishingImages/Articles/android-java-release-train-01.png" data-themekey="#" /> <h2>Can we use new Java language features?</h2><p>Since Java 9, new language features are usually implemented in the Java compiler without any impact on bytecode or JVM. These can be easily handled by Android’s D8 tool and can be used in Android projects without a problem.</p><p>Example:</p><pre><code class="java hljs"> public void sayIt() { var message = "I am a Java 10 inferred type running on Android"; System.out.println(message); } </code></pre><p>You just need to tell Gradle that the project is targeting new Java versions. This can be configured in <span class="pre-inline">build.gradle.kts</span>:</p><pre><code class="kotlin hljs"> android { compileOptions { sourceCompatibility = JavaVersion.VERSION_11 targetCompatibility = JavaVersion.VERSION_11 } } </code></pre><p>When <span class="pre-inline">compileOptions</span> are not defined, the defaults come into play. Up until AGP 4.1, the default Java compatibility level was set to a very ancient Java 6. Since AGP 4.2, it has been bumped to (only slightly less ancient) Java 8.</p><h2>Can we use new Java APIs?</h2><p>Regrettably, Java library APIs are a completely different thing.</p><p>Java 8 APIs are available starting with Android 8 (API level 26). Some Java 9 APIs (like <a href="">List.of()</a>) are available starting with Android 11 (API level 30). These APIs might also be available on older Android versions through <a href="">Java APIs desugaring</a>.</p><p>Hopefully, every future Android version will adopt more of these new Java APIs and make them available for use on older Android versions via desugaring.</p><h2>Can we use the latest version - Java 15?</h2><p>We can use JDK 15 for running Gradle builds as it supports the latest Java version <a href="">since Gradle 6.7</a>. </p><p>Unfortunately, we cannot use Java 15 language features in our code. In fact, we cannot use Java 14, 13 and 12 language features either, as the highest supported <span class="pre-inline">sourceCompatibility</span> level is still Java 11. However, the limitation of R8 not being able to parse the latest Java version class files <a href="">was resolved</a> at the beginning of December 2020, so we can hope for Java 15 support arriving soon.</p><h2>How does this affect Kotlin projects?</h2><p>Not much. Kotlin compiler and toolchain are not affected by JDK used for Gradle build nor the Java compatibility level set for a project.</p><p>However, when you use JDK 11 for build and Java 11 as a source compatibility level for Java compiler, it is reasonable to use the same level as Kotlin target for JVM. This allows Kotlin to generate code optimized for newer Java language versions:</p><pre><code class="kotlin hljs"> kotlinOptions { jvmTarget = JavaVersion.VERSION_11.toString() } </code></pre><p>When <span class="pre-inline">kotlinOptions</span> are not defined, the default <span class="pre-inline">jvmTarget</span> is again set to a very ancient Java 6. Please define your <span class="pre-inline">kotlinOptions</span>!</p><h2>The bottom line</h2><p>Better late than never, Java 11 has just arrived in the Android world. It won’t much change your day to day work of writing Java code. It may not change your work of writing Kotlin code at all. Nevertheless, it <em>is</em> a big thing for the ecosystem which promises easier upgrades in the future, that will in turn allow for the depreciation of lots of outdated stuff. Both, the Android team at Google and the Kotlin team at JetBrains may finally drop support for Java 6 and Java 8 and focus on more contemporary language versions. That is something we would all profit from.</p><p> <br> </p><p>Pavel Švéda<br></p><p>Twitter: <a href="">@xsveda​</a><br><br></p> ​<br>#android;#java;#kotlin;#gradle
Jetpack Compose: What you need to know, pt. 2 Compose: What you need to know, pt. 2<p>This is the second and final part of the Jetpack Compose series that combines curious excitement with a healthy dose of cautious skepticism. Let’s go!</p><h2>Ecosystem</h2><p><strong>Official documentation doesn’t cover enough.</strong></p><p>That’s understandable in this phase of development, but it absolutely needs to be significantly expanded before Compose hits 1.0.</p><p>On top of that, Google is once again getting into the bad habits of 1) mistaking developer marketing for advocacy and 2) scattering useful bits of information between <a href="">official docs</a>, KDoc, semi-official <a href="">blogs</a>, <a href="">code samples</a>, or other sources with unknown relevance. Although these can be useful, they’re difficult to find and are not usually kept up-to-date. </p><p><strong>Interoperability is good.</strong></p><p>We can use <a href="">legacy Views</a> in our Compose hierarchy and composables as <a href="">parts</a> of View-based UIs. It works, we can migrate our UIs gradually. This feature is also important in the long term, as I wouldn’t expect a Compose version of WebView or MapView written from scratch any time soon, if ever.</p><p>Compose also plays nicely with other libraries—it integrates well with Jetpack <a href="">ViewModel</a>, <a href="">Navigation</a>, or <a href="">reactive streams</a> (LiveData, RxJava, or Kotlin Flow—<a href="">StateFlow</a> is especially well suited for the role of a stream of states coming from the view model to the root composable). Popular 3rd party libraries such as <a href="">Koin</a> also have support for Compose.</p><p>Compose also gives us additional options. Its simplicity allows for much. For example, it is very well possible to completely get rid of fragments and/or Jetpack Navigation (although in this case, I think one vital piece of the puzzle is still missing—our DI frameworks need the ability to create scopes tied to composable functions), but of course you don’t have to. Choose what’s best for your app.</p><p>All in all, the future of the Compose ecosystem certainly looks bright.</p><p><strong>Tooling is a work in progress, but the fundamentals are already done.</strong></p><p>Compose alphas basically require <a href="">canary builds of Android studio</a>, which are expected to be a little bit unstable and buggy. Nevertheless, specifically for Compose, the Android tooling team has already added custom syntax and error highlighting for composable functions, a bunch of live templates, editor intentions, inspections, file templates, and even color previews in the gutter (Compose has its own color type).</p><p>Compose also supports <a href="">layout previews</a> in the IDE, but these are more cumbersome than their XML counterparts. A true hot reload doesn’t seem to be possible at the moment.</p><p>The IDE also sometimes struggles when a larger file with lots of deeply nested composable functions is opened in the editor. That said, the tooling won’t hinder your progress in a significant way.</p><p><strong>UI testing is perhaps more complicated than it was with the legacy toolkit.</strong></p><p>In Compose, there are no objects with properties in the traditional sense, so to facilitate UI tests, Compose (mis)uses its accessibility framework to expose information to the tests. </p><p>To be honest, it all feels a little bit hacky, but at least we have support for running the tests on JUnit 4 platform (with the help of a custom rule), <a href="">Espresso-like APIs</a> for selecting nodes and asserting things on them, and a helper function to print the UI tree to the console.</p><p>The situation is thus fairly similar to the legacy toolkit, and so is my advice: Mind the <a href="">test pyramid</a>, don’t rely too much on UI tests, and structure your app in such a way that the majority of the code can be tested by simple unit tests executed on the JVM.</p><h2>Performance and stability</h2><p><strong>Build speeds can be surprising.</strong></p><p>In a good way! One would think that adding an additional compiler to the build pipeline would slow things down (and on its own, it would), but Compose replaces the legacy XML layout system, which has its own performance penalties (parsing XMLs, compiling them as resources, etc.). </p><p>It turns out, even now when Compose is still in a very early stage of development, the build time of a project written with Compose is at least comparable to the legacy UI toolkit version—and it might be even faster, as measured <a href="">here</a>. </p><p><strong>Runtime performance is a mixed bag.</strong></p><p>UIs made with Compose can be laggy sometimes, but this is totally expected since we are still in alpha. Further optimizations are promised down the line, and because Compose doesn’t come with the burden of <a href="">tens of thousands of LOC</a> full of compatibility hacks and workarounds in each component, I hope someday Compose will actually be faster than the legacy toolkit.</p><p><strong>It crashes (it’s an alpha, I know).</strong></p><p>In my experience, Compose crashes both at compile time (the compiler plugin) and at runtime (usually because of a corruption of Compose’s internal data structure called “slot table”, especially when animations are involved). When it does crash, it leaves behind a very, very long stack trace that is full of synthetic methods, and which is usually also totally unhelpful. </p><p>We definitely need special debugging facilities for Compose (similar to what coroutines have), and yes, I know, the majority of these bugs will be ironed out before 1.0. The thing is, Compose simply must be reliable and trustworthy at runtime because we are not used to hard crashes from our UI toolkit—for many teams, that would be an adoption blocker. </p><h2>Expectations</h2><p><strong>Compose is meant to be the primary UI toolkit on Android.</strong></p><p>Several Googlers confirmed that if nothing catastrophic happens, this is the plan. Of course, it will take years, and as always, it won’t be smooth sailing all the way, but Google and JetBrains are investing heavily in Compose.</p><p><strong>Compose is no silver bullet.</strong></p><p>Yes, Compose in many ways simplifies UI implementation and alleviates a significant amount of painful points of the legacy UI toolkit.</p><p>At the same time, it’s still possible to repeat some horrible old mistakes regarding Android’s lifecycle (after all, your root composable must still live in some activity, fragment, or view), make a huge untestable and unmaintainable mess eerily similar to the situation when the whole application is written in one single Activity, or even invent completely new and deadly mistakes.</p><p>Compose is <em>not</em> an architecture. Compose is just a UI framework and as such it must be isolated behind strict borders. </p><p><strong>Best practices need to emerge.</strong></p><p>Compose is architecture-agnostic. It is well suited to clean architecture with MVVM, but that certainly isn’t the only possible approach, as it’s evident from the <a href="">official samples repo</a>. However, in the past, certain ideas proved themselves better than others, and we should think very carefully about those lessons and our current choices.</p><p>Just because these are official samples by Google (or by anyone else for that matter), that doesn’t mean you should copy them blindly. We are all new to this thing and as a community, we need to explore the possibilities before we arrive at a set of reasonable, reliable, and tried-and-proven best practices.</p><p>Just because we can do something doesn’t mean we should.</p><p><strong>There are a lot of open questions.</strong></p><p>The aforementioned official samples showcase a variety of approaches, but in my book, some are a little bit arguable or plainly wrong. For example, ask yourself: </p><p>How should the state be transformed while passed through the tree, if ever? How should internal and external states be handled? How smart should the composable functions be? Should a view model be available to any composable function directly? And what about repositories? Should composable functions have their own DI mechanism? Should composable functions know about navigation? And data formatting, or localization? Should they handle the process death themselves? The list goes on.</p><p><strong>Should you use it in production?