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Flutter Android 端 Activity/Fragment 流程源码分析

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Flutter 系列文章连载~《Flutter Android 工程结构及应用层编译源码深入分析》《Flutter 命令本质之 Flutter tools 机制源码深入分析》《Flutter 的 runApp 与三棵树诞生流程源码分析》《Flutter Android 端 Activity/Fragment 流程源码分析》

背景

前面文章我们分析了 flutter 在 android 端编译命令相关流程,我们接下来需要先分析一下 Flutter Android 端 framework 平台实现代码(非 native engine 引擎部分),下面以一个纯 Flutter App 为例展开分析。

工程结构及 API 变更

对于一个纯 flutter app 来说,当我们在 yaml 依赖中添加两个 flutter plugin 依赖,然后铜过 Android Studio 导入安卓工程后可以看到如下结构:



gradle sync 后对应的安卓依赖如下:



可以看到,上面依赖就是我们《Flutter Android 工程结构及应用层编译源码深入分析》一文中分析 Flutter Android App 编译流程中提到过的动态追加依赖。


对于 Android 平台来说,Flutter 依赖其实蛮简单的,我们以启用 androidx 为例展示 release 模式下的依赖关系,如下:


releaseRuntimeClasspath - Resolved configuration for runtime for variant: release+--- io.flutter:flutter_embedding_release:1.0.0-241c87ad800beeab545ab867354d4683d5bfb6ce|    +--- androidx.lifecycle:lifecycle-common:2.2.0|    |    \--- androidx.annotation:annotation:1.1.0|    +--- androidx.lifecycle:lifecycle-common-java8:2.2.0|    |    +--- androidx.lifecycle:lifecycle-common:2.2.0 (*)|    |    \--- androidx.annotation:annotation:1.1.0|    +--- androidx.lifecycle:lifecycle-runtime:2.2.0|    |    +--- androidx.lifecycle:lifecycle-common:2.2.0 (*)|    |    +--- androidx.arch.core:core-common:2.1.0|    |    |    \--- androidx.annotation:annotation:1.1.0|    |    \--- androidx.annotation:annotation:1.1.0|    +--- androidx.fragment:fragment:1.1.0|    |    +--- androidx.annotation:annotation:1.1.0|    |    +--- androidx.core:core:1.1.0|    |    |    +--- androidx.annotation:annotation:1.1.0|    |    |    +--- androidx.lifecycle:lifecycle-runtime:2.0.0 -> 2.2.0 (*)|    |    |    +--- androidx.versionedparcelable:versionedparcelable:1.1.0|    |    |    |    \--- androidx.collection:collection:1.0.0 -> 1.1.0|    |    |    |         \--- androidx.annotation:annotation:1.1.0|    |    |    \--- androidx.collection:collection:1.0.0 -> 1.1.0 (*)|    |    +--- androidx.collection:collection:1.1.0 (*)|    |    +--- androidx.viewpager:viewpager:1.0.0|    |    |    +--- androidx.annotation:annotation:1.0.0 -> 1.1.0|    |    |    +--- androidx.core:core:1.0.0 -> 1.1.0 (*)|    |    |    \--- androidx.customview:customview:1.0.0|    |    |         +--- androidx.annotation:annotation:1.0.0 -> 1.1.0|    |    |         \--- androidx.core:core:1.0.0 -> 1.1.0 (*)|    |    +--- androidx.loader:loader:1.0.0|    |    |    +--- androidx.annotation:annotation:1.0.0 -> 1.1.0|    |    |    +--- androidx.core:core:1.0.0 -> 1.1.0 (*)|    |    |    +--- androidx.lifecycle:lifecycle-livedata:2.0.0|    |    |    |    +--- androidx.arch.core:core-runtime:2.0.0|    |    |    |    |    +--- androidx.annotation:annotation:1.0.0 -> 1.1.0|    |    |    |    |    \--- androidx.arch.core:core-common:2.0.0 -> 2.1.0 (*)|    |    |    |    +--- androidx.lifecycle:lifecycle-livedata-core:2.0.0|    |    |    |    |    +--- androidx.lifecycle:lifecycle-common:2.0.0 -> 2.2.0 (*)|    |    |    |    |    +--- androidx.arch.core:core-common:2.0.0 -> 2.1.0 (*)|    |    |    |    |    \--- androidx.arch.core:core-runtime:2.0.0 (*)|    |    |    |    \--- androidx.arch.core:core-common:2.0.0 -> 2.1.0 (*)|    |    |    \--- androidx.lifecycle:lifecycle-viewmodel:2.0.0 -> 2.1.0|    |    |         \--- androidx.annotation:annotation:1.1.0|    |    +--- androidx.activity:activity:1.0.0|    |    |    +--- androidx.annotation:annotation:1.1.0|    |    |    +--- androidx.core:core:1.1.0 (*)|    |    |    +--- androidx.lifecycle:lifecycle-runtime:2.1.0 -> 2.2.0 (*)|    |    |    +--- androidx.lifecycle:lifecycle-viewmodel:2.1.0 (*)|    |    |    \--- androidx.savedstate:savedstate:1.0.0|    |    |         +--- androidx.annotation:annotation:1.1.0|    |    |         +--- androidx.arch.core:core-common:2.0.1 -> 2.1.0 (*)|    |    |         \--- androidx.lifecycle:lifecycle-common:2.0.0 -> 2.2.0 (*)|    |    \--- androidx.lifecycle:lifecycle-viewmodel:2.0.0 -> 2.1.0 (*)|    \--- androidx.annotation:annotation:1.1.0+--- io.flutter:armeabi_v7a_release:1.0.0-241c87ad800beeab545ab867354d4683d5bfb6ce+--- io.flutter:arm64_v8a_release:1.0.0-241c87ad800beeab545ab867354d4683d5bfb6ce\--- io.flutter:x86_64_release:1.0.0-241c87ad800beeab545ab867354d4683d5bfb6ce
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可以看到依赖主要都是 androidx,还是蛮香的。

