Modern Flutter Plugin Development

by Amir Hardon, Chris Sells, Collin Jackson, Harry Terkelsen and Matt Carroll 2019 was a big year for advancements in technology for Flutter plugin authors. We introduced the Android Plugin API 2.0, which provides a more robust and full-featured way to implement Android support in your plugins. We updated the pubspec.yaml format to make it possible to clearly specify Android and iOS support but also web, macOS, Windows, and Linux. Furthermore, as we push Flutter to support multiple platforms, we enabled federation, making it possible for multiple teams with different expertise to bring their code together into a seamless experience for the Flutter developer using the plugin. And finally, we made great strides in testing plugins, with more to come. Android Plugin API 2.0 In December of 2019, Flutter published a new version of its Android embedding. This is the Android code that is responsible for integrating Flutter within an Android app. It includes classes like FlutterActivity, FlutterFragment, FlutterView, and FlutterEngine. The v2 Android embedding includes support for standard Android lifecycle events and the separation of Flutter execution from the Android UI, which were missing in the v1 Android embedding. During the development of the v2 Android embedding it became clear that the existing API for Flutter plugins was insufficient to handle the new capabilities of the v2 Android embedding. A new Android plugin API was needed. We’ll discuss that API and how to use it. First, it is important to understand the FlutterEngine class within the v2 Android embedding. A FlutterEngine object represents a single Flutter execution context. This means that a FlutterEngine controls a Dart isolate (your Dart code that begins with an entrypoint like main). It also means that a FlutterEngine sets up a number of standard platform channels that all Flutter apps require; it includes support for platform views, it knows how to paint a texture with a Flutter UI, and it handles all other fundamental requirements to execute a single Flutter/Dart app. Additionally, an Android app may contain multiple FlutterEngines at the same time. The fundamental concept of “adding a plugin” to a Flutter app means applying that plugin to a single FlutterEngine. For example, if a Flutter app requires access to a camera, that capability is achieved by registering a camera plugin with a specific FlutterEngine instance. This registration is done for you automatically with a GeneratedPluginRegistrant, but it is important to understand that every FlutterEngine maintains its own set of Flutter plugins. In the old v1 Android embedding, all plugins were initialized and configured at the very beginning of the Android app and there was only ever one Flutter experience. In the v2 embedding, we make no assumption about when a plugin is initialized, and a plugin must be initialized once per FlutterEngine. As a result, all Flutter plugins for Android must now support instantiation instead of static initialization, and they must support being attached to, and detached from a FlutterEngine. The following code samples demonstrate the difference between the old v1 plugin initialization implementation and the new v2 plugin initialization process. Old plugin initialization class MyOldPlugin { public static void registerWith(PluginRegistrar registrar) { // Obtain any references that the plugin requires from the // registrar. // // This plugin is now considered "initialized" and "attached" // to a Flutter experience. } } New plugin initialization class MyNewPlugin implements FlutterPlugin { public MyNewPlugin() { // All Android plugin classes must support a no-args // constructor. A no-arg constructor is provided by // default without declaring one, but we include it here for // clarity. // // At this point your plugin is instantiated, but it // isn't attached to any Flutter experience. You should not // attempt to do any work here that is related to obtaining // resources or manipulating Flutter. } @override public void onAttachedToFlutterEngine(FlutterPluginBinding binding) { // Your plugin is now attached to a Flutter experience // represented by the given FlutterEngine. // // You can obtain the associated FlutterEngine with // binding.getFlutterEngine() // // You can obtain a BinaryMessenger with // binding.getBinaryMessenger() // // You can obtain the Application context with // binding.getApplicationContext() // // You cannot access an Activity here because this // FlutterEngine is not necessarily displayed within an // Activity. See the ActivityAware interface for more info. } @override public void onDetachedFromFlutterEngine(FlutterPluginBinding binding) { // Your plugin is no longer attached to a Flutter experience. // You need to clean up any resources and references that you // established in onAttachedToFlutterEngine(). } } As shown in the new plugin API, your plugin must wait until onAttachedToFlutterEngine() completes before taking any meaningful actions, and it must respect onDetachedFromFlutterEngine() by releasing all resources. It’s possible that your plugin will be attached and detached multiple times. Additionally, your plugin must not depend upon an Activity reference within onAttachedToFlutterEngine(). Just because your plugin is attached to a Flutter experience doesn’t mean that the Flutter experience is being displayed in an Activity. This is one of the most significant differences between the old and new plugin APIs. In the old v1 plugin API, plugin authors could depend upon an Activity being available immediately and permanently. This is no longer true. Plugins that require access to an Activity must implement a 2nd interface called ActivityAware. The