</strong></p><p>Well, it entirely depends on your project. There are several important factors to consider:</p><ul><li>Being still in alpha, the APIs will change, sometimes significantly. Can you afford to rewrite big parts of your UI, perhaps several times? </li><li>There are features missing. This situation will get better over time, but what you need now matters the most. </li><li>Runtime stability might be an issue. You can work around some things, but there’s no denying that Compose right now is less stable than the legacy toolkit. </li><li>What is the lifespan of your application? If you’re starting an app from scratch next week, with plans to release v1.0 in 2022 and support it for 5 years, then Compose might be a smart bet. Another good use might be for proof of concept apps or prototypes. But should you rewrite all your existing apps in Compose right now? Probably not. </li></ul><p>As always with new technology, all these questions lead us to these: Are you an early adopter? Can you afford to be?</p><h2>Under the hood</h2><p><strong>Compose is very cutting edge (and in certain aspects quite similar to how coroutines work).</strong></p><p>In an ideal world, no matter how deeply composable functions were nested and how complex they were, we could call them all on each and every frame (that’s 16 milliseconds on 60 FPS displays, but faster displays are becoming more prevalent). However, hardware limitations of real world devices make that infeasible, so Compose has to resort to some very intricate optimizations. At the same time, Compose needs to maintain an illusion of simple nested function calls for us developers.</p><p>Together, these two requirements result in a technical solution that’s as radical as it’s powerful—changing language semantics with a custom Kotlin compiler plugin.</p><p><strong>Compose compiler and runtime are actually very interesting, general-purpose tools.</strong></p><p>Kotlin functions annotated with @Composable behave very differently to normal ones (as it’s the case with suspending functions). This is possible thanks to the <a href="">IR code</a> being generated for them by the compiler (Compose uses the Kotlin IR compiler backend, which itself is in alpha).</p><p>Compose compiler tracks input argument changes, inner states, and other stuff in an internal data structure called <em>slot table</em>, with the intention to execute only the necessary composable functions when the need arises (in fact, composable functions can be executed in any order, in parallel, or even not at all).</p><p>As it turns out, there are other use cases when this is very useful—composing and rendering UI trees is just one of them. Compose compiler and runtime can be used for <a href="">any programming task</a> where working efficiently with tree data structures is important.</p><p><strong>Compose is the first big sneak peek at Kotlin’s exciting future regarding compiler plugins.</strong></p><p>Kotlin compiler plugins are still very experimental, with the API being unstable and mostly undocumented (if you’re interested in the details, read <a href="">this blog series</a> before it becomes obsolete), but eventually the technology will mature—and when it does, something very interesting will happen: Kotlin will become a language with more or less stable, fixed <em>syntax</em>, and vastly changeable, explicitly pluggable <em>behavior</em>.</p><p>Just look at what we have at our disposal even now, when the technology is in its infancy: There is Compose, of course (with a <a href="">desktop port</a> in the works), a plugin to <a href="">make classes open</a> to play nice with certain frameworks or tests, <a href="">Parcelable generator</a> for Android, or <a href="">exhaustive when for statements</a>, with <a href="">more plugins</a> coming in the future.</p><p>Last but not least, I think that the possibility to modify the language with external, independent plugins will lower the pressure on language designers, reducing the risk of bloating the language—when part of the community demands some controversial feature, why not test-drive it in the form of a compiler plugin first?</p><h2>Final words</h2><p>Well, there you have it—I hope this series helped you to create an image of Compose in your head that is a little bit sharper than the one you had before. Compose is certainly going to be an exciting ride!</p>#android;#jetpack;#compose;#ui
Architecture tests with ArchUnit, pt. 1: Reasons tests with ArchUnit, pt. 1: Reasons<p>Good architecture is essential for a codebase to enjoy a long and happy life (the other crucial ingredient is running automated unit tests, but that’s a different blog post). Nowadays there are many sensible options, including our favorite for mobile apps, Clean arch, but no matter which one you choose, you’ll need to document and enforce it somehow.</p><p>The former is traditionally accomplished by some form of oral history passed from one developer to another (sometimes augmented by blurry photos of frantically scribbled whiteboard diagrams), while the latter is sporadically checked during code reviews (if there’s time—which there isn’t—and you remember all the rules yourself—which you don’t).</p><p>Or maybe you even have a set of gradually outdated wiki pages and fancy UML models created in some expensive enterprise tool. Or complicated but incomplete rules written for rather arcane static analysis frameworks. Or any other form of checks and docs, which are usually hard to change, detached from the actual code and difficult and rather expensive to maintain.</p><p>But fret not! There is a new architectural sheriff in this JVM town of ours and he’s going to take care of all of this—say hello to your new best friend, <a href="">ArchUnit</a>! </p><h2>What’s all the fuss about?</h2><p>ArchUnit is a library to, well, unit test your architecture. There are other tools to <em>check</em> your architecture, but the “unit testing” part of ArchUnit is actually its killer feature.</p><p>While “normal” unit tests should describe behavior (not structure!) of the system under test, ArchUnit cleverly leverages JVM and existing unit test frameworks to let you document <em>and</em> check your architecture in a form of runnable unit tests, executable in your current unit test environment (because you already have a strong suite of unit tests, right?). Why exactly is this such a welcome improvement?</p><p>Well, it all boils down to the fundamental benefits of all unit tests: Because unit tests are code, they are a precise, up-to-date, unambiguous, executable specification of the system. Docs can be outdated and misleading, but unit tests either compile or don’t; they either pass or not. Imagine opening a project you don’t know anything about, running its unit tests and seeing this:</p> <img alt="ArchUnit test results in Android Studio" src="/Blog/PublishingImages/Articles/arch-unit-1-01.png" data-themekey="#" /> <p>Suddenly the whole onboarding situation looks much brighter, doesn’t it?</p><h2>Show me the code</h2><p>Enough talk, let’s get down to business! If your test framework of choice is JUnit 4, put this in your <span class="pre-inline">build.gradle.kts</span>:</p><pre><code class="kotlin hljs">dependencies { testImplementation("com.tngtech.archunit:archunit-junit4:0.14.1") } </code></pre><p>There are artifacts for other test frameworks as well, just refer to the <a href="">docs</a>. Be careful not to use older versions as this version contains important fixes for multi-module projects containing Android libraries in a CI environment.</p><p>Now we can write our first architecture test:</p><pre><code class="kotlin hljs">@RunWith(ArchUnitRunner::class) @AnalyzeClasses(packages = ["com.example.myapp"]) internal class UiLayerTest { @ArchTest val `view model subclasses should have correct name` = classes().that().areAssignableTo( .should().haveSimpleNameEndingWith("ViewModel") } </code></pre><p>And just like that, you now have one small naming convention documented and automatically verified across your whole project. The API does a great job at being self-explanatory and we’ll get into the details later, but let’s quickly recap what we have here:</p><p><span class="pre-inline">@AnalyzeClasses</span> annotation is one of the ways to specify what to check. Here, we simply want to test all code in the <span class="pre-inline">com.example.myapp</span> package and its subpackages. ArchUnit imports and checks Java bytecode (not source files), which is why it works with Kotlin (or any other JVM language), although it’s itself a pure Java library—another example of Kotlin’s stellar interoperability with Java. <em>Where</em> ArchUnit actually gets this bytecode is a slightly more complicated question, but that’s not important right now.</p><p>Anyway, we annotate our test cases with <span class="pre-inline">@ArchTest</span> and for the shortest syntax, we use properties instead of functions. As with other unit tests, it’s a good idea to leverage Kotlin’s escaped property names for more readable test outputs.</p><p>And then finally for the main course: ArchUnit has a comprehensive, very expressive and really rather beautiful fluent API for specifying the predicates and their expected outcomes. It’s not Java reflection and being a pure Java library, ArchUnit doesn’t have constructs for Kotlin-exclusive language elements, but it’s still more than powerful enough.</p><h2>Test the tests</h2><p>Now run the test. Most projects probably stick to this naming convention, so the result bar in your favorite IDE might be green already. But wait! How do we know that the tests actually work?</p><p>Although they may appear a bit strange, ArchUnit tests are still unit tests and we should treat them as such. That means we should follow the famous red-green-refactor cycle, albeit modified, because you absolutely need to see the test fail and it must fail for the correct reason. This is the only time when you actually test your tests!</p><p>What does this mean for ArchUnit tests? The difference from normal TDD for our specific test case is that we cannot simply write the test first and watch it fail, because if there are no view models in the project yet, the test will pass. So we need to cheat a little and break the architecture on purpose, manually, by creating a temporary class violating the naming convention in the main source set. Then we run the test, watch it fail, delete the class and watch the test go green (the refactoring part isn’t really applicable here).</p><p>This looks like extra work and granted, it can be a bit tedious, but the red part of the cycle simply cannot be skipped, ever. There is a myriad of logical and technical errors that can result in the test being wrong or not executed at all and this is your only chance to catch them. There’s nothing worse than a dead lump of code giving you a false sense of security.</p><p>And there’s one more thing to borrow from TDD playbook: Perhaps you are doing a code review or approving pull request and you discover some construction violating a rule that you haven’t thought of before. What to do with that? As with all new bugs, don’t rush fixing the bug! The first thing you should do is write a test exposing the bug (the red part of the cycle)—that means writing an ArchUnit rule which will fail with the offending code. Only after that, make the test green. This way, you’ll slowly make your test suite more precise, with the added bonus that future regressions will be prevented as well.</p><h2>Be careful what you test for</h2><p>We’ll take a look at all ArchUnit’s fluent API constructs in a future post, but there’s an important detail we need to discuss before that.</p><p>Basically all simple ArchUnit rules follow the form <span class="pre-inline">(no) LANGUAGE_ELEMENT that PREDICATE should (not) CONDITION</span>. From a mathematical point of view, these rules are <em>implications</em>.</p><p>An implication looks like this:</p> <img alt="Venn diagram of an implication" src="/Blog/PublishingImages/Articles/arch-unit-1-02.png" data-themekey="#" /> <p>For our example test above (and many other tests that you’ll write), it means that the test will pass for <em>all</em> these variants:</p><ul><li>class is not assignable to <span class="pre-inline">ViewModel::class</span> and does not have a simple name ending with <span class="pre-inline">ViewModel</span> (that’s OK) </li><li>class is assignable to <span class="pre-inline">ViewModel::class</span> and has a simple name ending with <span class="pre-inline">ViewModel</span> (that’s also OK) </li><li>class is not assignable to <span class="pre-inline">ViewModel::class</span> and has a simple name ending with <span class="pre-inline">ViewModel</span> (the criss-crossed part of the diagram; we don’t really want to allow this) </li></ul><p>It seems that what we really want is an equivalence:</p> <img alt="Venn diagram of an equivalence" src="/Blog/PublishingImages/Articles/arch-unit-1-03.png" data-themekey="#" /> <p>Although ArchUnit doesn’t (yet?) have API elements to specify equivalences, they are fairly simple to create: Because A ↔ B is the same as (A → B) AND (B → A), we just need to add another test to our suite:</p><pre><code class="kotlin hljs">@ArchTest val `classes named ViewModel should have correct super class` = classes().that().haveSimpleNameEndingWith("ViewModel") .should().beAssignableTo( </code></pre><p>This way, the offending case which the first test didn’t catch (class name ends with <span class="pre-inline">ViewModel</span>, but it is not assignable to <span class="pre-inline"></span>) is prevented.</p><h2>Best thing since sliced bread</h2><p>I don’t want to use the word game-changer, but I just did. Since we started adding ArchUnit tests to our projects, we have seen significant improvements in developer productivity and the health of our codebases. Compared to similar solutions, ArchUnit’s simple integration, ease of use and expressive powers are unmatched.</p><p>We’ve only scratched the surface of what’s possible, so <a href="/Blog/Pages/arch-unit-2.aspx">next time</a>, we’ll dive into ArchUnit APIs to discover some nifty architecture testing goodness! </p> #architecture;#jvm;#tdd;#android