源码分析

Flutter Android 端源码主要依赖于 gradle maven 下载,也就是上图中的io.flutter:flutter_embedding_XXX代码,另一个 ABI 依赖是libflutter.so引擎依赖。


本文以 Flutter 2.2.3 版本为例分析,由于新旧版本的 Android 入口存在差异,譬如新版本不再默认使用FlutterApplication配合io.flutter.app.FlutterActivity模式,而是直接使用io.flutter.embedding.android.FlutterActivty,所以我们的入口要从新的看起(注意新旧同类名不同包名的区别)。相关新旧版本迁移变更可以参考官方说明的Upgrading-pre-1.12-Android-projects

FlutterActivity 相关分析

Android App 中默认 Flutter UI 界面呈现在平台层的实现都继承自io.flutter.embedding.android.FlutterActivity,主题默认也是一个全屏无 ActionBar 形式,源码如下:


public class FlutterActivity extends Activity    implements FlutterActivityAndFragmentDelegate.Host, LifecycleOwner {  ......}
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依据这个继承关系可以看到,FlutterActivity 直接继承自 Activity,而不是兼容包的 AppCompatActivity,这对于有历史包袱的 App 来说是一件好事,也秉承了一个优质 SDK 去依赖的优质特点。同时其实现了两个接口,LifecycleOwner 为标准 AAC 框架成员,我们不再解释;重点看下FlutterActivityAndFragmentDelegate.Host,源码如下:


class FlutterActivityAndFragmentDelegate implements ExclusiveAppComponent<Activity> {  ......  //这里的一堆extends接口不多解释,注释都很明确  interface Host      extends SplashScreenProvider,          FlutterEngineProvider,          FlutterEngineConfigurator,          PlatformPlugin.PlatformPluginDelegate {    //1、获取宿主Activity或Fragment的context。    //对应上面FlutterActivity实现此接口就是返回他自己的context,也就是this。    @NonNull    Context getContext();    //2、是否可以通过deeplink调起initial route路由。    //主要是在AndroidManifest.xml中Activity配置flutter_deeplinking_enabled meta值。    //参见官方文档 https://flutter.dev/docs/development/ui/navigation/deep-linking    @Nullable    boolean shouldHandleDeeplinking();  //3、获取宿主Activity或Fragment的attached Activity。    //对应上面FlutterActivity实现此接口就是返回他自己,也就是this。    @Nullable    Activity getActivity();  //4、获取宿主Activity或Fragment的Lifecycle。    @NonNull    Lifecycle getLifecycle();  //5、获取宿主启动Flutter携带的参数,通过intent解析,譬如enable-dart-profiling等。    @NonNull    FlutterShellArgs getFlutterShellArgs();  //6、获取静态缓存的EngineId,如果没有就返回空,通过intent的cached_engine_id参数传递。    @Nullable    String getCachedEngineId();    //7、当FlutterActivity destory后是否销毁引擎实例,默认要销毁。    //当FlutterEngine属于Activity自己则需要销毁返回true,当FlutterEngine属于静态缓存的,则不用销毁,这里应该返回false。    boolean shouldDestroyEngineWithHost();    //8、当FlutterEngine已经attach到另一个Activity时这个Activity就需要与FlutterEngine断开。    void detachFromFlutterEngine();    //9、获取dart主入口,默认时main。    //可以在AndroidManifest.xml中给Activity设置io.flutter.Entrypoint的meta自定义。    @NonNull    String getDartEntrypointFunctionName();    //10、返回app bundle dart代码存在的路径。    @NonNull    String getAppBundlePath();    //11、获取初始路由地址。    //默认先从intent中解析route的值,没有就去meta-data解析io.flutter.InitialRoute的值,没有就返回null。    @Nullable    String getInitialRoute();    //12、获取渲染模式,用在FlutterView呈现FlutterEngine引擎渲染效果。    @NonNull    RenderMode getRenderMode();    //13、获取Transparency模式,用在FlutterView呈现FlutterEngine引擎渲染效果。    @NonNull    TransparencyMode getTransparencyMode();  //14、提供一个Flutter开屏图片,默认有配置。  //通过meta-data配置io.flutter.embedding.android.SplashScreenDrawable可生效。    @Nullable    SplashScreen provideSplashScreen();    //15、返回一个用来渲染FlutterView的FlutterEngine引擎。    //如果返回null则框架会自动新建一个FlutterEngine引擎实例,默认就是自动新建,我们可以重写复用等。    @Nullable    FlutterEngine provideFlutterEngine(@NonNull Context context);    //16、创建和配置platform plugin。    @Nullable    PlatformPlugin providePlatformPlugin(        @Nullable Activity activity, @NonNull FlutterEngine flutterEngine);    void configureFlutterEngine(@NonNull FlutterEngine flutterEngine);    void cleanUpFlutterEngine(@NonNull FlutterEngine flutterEngine);    boolean shouldAttachEngineToActivity();    void onFlutterSurfaceViewCreated(@NonNull FlutterSurfaceView flutterSurfaceView);    void onFlutterTextureViewCreated(@NonNull FlutterTextureView flutterTextureView);    void onFlutterUiDisplayed();    void onFlutterUiNoLongerDisplayed();    boolean shouldRestoreAndSaveState();  }}
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可以看到FlutterActivityAndFragmentDelegate.Host是 Flutter Android 平台层实现与标准 Activity/Fragment 之间的一个接口约定层,FlutterActivity 实现了这个接口的一系列方法,这些接口的含义上面也列举了,所以我们下面重心回到标准 Android Activity 生命周期线,如下:


public class FlutterActivity extends Activity    implements FlutterActivityAndFragmentDelegate.Host, LifecycleOwner {  ......  //步骤1、重点!关联Activity、Fragment与Flutter的接口桥梁委托实现  protected FlutterActivityAndFragmentDelegate delegate;  ......  @Override  protected void onCreate(@Nullable Bundle savedInstanceState) {    //步骤2、将清单文件中标准配置的主题切换回普通主题。很巧妙的让launch app和initialized后activity主题发生变化。    //普通主题通过io.flutter.embedding.android.NormalTheme的meta-data配置。    //清单标准配置的是一个图标、启动后执行到这里被换成标准纯背景色,两个主题不要对窗体size进行差异配置,不然会抖动。    switchLaunchThemeForNormalTheme();
super.onCreate(savedInstanceState); //步骤3、创建一个委托代理类实例,然后调用他的一系列方法。 delegate = new FlutterActivityAndFragmentDelegate(this); delegate.onAttach(this); delegate.onRestoreInstanceState(savedInstanceState); //步骤4、标准AAC操作,不解释。 lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE); //步骤5、从Activity的Intent中解析background_mode字段看是transparent还是opaque。 //透明则通过getWindow().setBackgroundDrawable设置为透明,否则保持NormalTheme的暗黑适配纯色背景。 configureWindowForTransparency(); //步骤6、创建一个View让Activity显示,这就是Flutter的View容器。 setContentView(createFlutterView()); //步骤7、官方对全屏状态栏的兼容配置。。。 configureStatusBarForFullscreenFlutterExperience(); } ......
//步骤8、创建并返回一个View用来给Activity显示。 @NonNull private View createFlutterView() { return delegate.onCreateView( null /* inflater */, null /* container */, null /* savedInstanceState */); }
//步骤9、官方对全屏状态栏的兼容配置。 //这些flag不用解释了吧,标准安卓操作。 //记住这个坑即可,我们如果想修改FlutterActivity的这玩意就在他之后覆盖即可。 private void configureStatusBarForFullscreenFlutterExperience() { if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) { Window window = getWindow(); window.addFlags(WindowManager.LayoutParams.FLAG_DRAWS_SYSTEM_BAR_BACKGROUNDS); window.setStatusBarColor(0x40000000); window.getDecorView().setSystemUiVisibility(PlatformPlugin.DEFAULT_SYSTEM_UI); } } //步骤10、生命周期回调处理,AAC调用、delegate对应方法触发等 @Override protected void onStart() { super.onStart(); lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_START); //判断delegate是否不为null if (stillAttachedForEvent("onStart")) { delegate.onStart(); } } //步骤11、同步骤10的一堆回调类似触发,省略实现。。。 @Override protected void onResume() {......} @Override public void onPostResume() {......} @Override protected void onPause() {......} @Override protected void onStop() {......} @Override protected void onSaveInstanceState(Bundle outState) {......} @Override protected void onDestroy() {......} @Override protected void onActivityResult(int requestCode, int resultCode, Intent data) {......} @Override protected void onNewIntent(@NonNull Intent intent) {......} @Override public void onBackPressed() {......} @Override public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {......} @Override public void onUserLeaveHint() {......} @Override public void onTrimMemory(int level) {......} ...... //步骤12、一堆前面介绍的FlutterActivityAndFragmentDelegate.Host接口实现方法。 ......}
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FlutterActivityAndFragmentDelegate 相关分析

上一小节我们看到 FlutterActivity 中的核心就是与 FlutterActivityAndFragmentDelegate 实例进行交互,那我们就继续看看这个实例对应的源码,重点先放到 FlutterActivity 中调用的实例化、onAttach、onRestoreInstanceState 上。如下:


class FlutterActivityAndFragmentDelegate implements ExclusiveAppComponent<Activity> {  private static final String TAG = "FlutterActivityAndFragmentDelegate";  private static final String FRAMEWORK_RESTORATION_BUNDLE_KEY = "framework";  private static final String PLUGINS_RESTORATION_BUNDLE_KEY = "plugins";
//步骤1、host实例就是FlutterActivity或FlutterFragment @NonNull private Host host; @Nullable private FlutterEngine flutterEngine; @Nullable private FlutterSplashView flutterSplashView; @Nullable private FlutterView flutterView; @Nullable private PlatformPlugin platformPlugin; private boolean isFlutterEngineFromHost; //步骤2、前面小节FlutterActivity的onCreate中实例化,host为FlutterActivity自己的this。 FlutterActivityAndFragmentDelegate(@NonNull Host host) { this.host = host; } //步骤3、前面小节FlutterActivity的onCreate中调用 void onAttach(@NonNull Context context) { //确保host不为空,否则抛出异常。 ensureAlive();
//步骤4、flutterEngine为空就创建。 if (flutterEngine == null) { setupFlutterEngine(); } //步骤5、前面小节FlutterActivity中接口实现,默认为true。 if (host.shouldAttachEngineToActivity()) { //步骤6、flutterEngine与FlutterActivity进行attach关联。 Log.v(TAG, "Attaching FlutterEngine to the Activity that owns this delegate."); flutterEngine.getActivityControlSurface().attachToActivity(this, host.getLifecycle()); }
//步骤7、通过host获取PlatformPlugin实例。 platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine); //步骤8、给host配置flutterEngine。 host.configureFlutterEngine(flutterEngine); }
//步骤9、前面小节FlutterActivity的onCreate中调用 void onRestoreInstanceState(@Nullable Bundle bundle) { Log.v( TAG, "onRestoreInstanceState. Giving framework and plugins an opportunity to restore state."); ensureAlive(); //步骤10、这两个变量都是在Activity的onSaveInstanceState方法进行set操作赋值的。 Bundle pluginState = null; byte[] frameworkState = null; if (bundle != null) { //说明来自异常终止恢复Activity pluginState = bundle.getBundle(PLUGINS_RESTORATION_BUNDLE_KEY); frameworkState = bundle.getByteArray(FRAMEWORK_RESTORATION_BUNDLE_KEY); } //步骤11、存放时也是这个规则,优先判断intent中的enable_state_restoration配置。 //不存在就看getCachedEngineId是不是不为null,是就不会要恢复,即此条件false,反之恢复。 if (host.shouldRestoreAndSaveState()) { flutterEngine.getRestorationChannel().setRestorationData(frameworkState); } //步骤12、FlutterActivity默认实现是true。 if (host.shouldAttachEngineToActivity()) { flutterEngine.getActivityControlSurface().onRestoreInstanceState(pluginState); } } ......}
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看完上面这段,接下来我们继续把目光挪到 FlutterActivityAndFragmentDelegate 的 onCreateView 方法上,这个方法的返回值在前面 FlutterActivity 的 onCreate 中被 setContent 设置为 Activity 的 View。如下:


class FlutterActivityAndFragmentDelegate implements ExclusiveAppComponent<Activity> {  ......  //步骤13、FlutterActivity中调用时参数均为null,返回一个androd view实例。  @NonNull  View onCreateView(      LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {    Log.v(TAG, "Creating FlutterView.");    //确保host属性不为null。    ensureAlive();  //步骤14、依据前面FlutterActivity的intent参数background_mode决定模式,默认surface模式。  //FlutterActivity默认背景是opaque、可以设置为transparent。  //当FlutterActivity的window背景不透明则RenderMode为surface模式、反之texture模式。    if (host.getRenderMode() == RenderMode.surface) {      //步骤15、创建一个FlutterSurfaceView。      //然后通过host接口调用FlutterActivity的onFlutterSurfaceViewCreated方法,默认空实现。      //getTransparencyMode模式和FlutterActivity的window背景模式完全一样逻辑。      FlutterSurfaceView flutterSurfaceView =          new FlutterSurfaceView(              host.getActivity(), host.getTransparencyMode() == TransparencyMode.transparent);      host.onFlutterSurfaceViewCreated(flutterSurfaceView);
//步骤15、创建一个FlutterView包含FlutterSurfaceView。 flutterView = new FlutterView(host.getActivity(), flutterSurfaceView); } else { //步骤16、创建一个FlutterTextureView。 //然后通过host接口调用FlutterActivity的onFlutterSurfaceViewCreated方法,默认空实现。 FlutterTextureView flutterTextureView = new FlutterTextureView(host.getActivity()); host.onFlutterTextureViewCreated(flutterTextureView);
//步骤17、创建一个FlutterView包含FlutterTextureView。 flutterView = new FlutterView(host.getActivity(), flutterTextureView); }
//步骤18、添加监听,当flutter渲染首帧时回调。 flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener); //步骤19、创建一个FlutterSplashView开屏view flutterSplashView = new FlutterSplashView(host.getContext()); if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) { flutterSplashView.setId(View.generateViewId()); } else { //这里不得不吐槽官方,不过确实没好的办法,直接写死了一个随机id,有可能冲突,且一个view树只能有一个FlutterSplashView了。 flutterSplashView.setId(486947586); } //步骤20、显示开屏图标,即io.flutter.embedding.android.SplashScreenDrawable配置的drawable图。 flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen()); //步骤21、FlutterView与flutterEngine关联attach。 Log.v(TAG, "Attaching FlutterEngine to FlutterView."); flutterView.attachToFlutterEngine(flutterEngine); //返回被开屏view包裹的FlutterView。 return flutterSplashView; }
//步骤22、一组回调监听定义,回调中本质是触发调用对应FlutterActivity或FlutterFragment的FlutterActivityAndFragmentDelegate.Host实现方法。 @NonNull private final FlutterUiDisplayListener flutterUiDisplayListener = new FlutterUiDisplayListener() { @Override public void onFlutterUiDisplayed() { //本质在FlutterActivity中调用Activity 5.0以上的reportFullyDrawn()安卓官方方法。 host.onFlutterUiDisplayed(); }
@Override public void onFlutterUiNoLongerDisplayed() { //本质在FlutterActivity中调用,默认空实现。 host.onFlutterUiNoLongerDisplayed(); } }; ......}
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接着继续看 FlutterActivity 生命周期及事件相关方法对 FlutterActivityAndFragmentDelegate 相关方法的调用,如下:


class FlutterActivityAndFragmentDelegate implements ExclusiveAppComponent<Activity> {  ......  void onStart() {    ......    doInitialFlutterViewRun();  }
//步骤23、开始在FlutterView中执行dart程序。 private void doInitialFlutterViewRun() { ...... //步骤24、判断是否dart已经运行中。 //这个情况只会发生在config change后我们没有保留Fragment实例的场景,尽量不要这样。 if (flutterEngine.getDartExecutor().isExecutingDart()) { return; } //步骤25、各种优先级获取初始跳转dart的路由地址。 String initialRoute = host.getInitialRoute(); if (initialRoute == null) { initialRoute = maybeGetInitialRouteFromIntent(host.getActivity().getIntent()); if (initialRoute == null) { initialRoute = DEFAULT_INITIAL_ROUTE; //值为 / } } //过滤这个tag可以调试flutter初始路由跳转信息。 Log.v( TAG, "Executing Dart entrypoint: " + host.getDartEntrypointFunctionName() + ", and sending initial route: " + initialRoute);
//步骤26、通过引擎的NavigationChannel设置初始路由信息。 flutterEngine.getNavigationChannel().setInitialRoute(initialRoute); //步骤27、按照优先级获取appBundlePath,默认从host获取,无则从FlutterLoader获取。 String appBundlePathOverride = host.getAppBundlePath(); if (appBundlePathOverride == null || appBundlePathOverride.isEmpty()) { appBundlePathOverride = FlutterInjector.instance().flutterLoader().findAppBundlePath(); }
//步骤28、配置dart的entrypoint并且执行,默认入口函数名为main,可通过meta-data的io.flutter.Entrypoint修改。 DartExecutor.DartEntrypoint entrypoint = new DartExecutor.DartEntrypoint( appBundlePathOverride, host.getDartEntrypointFunctionName()); flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint); } ...... //步骤29、FlutterActivity的onResume调用它。 void onResume() { ...... //调用引擎的LifecycleChannel触发方法。 flutterEngine.getLifecycleChannel().appIsResumed(); }
//步骤30、雷同onResume不做说明,都是调用flutterEngine.getLifecycleChannel()的对应方法。 void onPause() {......} void onStop() {......} void onUserLeaveHint() {......} ...... void onPostResume() { ...... if (flutterEngine != null) { if (platformPlugin != null) { //步骤31、即FlutterActivity中providePlatformPlugin方法返回的PlatformPlugin实例。 platformPlugin.updateSystemUiOverlays(); } } else { Log.w(TAG, "onPostResume() invoked before FlutterFragment was attached to an Activity."); } } ...... //步骤32、返回按钮触发,通过NavigationChannel通道退栈route。 void onBackPressed() { ensureAlive(); if (flutterEngine != null) { Log.v(TAG, "Forwarding onBackPressed() to FlutterEngine."); flutterEngine.getNavigationChannel().popRoute(); } else { Log.w(TAG, "Invoked onBackPressed() before FlutterFragment was attached to an Activity."); } }
//步骤33、动态权限申请返回处理,转发到感兴趣的Flutter Plugin去。 void onRequestPermissionsResult( int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) { ensureAlive(); if (flutterEngine != null) { ...... flutterEngine .getActivityControlSurface() .onRequestPermissionsResult(requestCode, permissions, grantResults); } else { Log.w( TAG, "onRequestPermissionResult() invoked before FlutterFragment was attached to an Activity."); } }
//步骤34、activity result返回处理,转发到感兴趣的Flutter Plugin去。 void onActivityResult(int requestCode, int resultCode, Intent data) { ensureAlive(); if (flutterEngine != null) { ...... flutterEngine.getActivityControlSurface().onActivityResult(requestCode, resultCode, data); } else { Log.w(TAG, "onActivityResult() invoked before FlutterFragment was attached to an Activity."); } }
//步骤35、通过SystemChannel发送低内存消息。 void onLowMemory() { Log.v(TAG, "Forwarding onLowMemory() to FlutterEngine."); ensureAlive(); flutterEngine.getDartExecutor().notifyLowMemoryWarning(); flutterEngine.getSystemChannel().sendMemoryPressureWarning(); } ......}
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通过这一小节的代码段分析,我们可以看出来 Flutter 在平台 SDK 层包装都是 FlutterActivityAndFragmentDelegate 来负责的,FlutterActivityAndFragmentDelegate 和核心是 FlutterView 和 FlutterEngine 的创建关联与管理调度。