ActivityAware interface adds callbacks to your plugin class that tell your plugin when it’s sitting in an Activity, when that Activity goes through config changes, and when your plugin is no longer sitting in an Activity. Your plugin must respect these callbacks. The following example shows the outline of an ActivityAware plugin: class MyNewPlugin implements FlutterPlugin, ActivityAware { @override public void onAttachedToFlutterEngine(FlutterPluginBinding binding) { // ... } @override public void onDetachedFromFlutterEngine(FlutterPluginBinding binding) { // ... } @override public void onAttachedToActivity(ActivityPluginBinding binding) { // Your plugin is now associated with an Android Activity. // // If this method is invoked, it is always invoked after // onAttachedToFlutterEngine(). // // You can obtain an Activity reference with // binding.getActivity() // // You can listen for Lifecycle changes with // binding.getLifecycle() // // You can listen for Activity results, new Intents, user // leave hints, and state saving callbacks by using the // appropriate methods on the binding. } @override public void onDetachedFromActivityForConfigChanges() { // The Activity your plugin was associated with has been // destroyed due to config changes. It will be right back // but your plugin must clean up any references to that // Activity and associated resources. } @override public void onReattachedToActivityForConfigChanges( ActivityPluginBinding binding ) { // Your plugin is now associated with a new Activity instance // after config changes took place. You may now re-establish // a reference to the Activity and associated resources. } @override public void onDetachedFromActivity() { // Your plugin is no longer associated with an Activity. // You must clean up all resources and references. Your // plugin may, or may not ever be associated with an Activity // again. } } The new plugin API explicitly recognizes that a plugin may or may not have an Activity associated with it, and that any such Activity might be destroyed and recreated at any time due to configuration changes. These concerns should be familiar to all Android developers. The key to writing a plugin for Flutter’s v2 Android embedding is to respect each plugin lifecycle callback that your plugin implements. As long as you wait until the right time to establish references, and you release those references at the appropriate time, your plugin will work as intended. Some plugins, like a camera plugin, only make sense when an Activity is available. So what are these plugins to do? In the case of UI-only plugins, those plugins can wait until onAttachedToActivity() executes before doing any work. Then, in onDetachedFromActivity(), those plugins can clean up all references and essentially deactivate themselves. There is no requirement that a plugin do anything special within onAttachedToFlutterEngine(). It’s OK for a plugin to only do work when attached to an Activity. For more details about how to migrate your Android plugin from the v1 API to the v2 API, see Supporting the new Android plugins APIs on flutter.dev. New pubspec format Traditionally a Flutter plugin was a single package that enabled Flutter applications running on Android and iOS to access platform-specific functionality; technically a plugin was composed of Dart code backed by Android-specific and iOS-specific code. Even though the assumption that any Flutter plugin supports Android and iOS wasn’t accurate (for example, the android_intent plugin only supports Android), it was chosen to initially design the plugin ecosystem under that assumption. The assumption was mostly correct, which meant the overall cost for the few instances where it was wrong was low, and the simplified assumption enabled rapid progress and focus. With Flutter growing to support many more platforms we have decided to lose this simplifying assumption as: We expect many plugins to only support a subset of Flutter’s supported platforms (even more so with federated plugins, described below). We want to unlock tooling features that require knowledge of a plugin’s supported platforms (such as a smarter pub.dev search and platform-informed tool operations). The core missing piece was a clear indication for which platforms a plugin supports, and so we’ve redesigned the Flutter plugin’s pubspec schema around multi-platform support. Under the previous pubspec schema the flutter.plugin key contained the different plugin configuration bits, instead we’ve introduced a new key per platform under the flutter.plugin.platforms key with the platform-specific plugin configuration. For example, here’s what the pubspec for a plugin that supports Android, iOS, macOS, and web looks like: flutter: plugin: platforms: android: package: com.example.hello pluginClass: HelloPlugin ios: pluginClass: HelloPlugin macos: pluginClass: HelloPlugin web: pluginClass: HelloPlugin fileName: hello_web.dart environment: sdk: ">=2.1.0 <3.0.0" # Flutter versions prior to 1.10 did not support # the flutter.plugin.platforms map. flutter: ">=1.10.0" A plugin that supports a subset of these platforms can omit platform keys from the platforms map, for example: flutter: plugin: platforms: android: package: com.example.hello pluginClass: HelloPlugin ios: pluginClass: HelloPlugin environment: sdk: ">=2.1.0 <3.0.0" # Flutter versions prior to 1.10 did not support # the flutter.plugin.platforms map. flutter: ">=1.10.0" Note that requiring a Flutter SDK greater than 1.10.0 is required when using the new schema as this is the first version where the Flutter tool supported this schema. Migrating an existing plugin to use the new schema This section uses the battery plugin as an example and walks through the migration of a