Architecture tests with ArchUnit, pt. 2: Rules tests with ArchUnit, pt. 2: Rules<p>In the <a href="/Blog/Pages/arch-unit-1.aspx">first part</a> of this series, we’ve had a glimpse of an architecture test written with the almighty ArchUnit, but of course there’s much more! Although ArchUnit’s API looks like Java Reflection API, it also contains powerful constructs to describe dependencies between code or predefined rules for testing popular architecture styles. Let’s see what we’ve got to play with!</p><h2>First things first</h2><p>ArchUnit rules follow the pattern <span class="pre-inline">(no) LANGUAGE_ELEMENT that PREDICATE should (not) CONDITION</span>. So what language elements can we use?</p><p>All tests begin with static methods in <span class="pre-inline">ArchRuleDefinition</span> class (but please import the class to make the rules more readable).</p><p>We can start with <span class="pre-inline">classes</span> or <span class="pre-inline">constructors</span> which are pretty self-explanatory. We also have <span class="pre-inline">theClass</span> if you want to be brief and specific. If possible, always use the overload that takes <span class="pre-inline">Class<*></span> argument instead of the overload that takes String to make your tests resistant to future refactorings; the same goes for other methods with these argument choices.</p><p>Next, we have <span class="pre-inline">fields</span>, <span class="pre-inline">methods</span> and <span class="pre-inline">members</span>. When testing Kotlin code, be extra careful with <span class="pre-inline">fields</span> because Kotlin properties are <em>not</em> Java fields. Remember that ArchUnit checks compiled bytecode and every Kotlin property is actually compiled to getter method by prepending the <span class="pre-inline">get</span> prefix, setter method by prepending the <span class="pre-inline">set</span> prefix (only for <span class="pre-inline">var</span> properties) and private field with the same name as the property name, but <em>only for properties with backing fields</em>. When testing Kotlin properties, it may sometimes be safer to test their generated getters or setters. Anyway, these subtle details show the importance of watching your test fail.</p><p>We also have a slightly mysterious <span class="pre-inline">codeUnits</span> method—it means simply anything that can access other code (including methods, constructors, initializer blocks, static field assignments etc.).</p><p>All methods mentioned above also have their negated variants. Now what can we do with all this?</p><h2>Packages, packages everywhere</h2><p>Consistent packaging is one of the most important things to get right in the project. We strongly prefer packaging by features first, then by layers. This concept sometimes goes by the name of “screaming architecture”: For example, when you open an Android project and you see top level packages such as <span class="pre-inline">map</span>, <span class="pre-inline">plannedtrips</span>, <span class="pre-inline">routeplanning</span>, <span class="pre-inline">speedlimits</span>, <span class="pre-inline">tolls</span>, <span class="pre-inline">vehicles</span> or <span class="pre-inline">voiceguidance</span>, you’ll get a pretty good idea about what the app is really about. But if instead you are looking at packages such as <span class="pre-inline">activities</span>, <span class="pre-inline">fragments</span>, <span class="pre-inline">services</span>, <span class="pre-inline">di</span>, <span class="pre-inline">data</span>, <span class="pre-inline">apis</span>, etc., it won’t tell you much about the application (every Android app will contain at least some of those things).</p><p>ArchUnit can enforce correct package structure, prevent deadly cyclic dependencies and much more. Let’s see a few examples (the actual packages mentioned are not important, use what is convenient for your project):</p><pre><code class="kotlin hljs">@ArchTest val `every class should reside in one of the specified packages` = classes().should().resideInAnyPackage( "..di", "..ui", "..presentation", "..domain", "" ) </code></pre><p>The two dots mean “any number of packages including zero”, so this test says that every class must exist in one of these predefined leaf packages.</p><p>This test however doesn’t say anything about the package structure <em>above</em> the leaves, so if you want to be more strict, you can write this, for example: </p><pre><code class="kotlin hljs">@ArchTest val `every class should reside in one of the specified packages` = classes().should().resideInAnyPackage( "com.example.myapp.*.di", "com.example.myapp.*.ui", "com.example.myapp.*.presentation", "com.example.myapp.*.domain", "com.example.myapp.*.data" ) </code></pre><p>The star matches any sequence of characters excluding the dot (for our sample packaging, in its place there would be a feature name), but you can also use <span class="pre-inline">**</span> which matches any sequence of characters <em>including</em> the dot. Together with the two dot notation, you can express pretty much any package structure conceivable (see the Javadoc for <span class="pre-inline">PackageMatcher</span> class).</p><h2>Building the walls</h2><p>One popular architectural style is to divide the code into layers with different levels. We can define layer level simply as the code’s distance from inputs/outputs—so things like UI, DB or REST clients are pretty low-level, whereas business logic and models are on the opposite side and the application logic sits somewhere in the middle.</p><p>In this case, it’s a good idea to isolate higher-level layers from external dependencies such as platform SDK or other invasive frameworks and libraries, since higher levels should be more stable and independent of the implementation details in lower layers. ArchUnit can help us with that:</p><pre><code class="kotlin hljs">@ArchTest val `higher-level classes should not depend on the framework` = noClasses().that().resideInAnyPackage("..presentation..", "..domain..") .should().dependOnClassesThat().resideInAnyPackage( "android..", "androidx..", "", "" /* and more */ ) </code></pre><p>Only a few lines and those pesky imports have no way of creeping in your pristine (and now even fairly platform-independent!) code.</p><h2>Piece(s) of cake</h2><p>Speaking of layers, we should not only handle their dependencies on the 3rd party code, but of course also the direct dependencies between them. Although we can use the constructs mentioned above, ArchUnit has another trick up to its sleeve when it comes to layered architectures.</p><p>Suppose we have defined these layers and their <em>code</em> dependencies:</p> <img alt="Example layer structure" src="/Blog/PublishingImages/Articles/arch-unit-2-01.png" data-themekey="#" /> <p>This is just an example, but let’s say that the domain layer is the most high-level, so it must not depend on anything else; presentation and data layers can depend on stuff from domain, UI can see view models in presentation layer (but view models must not know anything about UI) and DI sees all to be able to inject anything (and ideally, no other layer should see DI layer, because classes should not know anything about how they are injected; alas this is not always technically possible).</p><p>Whatever your actual layers are, the most important thing is that all dependencies go in one direction only, from lower level layers to higher level layers (this is the basic idea of Clean architecture). ArchUnit can encode these rules in one succinct test:</p><pre><code class="kotlin hljs">@ArchTest val `layers should have correct dependencies between them` = layeredArchitecture().withOptionalLayers(true) .layer(DOMAIN).definedBy("..domain") .layer(PRESENTATION).definedBy("..presentation") .layer(UI).definedBy("..ui") .layer(DATA).definedBy("") .layer(DI).definedBy("..di") .whereLayer(DOMAIN).mayOnlyBeAccessedByLayers(DI, PRESENTATION, DATA) .whereLayer(PRESENTATION).mayOnlyBeAccessedByLayers(DI, UI) .whereLayer(UI).mayOnlyBeAccessedByLayers(DI) .whereLayer(DATA).mayOnlyBeAccessedByLayers(DI) .whereLayer(DI).mayNotBeAccessedByAnyLayer() </code></pre><p>How does it work? <span class="pre-inline">layeredArchitecture()</span> is a static method in the <span class="pre-inline">Architectures</span> class (again, please import it). First we need to actually define our layers: <span class="pre-inline">layer</span> declares the layer (the argument is simply any descriptive String constant) and <span class="pre-inline">definedBy</span> specifies a package by which the layer is, well, defined (you can use package notation which we’ve seen before; you can also use a more general predicate). Without <span class="pre-inline">withOptionalLayers(true)</span> call, ArchUnit will require that all layers exist, which in a multi-module project might not necessarily be true (some modules might for example contain only domain stuff).</p><p>This rather short test will have an enormous impact on your codebase—correctly managed dependencies are what prevents your project from becoming a giant mess of spaghetti code.</p><h2>Inner beauty</h2><p>We’ve sorted the layers and packages, but what about their content? Take for example the domain layer: Continuing our rather simplified example, we want only <span class="pre-inline">UseCase</span> classes and <span class="pre-inline">Repository</span> interfaces in there. Furthermore, we want for these classes to follow certain name conventions and to extend correct base classes.</p><p>We can express all these requirements by the following set of ArchUnit tests:</p><pre><code class="kotlin hljs">@ArchTest val `domain layer should contain only specified classes` = classes().that().resideInAPackage("..domain..") .should().haveSimpleNameEndingWith("UseCase") .andShould().beTopLevelClasses() .orShould().haveSimpleNameEndingWith("Repository") .andShould().beInterfaces() @ArchTest val `classes named UseCase should extend correct base class` = classes().that().haveSimpleNameEndingWith("UseCase") .should().beAssignableTo( @ArchTest val `use case subclasses should have correct name` = classes().that().areAssignableTo( .should().haveSimpleNameEndingWith("UseCase") </code></pre><p>And as a bonus example for Android fans, you can, of course, be even more specific:</p><pre><code class="kotlin hljs">@ArchTest val `no one should ever name fields like this anymore ;)` = noFields().should().haveNameMatching("m[A-Z]+.*") </code></pre><h2>Endless power</h2><p>We’ve seen only a smart part of the ArchUnit API, but there’s almost nothing that ArchUnit tests cannot handle. You can examine all Java constructs and their wildest combinations (but always be aware of Kotlin-Java interoperability details and test your tests), go explore!</p><p>Next time, we’ll take a look at some advanced features and configuration options.</p>#architecture;#jvm;#tdd;#android