FlutterFragment 相关分析

分析完上面 FlutterActivity 相关流程,我们接着看看 FlutterFragment,继承自androidx.fragment.app.Fragment,言外之意就是说 Activity 也得最好是用配套的 FlutterFragmentActivity。关于 FlutterFragmentActivity 我们会在下一个小节分析,这里重点关注 FlutterFragment。


其实官方注释也明确说明了,他们不是特别优先推荐使用 FlutterFragment,建议尽可能优选 FlutterActivity。因为使用 FlutterFragment 如果配合的 Activity 不是 FlutterFragmentActivity,需要我们自己去关联一些 Activity 与 FlutterFragment 的方法,这些方法在 FlutterFragment 的源码中都以@ActivityCallThrough进行了注释声明,让使用变的没那么方便,譬如:


  • onPostResume()

  • onBackPressed()

  • onRequestPermissionsResult(int, String[], int[])} ()

  • onNewIntent(Intent)} ()

  • onUserLeaveHint()


如上 FlutterFragment 中的方法都需要主动与 Acitivty 关联调用(FlutterFragmentActivity 已实现)。下面是 FlutterFragment 的主要源码:


//步骤36、FlutterFragment也实现了前面分析的FlutterActivityAndFragmentDelegate.Host接口,含义不解释。public class FlutterFragment extends Fragment    implements FlutterActivityAndFragmentDelegate.Host, ComponentCallbacks2 {  ......  @Override  public void onAttach(@NonNull Context context) {    super.onAttach(context);    //步骤37、与FlutterActivity的区别在于FlutterFragment在他自己的onAttach中实例化FlutterActivityAndFragmentDelegate并调用onAttach方法。    delegate = new FlutterActivityAndFragmentDelegate(this);    delegate.onAttach(context);  }
@Override public void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); //步骤38、与FlutterActivity类似,不做过多分析说明。 delegate.onRestoreInstanceState(savedInstanceState); }
@Nullable @Override public View onCreateView( LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { //步骤39、与FlutterActivity类似,只不过是在FlutterFragment对应生命周期回调。 return delegate.onCreateView(inflater, container, savedInstanceState); } ...... //步骤39、与FlutterActivity类似,只是这个方法不是Fragment自己框架回调,需要依赖在Activity中调用。 //譬如FlutterFragmentActivity中对应同名方法的实现。 //注意这里的@ActivityCallThrough注解就是这个含义。 @ActivityCallThrough public void onPostResume() { delegate.onPostResume(); } ......}
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可以看到,FlutterFragment 和 FlutterActivity 基本没啥区别,核心都是实现了FlutterActivityAndFragmentDelegate.Host接口,在自己生命周期内先实例化一个 FlutterActivityAndFragmentDelegate,接着调用其一系列方法进行交互,前面 FlutterActivity 已经解释过了,这里不多解释。

FlutterFragmentActivity 相关分析

通过上面 FlutterFragment 源码我们可以看到,FlutterFragmentActivity 可以说是 FlutterFragment 的一个承载 Activity,这个 Activity 继承自androidx.fragment.app.FragmentActivity,所以使用 FlutterFragment 及 FlutterFragmentActivity 对你应用的基类 Activity 还是有一点限制的,没有前面介绍的 FlutterActivity 香。


由于 FlutterFragmentActivity 继承自androidx.fragment.app.FragmentActivity,所以相比 FlutterActivity 来说,AAC 架构那套就不用自己显式关联了,因为androidx.fragment.app.FragmentActivity内部已做好关联处理。