sample plugin from the previous schema to the new one. The most important thing to do when migrating is to only declare support for platforms that are supported by the plugin (this was not possible previously which meant that plugins which only supported Android had to include a no-op iOS implementation and vice versa). These are the relevant parts of a sample plugin’s pubspec.yaml file prior to the migration: name: sample version: 0.3.1+5 flutter: plugin: androidPackage: io.flutter.plugins.sample iosPrefix: FLT pluginClass: SamplePlugin environment: flutter: ">=1.6.7 <2.0.0" Assuming that the plugin supports Android and iOS, upgrading to the new schema involves: Increasing the minimally required Flutter version to 1.10.0 (which is the first version that supports the new schema). A minor version bump Replacing the current fields in flutter.plugin with the new platforms field. If the iosPrefix field was previously used, renaming the main iOS plugin file (more details below). The updated pubspec for this plugin looks like: name: sample version: 0.3.2 flutter: plugin: platforms: android: package: io.flutter.plugins.sample pluginClass: SamplePlugin ios: pluginClass: FLTSamplePlugin environment: flutter: ">=1.10.0 <2.0.0" Note that since the plugin supports Android and iOS these are the only keys under the flutter.plugin.platforms. The value of flutter.plugin.androidPackage field in the old schema becomes the value of flutter.plugin.platforms.android.package. The new schema has no equivalent for the iosPrefix field, as the pluginClass for iOS has a dedicated key we can use the prefix in the flutter.plugin.platforms.ios.pluginClass field which is set to FLTSamplePlugin. Plugins that were previously using the iosPrefix field The previous schema implied an inconsistency between the iOS plugin’s main interface name and its file name, e.g. for this sample plugin as defined with the previous schema, there would be a SamplePlugin.h file that declares an FLTSamplePlugin interface. This kind of inconsistency is no longer supported, which means the SamplePlugin.h file must be renamed to FLTSamplePlugin.h when upgrading to the new schema. Plugins that did not use the iosPrefix key don’t require renaming any files. For more information about development plugins that support any number of platforms, see Developing plugin packages on flutter.dev. Federation Not only does the new pubspec schema allow you to specify exactly which platforms your plugin supports, it also gives you the flexibility to spread implementations across multiple packages. In the past, the plugin’s Dart code, the Android Java (or Kotlin) code, and the iOS Objective-C (or Swift) code all needed to be in the same Dart package. Now, if we want to add support for another platform (Web, Mac OS, Windows, etc.), it doesn’t need to be in the same package. Plugins that are distributed across multiple packages are called federated plugins . Federated plugins have several advantages over single-package plugins, including: A plugin author does not need to have domain expertise for every supported Flutter platform (Android, iOS, Web, Mac OS, etc.). You can add support for a new platform without the original plugin author needing to review and pull in your code. Each package can be maintained and tested separately. So, how exactly do you create a federated plugin ? Let’s start with some terminology: app-facing package: This is the package you would import in your app in order to use the plugin. For example, package:url_launcher is an app-facing package. The app-facing package declares the app-facing API and works with various platform packages to perform platform-specific functionality. platform package : This is a package that implements the platform-specific functionality needed by the app-facing package . An example is package:url_launcher_web: this package is used by package:url_launcher to launch URLs when running the Flutter app on the web platform. Platform packages should not be imported in the app, they are only for use by the app-facing package to call into the required platform-specific code. platform interface package : This is the glue that holds the app-facing package and the platform packages together. Whereas the app-facing package declares an API that can be called from a Flutter app, the platform interface package declares an interface that each platform package must implement in order to support the app-facing package . Having a single package that defines this interface ensures that all platform packages implement the same functionality in a uniform way. The figure above shows the dependency graph between the app, the app-facing package, the platform packages, and the platform interface package. The app only ever imports the app-facing package (in this case package:url_launcher). How does the platform interface glue together the app-facing package and the correct platform package? In the past, there were no “platform packages”, only a subfolder for the Android code and another subfolder for the iOS code. The app-facing package communicated to the platform code via a MethodChannel. You can think of the MethodChannel as the de-facto “platform interface” because the app-facing package calls into the MethodChannel, and the corresponding platform code must be listening on a MethodChannel for the correct methods with the correct arguments. There is no way to statically confirm that the Android code or iOS code is listening for the correct MethodChannel calls. Old way of launching a URL Future<void> launch(String url) { channel.invokeMethod('launch', { 'url': url, }); } In the federated plugin structure, the platform interface package replaces the MethodChannel. The platform-specific functionality that