Architecture tests with ArchUnit, pt. 3: Advanced stuff tests with ArchUnit, pt. 3: Advanced stuff<p>​​​ArchUnit has many tricks up to its sleeve. We’ve <a href="/Blog/Pages/arch-unit-2.aspx">already seen</a> how to check package structure, language elements such as classes, fields, and methods, and how to make sure your layered architecture is sound. But there’s more! Let’s take a look at some advanced conce​pts.</p><h2>Slicing and dicing</h2><p>As we’ve seen in the previous part of this series, ArchUnit makes it easy to test layers and their relationships. However, dividing your codebase only horizontally isn’t enough—in all but very tiny projects, you’d end up with huge layers containing too many unrelated classes. Thus we need to divide our code vertically as well, usually by user-facing features and project-specific or general-purpose libraries. Those features or libraries are often compilation units (e.g., Gradle modules), but that doesn’t need to concern us at this moment.</p> <img alt="Package and module structure" src="/Blog/PublishingImages/Articles/arch-unit-3-01.png" data-themekey="#" /> <p>So, if we have defined several vertical slices of our codebase, we would like to test their relationships as well. Horizontal layer tests work <em>across</em> all slices, so they won’t help us in this case, but ArchUnit has us covered with its high-level slices API:</p><pre><code class="kotlin hljs">@ArchTest val `feature slices should not depend on each other` = slices().matching("com.example.myapp.feature.(*)..") .should().notDependOnEachOther() </code></pre><p>This is a good rule to have, as you usually want your features to be isolated from each other. How does it work?</p><p>First, we define the matcher which slices the codebase vertically: It takes package notation which we’ve seen in the previous rules. The matcher group denoted by parentheses specifies the actual slicing point as well as the slice’s name shown in error messages.</p><p>In this case, code units residing in the following example packages <em>and</em> their subpackages would constitute separate slices: <span class="pre-inline">com.example.myapp.feature.login</span>, <span class="pre-inline"></span> or <span class="pre-inline">com.example.myapp.feature.navigation</span>. </p> <img alt="Feature slices" src="/Blog/PublishingImages/Articles/arch-unit-3-02.png" data-themekey="#" /> <p>In contrast, writing <span class="pre-inline">matching("com.example.myapp.feature.(**)")</span>, then <span class="pre-inline">com.example.myapp.feature.login.model</span> and <span class="pre-inline"></span> would constitute <em>different</em> slices.</p> <img alt="Layer slices" src="/Blog/PublishingImages/Articles/arch-unit-3-03.png" data-themekey="#" /> <p>Be careful, as this distinction might be rather subtle!</p><p>As always when practicing TDD, you need to see your test fail for the right reason—in this case that means creating a temporary file that intentionally breaks the test and deleting it afterwards.</p><p>The rest of the rule is simple: After the usual <span class="pre-inline">should()</span> operator, we have only two options: <span class="pre-inline">notDependOnEachOther()</span> tests that, well, no slice depends on any other (unlike the layer dependency tests, these tests are bi-directional), whereas <span class="pre-inline">beFreeOfCycles()</span> allows dependencies between the slices, but only in one direction at most.</p><p>Generally speaking, it may be a good idea to run the <span class="pre-inline">beFreeOfCycles()</span> test on every slice (using one of the two test variants mentioned above) in your codebase, whereas some types of slices (typically libraries, but not features) may be permitted to depend on each other in one direction. </p><p>But what if your codebase isn’t structured in such a convenient way? For example, there might be no middle <span class="pre-inline">feature</span> package distinguishing features from libraries, or worse, the package structure may be completely inconsistent.</p><p>For such cases, ArchUnit contains handy <span class="pre-inline">SliceAssignment</span> interface which you can use to assign slices to classes in a completely arbitrary way:</p><pre><code class="kotlin hljs">@ArchTest private val features = object : SliceAssignment { override fun getIdentifierOf(javaClass: JavaClass) = when { javaClass.packageName.startsWith("com.example.myapp.login") -> SliceIdentifier.of("feature-login")"map") -> SliceIdentifier.of("feature-map") /* ... whatever you need ... */ else -> SliceIdentifier.ignore() override fun getDescription() = "this will be added to the error message" } </code></pre><p>Strings given to <span class="pre-inline">SliceIdentifier</span> are arbitrary constants identifying the slice and are also shown in error messages.</p><p>There is an important difference in what you write in that <span class="pre-inline">else</span> branch: If you return <span class="pre-inline">SliceIdentifier.of("remaining")</span>, then all classes not matching the previous cases will be assigned to the <span class="pre-inline">"remaining"</span> slice (which means they will be tested against other slices), whereas if you return <span class="pre-inline">SliceIdentifier.ignore()</span>, those classes won’t participate in the test at all (both options have their uses, but be careful not to confuse them).</p><p>We can then use our slice assignment like this:</p><pre><code class="kotlin hljs">slices().assignedFrom(features).should().notDependOnEachOther()</code></pre><h2>Why be in when you could be out?</h2><p>As we’ve learned, ArchUnit runs its tests on compiled bytecode. But where do these classes come from?</p><p>There is more than one way to specify that, but probably the most succinct is to use this annotation:</p><pre><code class="kotlin hljs">@RunWith(ArchUnitRunner::class) @AnalyzeClasses(packages = ["com.example.myapp"]) internal class MyArchTest </code></pre><p>Besides using String literals, we can specify packages with Classes or, if that’s not enough, completely customize the sources using ArchUnit’s <span class="pre-inline">LocationProvider</span>. In every case, please note that ArchUnit looks for packages within the current classpath and <em>all</em> classes must be imported for ArchUnit to be able to work correctly—if you import class <span class="pre-inline">X</span>, you need to import all its dependencies as well, transitively, otherwise ArchUnit will have only a limited amount of information to work with and the tests might yield false positives, giving you a false sense of security.</p><p>Now we have all the class locations that ArchUnit needs, but there are situations when we don’t necessarily need to test against all of the classes in there. We can filter the classes with <span class="pre-inline">importOptions</span>:</p><pre><code class="kotlin hljs">@RunWith(ArchUnitRunner::class) @AnalyzeClasses( packages = ["com.example.myapp"], importOptions = [ DoNotIncludeArchives::class, DoNotIncludeTests::class, DoNotIncludeAndroidGeneratedClasses::class ] ) internal class MyArchTest </code></pre><p>ArchUnit comes with a couple of handy predefined import options, such as the first two, or we can write our own, which is simple enough:</p><pre><code class="kotlin hljs">internal class DoNotIncludeAndroidGeneratedFiles : ImportOption { companion object { private val pattern = Pattern.compile(".*/BuildConfig\\.class|.*/R(\\\$.*)?\\.class|.*Binding\\.class") } override fun includes(location: Location) = location.matches(pattern) } </code></pre><p>This import option rejects Android <span class="pre-inline">BuildConfig</span>, <span class="pre-inline">R</span>, and <span class="pre-inline">Binding</span> classes. The location argument passed here is platform-independent, so you don’t have to worry about path separators and such things.</p><p>But what if we need to be more granular? For example, sometimes we might need to ignore certain classes on a per-test basis, basically adding ad-hoc exceptions to our pristine rules, because, you know, the real world happens. This is a slippery slope, so don’t forget to document such situations, but it’s simple enough to do by adding <span class="pre-inline">or</span> clause to the rule:</p><pre><code class="kotlin hljs">@ArchTest val `domain layer should contain only specified classes` = classes().that().resideInAPackage("..domain..") .should().haveSimpleNameEndingWith("Repository").andShould().beInterfaces() .orShould().haveSimpleNameEndingWith("Controller").andShould().beInterfaces() .orShould().haveSimpleNameEndingWith("UseCase") .orShould().be( .because("only repositories, controllers and use cases are permitted in domain and Data is special wrapper for results from those classes") </code></pre><p>Be specific as possible with your exceptions as you don’t want them to accidentally match more language elements than intended—here, using class literal instead of String is safe and future refactoring-proof.</p><p><span class="pre-inline">Because</span> clause allows you to add more detail to the default error message generated from the test case name. There is also <span class="pre-inline">`as`</span> clause (ArchUnit being written in Java, it accidentally overloads Kotlin’s keyword, so don’t forget to escape it or create an alias extension function with a Kotlin-friendly name) that allows you to completely override the error message.</p><h2>Godspeed</h2><p>Because ArchUnit does a lot under the hood (bytecode examination and cycle checks tend to be expensive, among other things), the tests themselves, although written using unit test infrastructure, seldom have the <em>speed</em> of normal unit tests. The speed penalty can be quite severe, so organize your test suites in a way that ArchUnit tests don’t slow you down when developing your code using the classic TDD cycle of red-green-refactor, which has to be always blazingly fast.</p><p>That said, all ArchUnit’s features working together allow us to express our architecture in a clean, succinct, and <em>executable</em> way, which is invaluable in larger projects. Consistency and clarity are virtues that may very well make the difference between a codebase that is easy to learn and maintain and one that becomes a big steaming pile of unreadable mess.</p>#architecture;#jvm;#tdd;#android