//步骤40、这里implements的一堆其实FlutterActivityAndFragmentDelegate.Host都有implements,算是Host的子集吧。//本质和FlutterActivityAndFragmentDelegate.Host对于实现方干的一样的事。public class FlutterFragmentActivity extends FragmentActivity    implements SplashScreenProvider, FlutterEngineProvider, FlutterEngineConfigurator {  @Override  protected void onCreate(@Nullable Bundle savedInstanceState) {    //步骤41、这里和FlutterActivity完全一样,只是不用实例化一个FlutterActivityAndFragmentDelegate,因为其内部的FlutterFragment会做这些事。    switchLaunchThemeForNormalTheme();    super.onCreate(savedInstanceState);    configureWindowForTransparency();    //步骤42、这里区别是调用createFragmentContainer生成了一个View设置给Activity的content。    setContentView(createFragmentContainer());    //步骤43、这里和FlutterActivity完全一样。    configureStatusBarForFullscreenFlutterExperience();    //步骤44、与FlutterActivity不一样,这里检验的是FlutterFragment是否添加OK。    ensureFlutterFragmentCreated();  }
//步骤45、上面步骤42设置的View,用来放置FlutterFragment的容器View,本质是一个FrameLayout且MATCH_PARENT。 @NonNull private View createFragmentContainer() { FrameLayout container = provideRootLayout(this); container.setId(FRAGMENT_CONTAINER_ID); container.setLayoutParams( new ViewGroup.LayoutParams( ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT)); return container; }
protected FrameLayout provideRootLayout(Context context) { return new FrameLayout(context); }
//步骤46、确保给Activity上attach一个FlutterFragment,不存在就创建一个通过FragmentManager添加。 private void ensureFlutterFragmentCreated() { FragmentManager fragmentManager = getSupportFragmentManager(); flutterFragment = (FlutterFragment) fragmentManager.findFragmentByTag(TAG_FLUTTER_FRAGMENT); if (flutterFragment == null) { flutterFragment = createFlutterFragment(); fragmentManager .beginTransaction() .add(FRAGMENT_CONTAINER_ID, flutterFragment, TAG_FLUTTER_FRAGMENT) .commit(); } }
//步骤47、创建一个FlutterFragment。 //如果我们自定义FlutterFragmentActivity子类,可以重写这个方法实现自己的自定义FlutterFragment。 @NonNull protected FlutterFragment createFlutterFragment() { final BackgroundMode backgroundMode = getBackgroundMode(); final RenderMode renderMode = getRenderMode(); final TransparencyMode transparencyMode = backgroundMode == BackgroundMode.opaque ? TransparencyMode.opaque : TransparencyMode.transparent; //步骤48、依据是否Engine缓存决定怎么创建FlutterFragment。 if (getCachedEngineId() != null) { ...... return FlutterFragment.withCachedEngine(getCachedEngineId()) .renderMode(renderMode) .transparencyMode(transparencyMode) .handleDeeplinking(shouldHandleDeeplinking()) .shouldAttachEngineToActivity(shouldAttachEngineToActivity()) .destroyEngineWithFragment(shouldDestroyEngineWithHost()) .build(); } else { ...... return FlutterFragment.withNewEngine() .dartEntrypoint(getDartEntrypointFunctionName()) .initialRoute(getInitialRoute()) .appBundlePath(getAppBundlePath()) .flutterShellArgs(FlutterShellArgs.fromIntent(getIntent())) .handleDeeplinking(shouldHandleDeeplinking()) .renderMode(renderMode) .transparencyMode(transparencyMode) .shouldAttachEngineToActivity(shouldAttachEngineToActivity()) .build(); } } ...... //步骤49、调用flutterFragment对应生命周期回调。 //可以看到,使用FlutterFragmentActivity配合FlutterFragment的好处就是这些FlutterFragment的@ActivityCallThrough都帮你调用好了。 @Override public void onPostResume() { super.onPostResume(); flutterFragment.onPostResume(); } ......}
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总结

到此我们其实就知道 FlutterActivity 和 FlutterFragment 的大致实现,相信你通过上面分析对 Flutter App 在安卓端的承载有一定认识,通常一个 engine 的整个 Flutter Dart 无论页面栈多少级,终归在安卓端都是一个 Activity 或者 Fragment 承载,抑或 View,端侧仅仅算是一个容器而已。


这也就是为什么我们在 Android 平台上查看一个标准 Flutter App 的 Activity 堆栈默认只有继承自 FlutterActivity 的一个 Activity,其内部 View 层级如下的原因:



关于本文更多样例可以参见官方文档:



这里不做过多演示。


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Flutter Android 端 Activity/Fragment 流程源码分析