the app-facing package needs from the platform packages is encapsulated in a platform interface. In our example, the app-facing package is package:url_launcher, and the only platform-specific functionality it needs is the ability to launch a URL on the given platform. A (very) simple platform interface would look like: abstract class UrlLauncherPlatform { /// Launches the given [url]. Future<void> launch(String url); /// The live "instance" of this platform interface. /// /// This is set when the platform package is registered, /// usually when the platform is initializing. /// /// For example, the web platform package /// (package:url_launcher_web) would extend this class with an /// implementation that launched the URL in a new tab, /// and at initialization time would set itself to be the live /// instance with: /// /// UrlLauncherPlatform.instance = WebUrlLauncher(); static UrlLauncherPlatform instance; } Now, instead of calling on the MethodChannel, the app-facing package would call into the platform interface. New way of launching a URL Future<void> launch(String url) { return UrlLauncherPlatform.instance.launch(url); } So, the app-facing package calls into the platform interface . How does the platform interface glue into the platform packages ? The platform packages implement the platform interface, and register themselves as the default instance of the platform interface when the platform is initializing. For example, if we wanted to write package:url_launcher_web, we would only need to write a class that extends UrlLauncherPlatform and launches a URL for the web platform. The code would look something like this: class UrlLauncherWeb extends UrlLauncherPlatform { /// The web platform automatically calls this when the app /// initializes. static void registerWith(Registrar registrar) { var webLauncher = UrlLauncherWeb(); UrlLauncherPlatform.instance = webLauncher; } @override Future<void> launch(String url) => window.open(url, ''); } The nice thing about migrating to a federated plugin structure, is that once you are set up with the app-facing package and platform interface package, adding support for a new platform is simple (and you don’t even have to do it yourself!) All that is required is to create a new platform package which extends the platform interface declared in the platform interface package. For more details about federated plugins, see Federate plugins on flutter.dev. Testing plugins As you write new cross-platform plugins or add platforms to existing ones, you can save yourself time and future headaches by writing tests. Automated tests protect your plugin from functional regressions so that you can rapidly develop new features and merge contributions. A well-tested plugin typically includes several styles of test spread across multiple packages. It’s possible to slow yourself down writing tests that are flaky or unlikely to never fail, so focus on the test writing that gives you confidence that the key use cases are still functional. AutomatedWidgetsFlutterBinding tests Tests that use AutomatedWidgetsFlutterBinding run on the development machine, rather than a device or browser. Because of this, they run faster and some functionality needs to be provided by mocks. In the app-facing package (such as myplugin), the package’s unit tests ensure that calls to the app-facing API result in expected interactions with the platform interface package. These tests typically import package:mockito to provide a fake platform interface and verify that it receives the correct calls. Here is an example test from package:url_launcher: test('returns true', () async { when(mock.canLaunch('foo')).thenAnswer((_) => Future<bool>.value(true)); final bool result = await canLaunch('foo'); expect(result, isTrue); }); In the platform interface package (such as myplugin_platform_interface), a platform interface is an abstract class and cannot be instantiated directly. However, the platform interface package typically contains the method channel implementation of the platform interface as well, so that’s what you should test. Tests for this package should focus on the method channel invocations that result from calls to the platform interface and the method channel. These tests typically use setMockMethodCallHandler with the isMethodCall matcher to verify behavior. test('canLaunch', () async { await launcher.canLaunch(' http://example.com/' ); expect( log, <Matcher>[ isMethodCall('canLaunch', arguments: <String, Object>{ 'url': ' http://example.com/' , }) ], ); }); In the platform tests (such myplugin_web), you can take advantage of platform-specific functionality. In the current Flutter SDK, flutter test provides an experimental —-platform flag that allows you to choose to run tests in a Chrome-like environment where dart:html is available. This test mode is useful for writing tests in the platform implementation package (for example, myplugin_web). test('cannot launch "tel" URLs', () { expect(canLaunch('tel:5551234567'), completion(isFalse)); }); In addition, you can run your GUI tests in Chrome using experimental support for ‘flutter drive’ testing on the web . For more information about plugin testing, see Testing your plugin on flutter.dev. Summary As you can see, there’s a lot of new features for the Flutter plugin developer, allowing you to build more full-featured and more robust plugins across more platforms. If you’re interested in some web-specific details, I recommend Harry Terkelsen’s two-part series on How to Write a Flutter Web Plugin ( part 1 and part 2 ). For more information on writing plugins in general, the Developing plugin packages documentation on flutter.dev is also a great resource. 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