Architecture tests with ArchUnit, pt. 4: Extensions & Kotlin support tests with ArchUnit, pt. 4: Extensions & Kotlin support<p>​ArchUnit is <a href="/Blog/Pages/arch-unit-3.aspx">immensely capable on its own</a> and that's a great merit on its own, but it doesn’t stop there—ArchUnit’s power can be augmented by adding custom matchers, language elements, and even whole new concepts. In this post, we’ll look at how we can achieve that and then we’ll see if we can leverage these capabilities to support even Kotlin-exclusive language elements in ArchUnit tests (spoiler alert: yes, we can!). Ready?</p><h2>Shiny new things</h2><p>As we’ve mentioned several times, ArchUnit rules look like this:</p><pre><code class="kotlin hljs">(no) LANGUAGE_ELEMENT that PREDICATE should (not) CONDITION</code></pre><p>As it turns out, by thoroughly following the open/closed principle, ArchUnit allows us to supply our own language elements, predicates and conditions. These can be simple aggregations of existing built-in predicates or conditions to facilitate reuse, or we can invent entirely new domain-specific concepts to utilize in our architecture tests. So how is it done?</p><p>To create a custom language element, predicate or condition, we need to extend <span class="pre-inline">AbstractClassesTransformer</span>, <span class="pre-inline">DescribedPredicate</span>, or <span class="pre-inline">ArchCondition</span> respectively. Each abstract base class takes one type argument—the language element it operates on (ArchUnit provides for example <span class="pre-inline">JavaClass</span>, <span class="pre-inline">JavaMember</span>, <span class="pre-inline">JavaField</span> or <span class="pre-inline">JavaCodeUnit</span> and we can even create our own; these are reflection-like models read from compiled bytecode). They also have one constructor argument, a String that is appended to the rule description in error messages.</p><p>The simplest one to implement is <span class="pre-inline">DescribedPredicate</span>—we need to override its <span class="pre-inline">apply</span> method which takes the examined language element and returns Boolean:</p><pre><code class="kotlin hljs">val myPredicate = object : DescribedPredicate("rule description") { override fun apply(input: JavaClass): Boolean = // … }</code></pre><p><span class="pre-inline">ArchCondition</span> is slightly more involved, as its <span class="pre-inline">check</span> function takes the language element as well. In addition, it also takes <span class="pre-inline">ConditionEvents</span> collection, which is used to return the result of the evaluation, as this function doesn’t directly return anything: </p><pre><code class="kotlin hljs">val myCondition = object : ArchCondition("condition description") { override fun check(item: JavaClass, events: ConditionEvents) { if (item.doesNotSatisfyMyCondition()) { events.add(SimpleConditionEvent.violated(item, "violation description")) } } }</code></pre><p><span class="pre-inline">AbstractClassesTransformer</span> has a <span class="pre-inline">doTransform</span> method which takes a collection of <span class="pre-inline">JavaClass</span>es and transforms it to another collection. Elements of the output collection can be <span class="pre-inline">JavaClass</span>es as well, different built-in types or even custom classes. The transformation may comprise any number of operations including mapping or filtering:</p><pre><code class="kotlin hljs">val myTransformer = object : AbstractClassesTransformer("items description") { override fun doTransform(collection: JavaClasses): Iterable = collection.filter { /* ... */ }.map { /* ... */ } }</code></pre><p>Anyway, we can use our custom transformers, predicates, and conditions like this:</p><pre><code class="kotlin hljs">all(encryptedRepositories) .that(handleGdprData) .should(useSufficientlyStrongCrypthographicAlgorithms)</code></pre><p>and they can, of course, be combined with the built-in ones.</p><p>Besides promoting reuse, custom transformers, predicates, and conditions are particularly good at increasing the level of abstraction of your tests—it’s better to describe your system using its domain language instead of low-level, opaque technical terms.</p><h2>Gimme some Kotlin lovin’</h2><p>As promised, now it’s the time to tackle the last thing we’d like to have in ArchUnit tests—Kotlin support.</p><p>Because ArchUnit reads compiled bytecode and Kotlin has killer Java interoperability, we can get pretty far out of the box, but we still can’t directly test for Kotlin stuff like sealed and data classes, objects, typealiases, suspending functions etc. To find out how that could be possible, we need to take a slight detour first.</p><p>When targeting JVM (or Android), Kotlin compiler outputs JVM bytecode. Adding custom bytecodes for Kotlin-exclusive constructs is of course out of the question, so the compiler must resort to clever tricks to convert Kotlin stuff to vanilla JVM bytecode. Now, there’s some wiggle room (JVM bytecode allows some things that Java as a language doesn’t), but still, to achieve Kotlin’s stellar level of Java interoperability, the compiler must mostly play by Java’s rules.</p><p>To achieve that, for example, Kotlin compiler generates getters, setters and backing fields for properties. It also creates encapsulating classes for top level functions and properties, adds new functions to existing classes (to support data classes) or adds parameters to existing functions (this is one of the tricks behind suspending functions).</p><p>As a result, when examining the bytecode alone, those Kotlin concepts effectively disappear. But for Kotlin compiler to be able to compile Kotlin code against another <em>already compiled</em> Kotlin code (and to see it as Kotlin code, not generic JVM code), this information must be preserved somewhere.</p><p>Take for example this simple data class:</p><pre><code class="kotlin hljs">data class Money(val amount: BigDecimal, val currency: Currency)</code></pre><p>IntelliJ Idea/Android Studio lets us see bytecode generated from this Kotlin code, which in turn can be (in most cases) decompiled to equivalent Java code. If we do that with the <span class="pre-inline">Money</span> class, we’ll see something like this:</p><pre><code class="java hljs">@Metadata( mv = {1, 1, 16}, bv = {1, 0, 3}, k = 1, d1 = {"\u0000,\n\u0002\u0018\u0002\n\ /* rest omitted */ }, d2 = {"Lcom/example/myapp/Money;", "", "amount", "Ljava/math/BigDecimal;", "currency", "Ljava/util/Currency;", "(Ljava/math/BigDecimal;Ljava/util/Currency;)V", "getAmount", "()Ljava/math/BigDecimal;", "getCurrency", "()Ljava/util/Currency;", "component1", "component2", "copy", "equals", "", "other", "hashCode", "", "toString", "", "app-main"} ) public final class Money { @NotNull private final BigDecimal amount; @NotNull private final Currency currency; @NotNull public final BigDecimal getAmount() { return this.amount; } @NotNull public final Currency getCurrency() { return this.currency; } /* rest omitted */</code></pre><p>Bingo! The Java part is pretty straightforward, but it looks like that strange <span class="pre-inline">@Metadata</span> stuff might be what we need. Indeed, the documentation for <span class="pre-inline">@Metadata</span> says that “This annotation is present on any class file produced by the Kotlin compiler and is read by the compiler and reflection.” Its arguments contain various interesting Kotlin-exclusive bits and pieces related to the class and because it has runtime retention, it will be stored in binary files, which means we can read them from our ArchUnit tests! If only we could make sense of that gibberish inside the annotation…</p><h2>Metadata dissection</h2><p>It turns out that we can! There’s a small official library to do just that.</p><p>First, add JVM metadata library to your build script:</p><pre><code class="kotlin hljs">dependencies { testImplementation("org.jetbrains.kotlinx:kotlinx-metadata-jvm:0.1.0") }</code></pre><p>Then, our plan of attack is this:</p><ol><li>The starting point is the input of our custom transformer, predicate, or condition, which in this case will be ArchUnit’s <span class="pre-inline">JavaClass</span> object. </li><li>ArchUnit can read annotations on the <span class="pre-inline">JavaClass</span> object, so we examine if Kotlin’s <span class="pre-inline">@Metadata</span> annotation is present. </li><li>If it is, we use the <a href="">kotlinx-metadata</a> library to read the actual metadata. (KotlinPoet has a <a href="">higher-level API</a> based on kotlinx-metadata, which presumably might be a little bit nicer to use; we’ll just use the basic API here, as the end result will be the same in either case.) </li><li>We expose the data in some easily digestible object so we can write simple and readable assertions about it. </li></ol><p>To make an already long story short, here is the first piece of the puzzle—transformation from ArchUnit’s <span class="pre-inline">JavaClass</span> to kotlinx-metadata <span class="pre-inline">KmClass</span> model:</p><pre><code class="kotlin hljs">private fun JavaClass.toKmClass(): KmClass? = this .takeIf { it.isAnnotatedWith( } ?.getAnnotationOfType( ?.let { metadata -> KotlinClassHeader( kind = metadata.kind, metadataVersion = metadata.metadataVersion, bytecodeVersion = metadata.bytecodeVersion, data1 = metadata.data1, data2 = metadata.data2, extraString = metadata.extraString, packageName = metadata.packageName, extraInt = metadata.extraInt ) ) } ?.let { (it as? KotlinClassMetadata.Class)?.toKmClass() }</code></pre><p>If a given <span class="pre-inline">JavaClass</span> is annotated with <span class="pre-inline">@Metadata</span>, this extension reads the annotation and converts it to a  <span class="pre-inline">KotlinClassMetadata</span> object (mapping the annotation attributes to the corresponding properties of <span class="pre-inline">KotlinClassHeader</span> along the way). </p><p><span class="pre-inline">KotlinClassMetadata</span> is a sealed class and its subclasses represent various different kinds of classes generated by the Kotlin compiler. There are a few of them, but to keep things simple we are interested only in “real” classes (<span class="pre-inline">KotlinClassMetadata.Class</span>) from which we finally extract the rich <span class="pre-inline">KmClass</span> model (and return null in all other cases).</p><p>To make our life easier later, we also add this handy extension:</p><pre><code class="kotlin hljs">private fun JavaClass.isKotlinClassAndSatisfies(predicate: (KmClass) -> Boolean): Boolean = this.toKmClass()?.let { predicate(it) } == true</code></pre><h2>Grand finale</h2><p>Now we can finally write our transformers, predicates, and conditions. Because they will be all quite similar, let’s create factory methods for them first:</p><pre><code class="kotlin hljs">fun kotlinClassesTransformer(description: String, predicate: (KmClass) -> Boolean) = object : AbstractClassesTransformer(description) { override fun doTransform(collection: JavaClasses): Iterable = collection.filter { it.isKotlinClassAndSatisfies(predicate) } }</code></pre><pre><code class="kotlin hljs">fun kotlinDescribedPredicate(description: String, predicate: (KmClass) -> Boolean) = object : DescribedPredicate(description) { override fun apply(javaClass: JavaClass) = javaClass.isKotlinClassAndSatisfies(predicate) }</code></pre><pre><code class="kotlin hljs">fun kotlinArchCondition( ruleDescription: String, violationDescription: String, predicate: (KmClass) -> Boolean ) = object : ArchCondition(ruleDescription) { override fun check(javaClass: JavaClass, events: ConditionEvents) { if (!javaClass.isKotlinClassAndSatisfies(predicate)) { events.add(SimpleConditionEvent.violated(javaClass, "$javaClass $violationDescription")) } } }</code></pre><p>And now, finally, we have everything ready to write things we can actually use in our ArchUnit tests—for example:</p><pre><code class="kotlin hljs">val kotlinSealedClasses = kotlinClassesTransformer("Kotlin sealed classes") { it.sealedSubclasses.isNotEmpty() }</code></pre><pre><code class="kotlin hljs">val areKotlinDataClasses = kotlinDescribedPredicate("are Kotlin data classes") { Flag.Class.IS_DATA(it.flags) }</code></pre><pre><code class="kotlin hljs">val beKotlinObjects = kotlinArchCondition("be Kotlin objects", "is not Kotlin object") { Flag.Class.IS_OBJECT(it.flags) }</code></pre><p>The predicate lambdas operate on <span class="pre-inline">KmClass</span> instances. <span class="pre-inline">KmClass</span> is quite a low-level but powerful API to examine <span class="pre-inline">@Metadata</span> annotation content. <span class="pre-inline">KmClass</span> has direct methods or properties for some Kotlin constructs, while others can be derived from its flags. Sometimes it takes a little bit of exploration, but all Kotlin-specific stuff is there. Or, for a higher-level API to do the same, see <a href="">KotlinPoet metadata</a>.</p><p>Now we can write tests such as:</p><pre><code class="kotlin hljs">all(kotlinSealedClasses) .that(resideInAPackage("..presentation")) .should(haveSimpleNameEndingWith("State"))</code></pre><pre><code class="kotlin hljs">classes() .that(resideInAPackage("..model")).and(areKotlinDataClasses) .should(implement(</code></pre><pre><code class="kotlin hljs">classes() .that().areTopLevelClasses().and().areAssignableTo( .should(beKotlinObjects)</code></pre><p>So there you have it—support for Kotlin constructs in ArchUnit. The sky’s the limit, now your codebase can be more robust than ever! </p><h2>Conclusion</h2><p>Well, this was quite a journey! The benefits of having even a small suite of ArchUnit tests in your project are immense—they prevent subtle, hard-to-catch bugs, act as the best possible documentation of your architecture, save you time during code reviews and keep your codebase clean, consistent, maintainable and healthy. They are easy to write, simple to integrate into your CI/CD pipeline and extendable even beyond the original language. What’s not to like? Start writing them now and reap the rewards for years to come!</p>#architecture;#jvm;#tdd;#android;#kotlin
Truce with fragments with fragments<p>​​​Once upon a time, one could write an entire Android app in a single humongous activity. Google provided us with a bunch of fairly low-level building blocks and basically no guidance on how to use them together in a maintainable and scalable way. (“So you want to build a house? Here is a shovel, a saxophone and a kitten, we thought they might be useful, but do absolutely what you want with them, go on! And please mind the lifecycles, thank you.”)</p><p>Fortunately, things have changed a bit and now the official docs mention stuff like view models, lifecycle observers, navigation graphs, repositories or single-activity applications. And there are even official <em>opinions</em> on how to combine them!</p><p>Nevertheless, activities and fragments, the remnants of those dark times, are apparently here to stay. When you look at API surfaces of these... <em>things</em>, one question surely comes to mind: How do I work with that and stay sane at the same time?</p><h2>A match made in a place other than heaven</h2><p>Before we get to that, a little disclaimer is needed: This article is very opinionated. Your mileage and needs may vary. The following recommendations are best suited to “ordinary” form-based apps containing quite a lot of screens and user flows, with modest performance requirements. Games, specialized single-purpose apps, apps with dynamic plug-ins, high-performance, or UI-less apps might benefit from completely different approaches.</p><p>With that out of the way, the first question we should ask is: Do we even <em>need</em> activities and fragments?</p><p>With activities being the entry points to the app's UI, the answer is obviously yes. There’s no way around the fact that we need at least one activity with <span class="pre-inline">android.intent.action.MAIN</span> and <span class="pre-inline">android.intent.category.LAUNCHER</span> in its intent filter. But do we need <em>more</em> than one? The answer to that is a resounding no and we’ll see why in a future post.</p><p>Fragments are a different matter. First introduced in Android 3.0, when tablets were a thing, they were hastily put together as a kind of reusable mini-activities so that larger tablet layouts could display several of them simultaneously (think master-detail flows and such). Unfortunately, they inherited many design flaws of activities and even added some very interesting new ones. To say they are controversial would be an understatement.</p><p>On top of that, we don’t really need them in the way that we need that one launcher activity. Bigger, reusable pieces of UI can be served using good old views and there are 3rd party frameworks that do just that (and even several others that achieve the same thing in other ways, like using RecyclerViews to compose the UI from different “cells” etc.); and don't even forget that Jetpack Compose is coming...<br></p><p>However! Fragments are still developed, supported, documented, advocated for, integrated with other highly useful libraries (like Jetpack’s Navigation component) and in some places, they’re quite irreplaceable (yes, this is more of a design flaw of such APIs, but we need to work with what we have). Love them or hate them, they are the standard, well-known official solution, so let’s just be pragmatic: We’ll give them a hand, but won’t let them take the whole arm.</p><p>And so, we’ve arrived at the second question: One activity and possibly many fragments—but what can we use them <em>for</em>? And if we can, <em>should</em> we?</p><h2>Less is more</h2><p>This is where the opinions begin, so brace yourself.</p><p>Architecture-wise, what is an activity (and to an extent, a fragment, since they share many similarities)? My answer is this: A textbook violation of the single responsibility principle. </p><p>The main problem with an activity/fragment is that it is:</p><ol><li>a giant callback for dozens of unrelated subsystems</li><li>which is created and destroyed outside of our control</li><li>and we cannot safely pass around or store references to it.</li></ol><p>Typical consequences of these issues (when not handled in a sensible way) include activity subclasses several thousand lines long, full of untestable spaghetti (1), UI glitches and crashes (2) and memory leaks (3).</p><p>Open any activity in your current project, type <span class="pre-inline">this.</span> and marvel at the endless list of methods. The humble activity handles UI lifecycle and components, view models, data and cache directories, action bars and menus, assets and resources, theming, permissions, windows and picture-in-picture, navigation and transitions, IPC, databases and much, much more.</p><p>How Android got to this point isn’t important right now, but your code doesn’t have to suffer the same bloated fate. We need to chip away at the responsibilities and one way to do that is this: Use fragments exclusively for UI duties and that single activity for system call(back)s (and absolutely no UI).</p><h2>Fragments of imagination</h2><p>Each fragment should represent one screen (or a significant part of one) of your application. A fragment should only be responsible for </p><ol><li>rendering the view model state to the UI and</li><li>listening for UI events and sending them to its view model.</li></ol><p>That’s all. Nothing more. Really.</p><p>View model states should be tailored to concrete UI, should be observable and idempotent. It’s alright for view models and fragments to be quite tightly coupled (but view models mustn’t know anything about the fragments).</p><p>Because fragments are much harder to test than view models, the view model should pre-format the displayed data as much as possible, so the fragment can be kept extremely simple and just directly assign state properties to its view properties. There shouldn’t be any traces of formatting or any other logic in the fragments.</p><p>The opposite way should be equally simple—the fragment just attaches listeners to its views (our current favorite is the <a href="">Corbind</a> library which transforms Android callbacks to handy and most importantly unified <span class="pre-inline">Flow</span>s) and sends these events directly to the view model.</p><p>That is what fragments should do. But what they shouldn’t do is perhaps even more important:</p><ul><li>Fragments shouldn’t handle navigation between screens, permissions nor any other system stuff, even if the APIs are conveniently accessible from right inside the fragment. </li><li>Fragments shouldn’t know about each other and shouldn’t depend on or interact with their parent activity. </li><li>Fragments shouldn’t know about how they are instantiated and how they are injected (if your DI framework allows this); they also shouldn’t know about fragment transactions, if possible. </li><li>Data should be passed to fragments <em>exclusively</em> through their view models and that should be just the data to be displayed in the UI—forget about fragment arguments and rich domain models in them. </li><li>This almost goes without saying, but fragments shouldn’t do any file, database or network IO (I know, inside the fragment, the almighty Context is sooooo close… Just a small peak into SharedPrefs, please? No, never!). </li><li>Since Android view models got <span class="pre-inline">SavedStateHandle</span>, fragments even shouldn’t persist their state to handle process death. </li><li>And for heaven’s sake, never ever use abominations such as headless or retained fragments. </li></ul><p>Some other tips include:</p><ul><li>Fragments should handle only the very basic lifecycle callbacks like <span class="pre-inline">onCreate</span>/<span class="pre-inline">onDestroy</span>, <span class="pre-inline">onViewCreated</span>/<span class="pre-inline">onDestroyView</span>, <span class="pre-inline">onStart</span>/<span class="pre-inline">onStop</span> and <span class="pre-inline">onPause</span>/<span class="pre-inline">onResume</span>. If you need the more mysterious ones, you’re probably going to shoot yourself in the foot in the near future. </li><li>If possible, don’t use the original <span class="pre-inline">ViewPager</span> with fragments—that road leads to madness and memory leaks. There's a safer and more convenient <span class="pre-inline">ViewPager2</span> which works much like <span class="pre-inline">RecyclerView</span>. </li><li>Make dialogs with <span class="pre-inline">DialogFragments</span> <em>integrated with Jetpack Navigation component</em>. It’s much easier to handle their lifecycle (those dismissed dialogs popping on the screen again after device rotation, anyone?) and they can have their own view models. This way, there’s almost no difference between part of your UI being a dialog or a whole screen. </li><li>Sometimes it’s OK for fragments to include other fragments (e.g., a screen containing a <span class="pre-inline">MapFragment</span>), but keep them separate—no direct dependencies and communication between them, no shared view models etc. </li><li>To make your life easier, your project probably has some sort of <span class="pre-inline">BaseFragment</span> which simplifies plumbing, sets up scopes, and what have you. That’s fine, but resist the temptation to pollute it with some “handy” little methods for random things like toasts, snackbars, toolbar handling etc. YAGNI! Don’t misuse inheritance as a means to share implementation—that’s what composition is for. </li><li>Our favorite way to access views from fragments is the relatively new and lovely ViewBinding library. It’s simple to integrate, straightforward to use, convenient, type-safe, and greatly reduces boilerplate. No other solution (findViewById, Butter Knife, kotlin-android-extensions plugin or Data Binding library) possesses all these qualities. </li><li>Speaking of Data Binding, even when it isn’t throwing a wrench into your build, we don’t think that making our XMLs smarter than they need to be is a good idea to begin with. And don’t let me start about the testability of such implementations. </li><li>Use <a href="">LeakCanary</a>! The recent versions require practically no setup and automatically watch Activity, Fragment, View and ViewModel instances out of the box. </li></ul><p>After following all this advice (and a little bit of coding), your <em>complete</em> fragment could look like this (the implementation details aren’t important, just look at the amount and <em>intention</em> of the code):</p><pre> <code class="kotlin hljs">internal class ItemDetailFragment : BaseFragment<ImteDetailViewModel, ItemDetailViewModel.State, ItemDetailFragmentBinding>() { // take advantage of reduced visibility if possible so you don’t pollute your project’s global scope // required by DI framework override val viewModelClass = ItemDetailViewModel::class // layout inflation with the lovely ViewBinding library override fun onCreateViewBinding(inflater: LayoutInflater) = ItemDetailFragmentBinding.inflate(inflater) // initialization of view properties that cannot be set in XML override fun ItemDetailFragmentBinding.onInitializeViews() { detailContainer.layoutTransition?.disableTransitionType(DISAPPEARING) } // render the view model state in the UI; kept as simple as possible // state properties should preferably be primitive or primitive-like types // no DataBinding :) // notice the receiver - we don’t have to reference the binding on every single line override fun ItemDetailFragmentBinding.onBindState(state: ItemDetailViewModel.State) { loading.isVisible = state.isLoadingVisible itemTitle.text = state.item.title itemCategory.textResId = state.item.categoryResId itemFavorite.isChecked = state.item.isFavorite itemPrice = state.item.price // price is a String and is already properly formatted /* ... */ } // the other way around: catch UI events and send them to the view model override fun ItemDetailFragmentBinding.onBindViews() { toolbar.navigationClicks().collect { viewModel.onBack() } { viewModel.onCheckout() } favorite.checkedChanges().collect { isChecked -> viewModel.setFavorite(isChecked) } addToWishList.clicks().collect { viewModel.onAddToWishList() } addToCart.clicks().collect { viewModel.onAddToCart() } /* ... */ } } </code></pre><p>That’s not that bad, is it?</p><h2>If you can’t beat them, join them</h2><p>Although hardly an elegant or easy-to-use API, fragments are here to stay. Let’s make the best of this situation: Pragmatically utilize them for their useful integrations and focus on the single real responsibility they have—handling the UI. Ignore the rest and KISS—this principle is extremely important when working with fragments. That way, you’re going to have small, simple, focused fragments—and more importantly, a lot less less headaches.</p>#architecture;#android;#jetpack
The past and the future of Android R class past and the future of Android R class<p>​​​Every Android application contains some resources like localized strings, icons, screen layouts, or navigation targets. And every native Android application accesses these resources using Android’s retrieval mechanism based on resource IDs listed in R class. Let’s deep dive into the world of almighty R to see whether there are any gotchas or possible improvements. Spoiler alert: There are!</p><h2>What is a R class?<br></h2><p>Android application resources are placed in a directory structure under <span class="pre-inline">res</span> root (do not confuse that with Java application’s <span class="pre-inline">resources</span> directory) but may differ in a format. Icons and layouts are placed in their individual files, but strings might (or might not) be stored all in a single file and navigation targets are part of much richer content of navigation graphs.</p><p>Nevertheless, all application resources have an unique resource ID generated by <a href="">aapt tool</a> during compilation. All resource IDs are listed in a Java class with a simple name—R. For each resource type, there is a nested class with the resource type name (e.g., <span class="pre-inline">R.string</span> for string resources), and for each resource of that type, there is a static integer (e.g., <span class="pre-inline">R.string.app_name</span>). </p><p>This resource retrieval mechanism is heavily used in many parts of Android SDK and is optimized for performance. Unfortunately, it is not optimized for developer happiness and safety. As all resource IDs are of type <span class="pre-inline">Int</span>, one can easily use a string resource ID to retrieve an icon which won’t end up well.</p><p>Let’s focus now on how the R class is generated and used by the rest of the application code.</p><h2>The origins</h2><p>Since the first public Android release in 2008, the principle of resource retrieval has stayed the same. Before the application code gets compiled, all resources under the <span class="pre-inline">res</span> directory must be found, the R class source code is then  generated and merged with the rest of application source code. Once done, the application code can reference resources IDs of R class like any other code.</p><p>At that time, it was the responsibility of Android Development Tools (ADT) Plugin to initiate aapt, generate the <span class="pre-inline"></span> source file, merge it with other application source files and compile them together.</p><pre> <code class="java hljs">public final class R { public static final class string { public static final int cancel = 17039360; public static final int copy = 17039361; public static final int cut = 17039363; public static final int no = 17039369; public static final int ok = 17039370; public static final int paste = 17039371; public static final int yes = 17039379; } public static final class layout { public static final int activity_list_item = 17367040; public static final int list_content = 17367060; public static final int preference_category = 17367042; public static final int select_dialog_item = 17367057; } public static final class drawable { public static final int btn_default = 17301508; public static final int btn_dialog = 17301527; public static final int btn_dropdown = 17301510; public static final int btn_minus = 17301511; public static final int btn_plus = 17301512; public static final int checkbox_off_background = 17301519; public static final int checkbox_on_background = 17301520; public static final int divider_horizontal_bright = 17301522; } } </code></pre><p>The first versions of appt were generating all resource IDs in R class as constants (<span class="pre-inline">public static final int</span> in Java). This appears to be very bad for build performance of modularized projects that have multiple library modules. Actual values of resource IDs in these modules might collide and as a result all library modules had to be recompiled more frequently. So from ADT version 14 the <em>library module’s</em> R classes have their fields generated as <span class="pre-inline">public static int</span> (non-final), but the leaf application modules kept their fields as <span class="pre-inline">final</span> since no other modules depend on them.</p><p>This change was quite an issue at that time since Java’s <span class="pre-inline">switch</span> statements require compile time constants and one cannot use a library module resource ID as a statement value. But with Kotlin’s <span class="pre-inline">when</span> expression this isn’t an issue anymore.</p><h2>Gradle times</h2><p>At Google I/O 2014, Android Studio with Gradle-based build system was announced as a replacement for ADT and Ant-based build. But since R class code generation is done by the aapt tool, not much has changed in the way resource IDs are generated and consumed.</p><h2>Elephant in the build</h2><p>As projects grow, their R classes grow too. It was around 2017 when some bigger development teams realized that the R classes grow much faster. Elin Nilsson in her <a href="">talk about app modularization</a> mentioned that in their 1,2 million LoC codebase the generated R classes together would sum up to 56 million LoC that need to be compiled and dexed on every clean build.</p><p>Wait, what? Do I actually have millions of unique resources in my modularized app? Of course not! The main reason why the R classes are so huge is that they contain duplicates. R classes are generated for every module of your build and the module specific R classes include references to all resources of its transitive dependencies.</p> <img alt="R classes in modules" src="/Blog/PublishingImages/Articles/r-class-01.png" data-themekey="#" /> <p>In this example, the MDC library contains <span class="pre-inline"></span> class with references to material resources.</p><p>Our <span class="pre-inline">:lib</span> module depends on the MDC library and contains few other resources. It also has its own <span class="pre-inline">com.example.myapp.lib.R</span> class where references to its own resources and transitive references to resources of MDC library are listed.</p><p>Finally, the <span class="pre-inline">:app</span> module has its own generated <span class="pre-inline">com.example.myapp.R</span> class and lists both <span class="pre-inline">:lib</span> module and MDC library reference IDs again.</p><p>Material Components library resources IDs are generated three times! And this is the story of how a small modularized application can achieve few millions LoC in R classes.</p><h2>The rescue</h2><p>The Android Developer Tools team noticed this problem and rolled out a few tweaks that can help to tame this monster. Let’s go through the list in the order in which they have been released. </p><p> <strong>Android Gradle Plugin 3.3 (January 2019)</strong> introduces a not very well documented flag you can enable in <span class="pre-inline"></span>:</p><pre> <code class="kotlin hljs">android.namespacedRClass=true</code></pre><p>It enables non-transitive R class namespacing where each library only contains references to its own resources without pulling references from dependencies. This has a huge effect on R classes size which leads to much faster builds.</p><p>As a bonus, it makes the module better isolated from an architectural point of view. Unless there is an explicit import for the R class from another module, the module can use only its own resources. It prevents the module from accidentally using a drawable or a string from another module.</p><p>Unfortunately, Android Studio isn't aware of this so it won't prevent you from making incorrect references, but at least it will fail at compile time.</p><p>This is all great (actually more than that!), but at the end of the day, the R class is just a list of Ints known at compile time and you still need to compile it. Wouldn't it be nice if AGP could generate Java bytecode directly? This dream comes true with another <span class="pre-inline"></span> flag:</p><pre> <code class="kotlin hljs">android.enableSeparateRClassCompilation=true</code></pre><p>This will make AGP to generate the R classes as compiled jar files instead of source code and merge them with the rest of the project automatically. Technically, instead of <span class="pre-inline"></span> file in <span class="pre-inline">build/generated</span> directory, the build process will generate <span class="pre-inline">R.jar</span> file in <span class="pre-inline">build/intermediates</span> directory.</p><p>This won’t speed up your build in the order of magnitudes, but it’s still better than nothing. And of course, you can use any of these flags separately if it is too hard for you to enable them together at once.</p><p> <strong>Android Gradle Plugin 3.4 (April 2019)</strong> doesn’t give us much but there is an interesting paragraph in the release notes:</p><p> <em>The correct usage of unique package names is currently not enforced but will become more strict on later versions of the plugin. On Android Gradle plugin version 3.4, you can opt-in to check whether your project declares acceptable package names by adding the line below to your <span class="pre-inline"></span> file:</em></p><pre> <code class="kotlin hljs">android.uniquePackageNames=true</code></pre><p>There were no details for this requirement at that time, but having one package name used only in one project module seems a good thing anyway, so why not enable this check right now and have no migration issues in the future, right?</p><p> <strong>Android Gradle Plugin 3.6 (February 2020)</strong> gives the answer to the requirement for a unique package name per module. Starting with this version, AGP simplifies the compile classpath by generating only one R class for each module to speed up the build.</p><p>AGP 3.6 also makes the <span class="pre-inline">android.enableSeparateRClassCompilation</span> flag enabled by default and it cannot be disabled anymore.</p><p></p><p>On the other hand, a new <span class="pre-inline">android.enableAppCompileTimeRClass</span> experimental flag was introduced. It is limited to Android <em>application modules</em> only. Before a compilation phase of an <em>application module</em> can begin, all R classes from all other modules need to be re-generated to create a final set of unique resource IDs that will be used in the <em>application module</em> at runtime. This new experimental flag solves this limitation by generating a fake <em>application module</em> R class in advance while updating it with real resource ID values afterwards. One limitation of this approach is that the <em>application modules</em> resource IDs cannot be final anymore (same as in <em>library modules</em>), therefore they cannot be used in Java’s switch statements or as annotation parameters. </p><p> <strong>Android Gradle Plugin 4.1 (release candidate in October 2020)</strong> makes another step to make R class namespacing enabled by default as it renames the flag from</p><pre> <code class="kotlin hljs">android.namespacedRClass=true</code></pre><p>to</p><pre> <code class="kotlin hljs">android.nonTransitiveRClass=true</code></pre><p>There is also a similar flag for app modules</p><pre> <code class="kotlin hljs">android.experimental.nonTransitiveAppRClass=true</code></pre><p>but according to the comments in AGP source code this seems to be just temporary and will be removed in the future so you don’t have to bother now.</p><h2>Conclusion</h2><p>Access to app resources through R class has a long history full of slow builds.</p><p>If you are a conservative developer, just try to stay with the current stable version of Android Gradle Plugin and things will get better over time. Sometimes you might be surprised with a new requirement or package limitation and the list of changes above might help you to overcome these.</p><p>If you are more progressive, just try the latest RC’s or betas and definitely try to enable <span class="pre-inline">android.nonTransitiveRClass</span> and <span class="pre-inline">android.enableAppCompileTimeRClass</span> flags. It is a game changer and my guess is that it is the future for all of us anyway.<br></p><p><br></p><p>Pavel Švéda<br></p><p>Twitter: <a href="">@xsveda​</a>​​</p>#android;#gradle
Jetpack Compose: What you need to know, pt. 1 Compose: What you need to know, pt. 1<p><a href="">Jetpack Compose</a> is coming sometime this year. Although it is still under heavy development, given its significance, I think now is the right time to look at what it brings to the table.</p><p>This isn’t a technical tutorial or introduction to Compose (there are many of these floating around, but be careful, as many of them are already out of date), but rather a collection of more or less random points, notes, and findings. Let’s find out if the hype is justified!</p><h2>Executive summary, but for developers</h2><p><strong>Compose is going to be one of the biggest changes Android development has ever seen.</strong></p><p>Yes, perhaps even bigger than reactive programming, Kotlin, or coroutines. UI is a crucial part of any application and a UI toolkit built on the mindset from the 2010s instead of the 1990s is indeed a very welcome upgrade.</p><p>Also, because it relies on Kotlin-exclusive features, Compose is another nail into Java’s coffin on Android.</p><p><strong>Making UIs is fun again!</strong></p><p>This is Compose’s equivalent of <span class="pre-inline">RecyclerView</span> <em>with different item types</em>:</p><pre><code class="kotlin hljs">LazyColumn { items(rows) { row -> when (row) { is Title -> TitleRow(row.title) is Item -> ItemRow(row.text) } } }</code></pre><p>Of course, everything isn’t that simple, but Compose really excels at its main goal—creating sophisticated and highly reusable custom components and complex layouts in a simple, effective, and safe manner.</p><p><strong>The mental model is radically different from what we are used to in Android.</strong></p><p>Unidirectional UI toolkits were the rage with web folks some time ago, and now they’ve finally arrived on mobile platforms.</p><p>The good news is that because we are late to the party, the paradigm has matured, and perhaps Compose won’t repeat at least some of the mistakes that caught up with early implementations on other platforms. The bad news is that the paradigm requires a significant mindset shift (say on a scale of reactive programming)—but it’s for the better, I promise.</p><p><strong>Compose has a huge multiplatform potential.</strong></p><p>Compose comprises <a href="">several artifacts</a>, and only some of them are Android-specific. JetBrains already work on <a href="">desktop port</a>, and covering other platforms is certainly not impossible.</p><p>Building on a combination of existing platform-specific UI toolkits and Kotlin Multiplatform features such as <span class="pre-inline">expect/actual</span> declarations, one can imagine a distant future where a single UI toolkit provides the holy grail of unified implementation, native performance, and platform-specific look’n’feel.</p><h2>Creating UI</h2><p><strong>There are no XML layouts, no inflaters and no objects representing the UI components.</strong></p><p>There are no setters to mutate the current UI state because there are no objects representing the UI views (<span class="pre-inline">@Composable</span> function calls only <em>look</em> like constructor calls, but don’t let that fool you), which means there cannot even be any internal UI state (well, the last point isn’t entirely true, but we’ll get to that later). Then you must think about states and events traveling through your UI tree in various directions and whatnot. </p><p>If you’ve never experienced a unidirectional toolkit, it will feel alien, strange, and maybe even ineffective, but the benefits are worth it.</p><p><strong>String, font, and drawable resources are staying.</strong></p><p>Compose doesn’t want to get rid of these and works with them just fine. However, only bitmap and vector drawables make sense with Compose. Other types such as layer list drawables, state list drawables, or shape drawables are superseded by more elegant solutions.</p><p><a href="">Colors</a> and <a href="">dimensions</a> should be defined entirely in Kotlin code if possible, but traditional resources still may be used if needed.</p><p><strong>There are no resource qualifiers.</strong></p><p>Compose has the power of Kotlin at its disposal. If you need to provide alternative values depending on the device configuration (or any other factor), simply add a condition to your composable function—it’s an explicit and unambiguous way to specify the value you want.</p><p>And of course remember to keep your code DRY—if you find yourself repeating the same bit of logic in many places, refactor.</p><p><strong>There are no themes and styles (sort of).</strong></p><p>Compose contains basic components that expose a number of parameters to change their look and behavior. Because everything is written in Kotlin, these parameters are rich, and most importantly, type-safe.</p><p>If you need to style a bunch of components in the same way, you simply <a href="">wrap</a> the original composable function call with your own composable, setting the parameters you need to change (or exposing new ones), and use this in your code.</p><p>Simple, efficient (because there is virtually no penalty for nested calls), and without hidden surprises.</p><p><strong>Compose comes with Material Design implementation out of the box.</strong></p><p>Although there are no themes or styles as such, there is a way to create and use application-wide themes.</p><p>Compose comes with <a href="">Material Design implementation</a>. Just wrap your root composable with <a href="">MaterialTheme</a>, customize colors, shapes, or typography to fit your brand, and you’re good to go. You can have different <span class="pre-inline">MaterialTheme</span> wrappers for different parts of your UI, effectively replacing theme overlays from the legacy system.</p><p>Often this is all you’ll ever need, but if your design system is more sophisticated or simply won’t fit the predefined Material Design attributes, you can <a href="">implement your own</a> from scratch. However, this is quite difficult and requires advanced knowledge of Compose to get it right.</p><p>See <a href="">this blog series</a> for valuable insights on custom design systems in Compose and <a href="">this post</a> for a comparison of different theming approaches.</p><p><strong>We can’t completely get rid of the legacy theme system (yet).</strong></p><p>Compose theming reaches only the parts of the UI that are managed by Compose. We might still need to set a legacy theme for our activities (to change window’s background, status bar, and navigation bar colors, etc.), or to style View-based components that don't have Compose counterparts.</p><p><strong>Don’t expect component or feature parity with legacy View-base components or Material Design specs any time soon.</strong></p><p>It’s the old story all over again: Writing a new UI toolkit from scratch means that there is going to be a long period in which a number of components (or at least their features) won’t be officially available.</p><p>For example, Compose’s <a href="">TextField</a> doesn’t have the same features (and API) that <a href="">TextInputLayout</a> has, and both of these implementations aren’t 100 % aligned with the <a href="">Material Design spec</a>.</p><p>This situation may be slightly annoying, but at least with Compose, it’s significantly easier to write custom components yourself.</p><p><strong>Finally, an animation system so simple that you’ll actually use it.</strong></p><p>Animating many things is as simple as wrapping the respective value in a <a href="">function</a> call, and for more complex animations, Compose superbly leverages the power of Kotlin.</p><p>Done right, animations are a great way to enhance user experience. With Compose animation APIs, their implementation is at last effective and fun.</p><h2>Internals</h2><p><strong>Composable functions are like a new language feature.</strong></p><p>Technically, <span class="pre-inline">@Composable</span> is an annotation, but you need to think about it more like a keyword (suspend is a good analogy, more on that later). This “soft keyword” radically changes generated code, and you need to have at least a basic idea of <a href="">what goes on under the hood</a>, otherwise, it’s very well possible to shoot yourself in the foot even with innocent-looking composable functions.</p><p><strong>The knowledge of the internals is important for creating performant UIs.</strong></p><p>Compose compiler does a lot in this regard (like positional memoization, and fine-grained recomposition), but there are situations when the developer has to provide optimization clues, or declare and then actually honor contracts that the compiler cannot infer on its own (such as marking data classes as truly immutable).</p><p>However, I expect the compiler to become smarter in the future, alleviating the need for many of these constructs.</p><h2>States</h2><p><strong>Compose UIs are declarative, but not truly stateless.</strong></p><p>UIs in Compose are declared by constructing deeply nested trees of composable functions where <a href="">states flows down and events up</a>. At the root of the tree, there is a comprehensive, “master” state coming from some external source (the best candidate for this task is the good old view model). When the state changes, parts of the UI affected by the change are re-rendered automatically.</p><p>In theory, we want the UI to be <em>perfectly</em> stateless. The root state should be completely externalized and should contain <em>everything</em> that must be set on the screen. That would mean not just obvious things like text field strings, checkbox states, and so on, but also, for example, <em>all</em> styling attributes for all the views, internal animation states including clock, current animated values, etc.</p><p>In practice, this would be too cumbersome (argument lists would grow unacceptably large and “interesting” arguments like user inputs would get mixed up with purely technical ones like animation states), so besides explicit state that is passed via composable function arguments, Compose has several other ways to provide data down the component tree.</p><p><strong>Composable functions can have their own internal state.</strong></p><p>Yes, function can have state encapsulated in it that survives between its invocations. This is a pragmatic decision that simplifies its signature and enables some optimizations, and is especially handy for animations and other things that don’t need to be changed and/or observed from outside.</p><p><strong>Ambients are like service locators for data passed through the UI tree.</strong></p><p> <a href="">Ambient</a> holds a kind of global variable defined in the tree node somewhere up in the hierarchy, statically accessible to nodes below it. If this rings an alarm bell in your head, you’re right—statically accessed global variables create invisible coupling and other problems.</p><p>However, this is a trade-off that is often worth it. Ambients are best suited for values that we need to set or change explicitly but don’t want to explicitly <em>pass</em> through the tree. Theme attributes and properties are a prime example of such things. </p><p><strong>State management is now more important than ever.</strong></p><p>So we have (at least) 3 ways to store and manipulate state in Compose, and they can even be combined along the way. The question of which method to use for which part of the state becomes essential. Sometimes, the answer can be difficult, and choosing the wrong one can lead to all kinds of messy problems.</p><p>Also, especially for larger screens, both the structure and the content of the state object is crucial.</p><h2>Until next time</h2><p>Well, that concludes part 1. In the second and final part of this series, we’ll look at the ecosystem, performance, stability, and even the magic that makes Compose possible. Take care and stay tuned!</p>#android;#jetpack;#compose;#ui