Flutter-Android-Embedder啟動流程

概述

Flutter的啟動包括Embedder、Engine、Framework三部分，本文僅描述Android平臺的Embdder模塊的啟動流程，Flutter通常通過啟動一個FlutterActivity啟動，純Flutter App則通過FlutterApplication啟動，本文以后一種情況為例分析，

FlutterApplication#onCreate

// io/flutter/app/FlutterApplication.java

  @Override
  @CallSuper
  public void onCreate() {
    super.onCreate();
    FlutterInjector.instance().flutterLoader().startInitialization(this);
  }

實際是呼叫FlutterLoader#startInitialization進行初始化，

FlutterLoader#startInitialization

  public void startInitialization(@NonNull Context applicationContext) {
    startInitialization(applicationContext, new Settings());
  }

  public void startInitialization(@NonNull Context applicationContext, @NonNull Settings settings) {
    if (this.settings != null) {
      return;
    }
    if (Looper.myLooper() != Looper.getMainLooper()) {
      throw new IllegalStateException("startInitialization must be called on the main thread");
    }

    final Context appContext = applicationContext.getApplicationContext();

    this.settings = settings;

    initStartTimestampMillis = SystemClock.uptimeMillis();
    flutterApplicationInfo = ApplicationInfoLoader.load(appContext);
    VsyncWaiter.getInstance((WindowManager) appContext.getSystemService(Context.WINDOW_SERVICE))
        .init();

    // Use a background thread for initialization tasks that require disk access.
    Callable<InitResult> initTask =
        new Callable<InitResult>() {
          @Override
          public InitResult call() {
            ResourceExtractor resourceExtractor = initResources(appContext);

            if (FlutterInjector.instance().shouldLoadNative()) {
              System.loadLibrary("flutter");
            }

            // Prefetch the default font manager as soon as possible on a background thread.
            // It helps to reduce time cost of engine setup that blocks the platform thread.
            Executors.newSingleThreadExecutor()
                .execute(
                    new Runnable() {
                      @Override
                      public void run() {
                        FlutterJNI.nativePrefetchDefaultFontManager();
                      }
                    });

            if (resourceExtractor != null) {
              resourceExtractor.waitForCompletion();
            }

            return new InitResult(
                PathUtils.getFilesDir(appContext),
                PathUtils.getCacheDirectory(appContext),
                PathUtils.getDataDirectory(appContext));
          }
        };
    initResultFuture = Executors.newSingleThreadExecutor().submit(initTask);
  }

主要做了以下幾件事：

1. 檢查是否在主執行緒

2. 注冊Vsync信號監聽

3. 啟動一個異步任務，執行以下操作：

4. 1. 加載flutter engine

   2. 異步初始化字體管理器

   3. 加載asset目錄下的資源

以上是Application會執行的操作，接著一般會進入一個FlutterActivity，那么依次查看其回呼，

FlutterActivity#onCreate

// io/flutter/embedding/android/FlutterActivity.java

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    switchLaunchThemeForNormalTheme();

    super.onCreate(savedInstanceState);

    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);

    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    delegate.onActivityCreated(savedInstanceState);

    configureWindowForTransparency();
    setContentView(createFlutterView());
    configureStatusBarForFullscreenFlutterExperience();
  }

主要做了以下幾件事：

1. 從Splash切換成真正要顯示的View

2. 創建一個delegate，并觸發對應回呼

3. 配置相關的視窗屬性

4. 創建一個新的FlutterView，作為Activity的ContentView

FlutterActivityAndFragmentDelegate#onAttach

  void onAttach(@NonNull Context context) {
    ensureAlive();
    if (flutterEngine == null) {
      setupFlutterEngine();
    }

    if (host.shouldAttachEngineToActivity()) {
      Log.v(TAG, "Attaching FlutterEngine to the Activity that owns this delegate.");
      flutterEngine.getActivityControlSurface().attachToActivity(this, host.getLifecycle());
    }

    platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);

    host.configureFlutterEngine(flutterEngine);
  }

主要做了以下幾件事：

1. 設定FlutterEngine，一般是新建一個

2. 把Engine和容器Activity關聯

3. 新建一個PlatformPlugin

4. engine初始化完畢，觸發對應回呼

其中，FlutterActivityAndFragmentDelegate#setupFlutterEngine行為比較復雜

  /* package */ void setupFlutterEngine() {
    Log.v(TAG, "Setting up FlutterEngine.");

    // First, check if the host wants to use a cached FlutterEngine.
    String cachedEngineId = host.getCachedEngineId();
    if (cachedEngineId != null) {
      flutterEngine = FlutterEngineCache.getInstance().get(cachedEngineId);
      isFlutterEngineFromHost = true;
      if (flutterEngine == null) {
        throw new IllegalStateException(
            "The requested cached FlutterEngine did not exist in the FlutterEngineCache: '"
                + cachedEngineId
                + "'");
      }
      return;
    }

    // Second, defer to subclasses for a custom FlutterEngine.
    flutterEngine = host.provideFlutterEngine(host.getContext());
    if (flutterEngine != null) {
      isFlutterEngineFromHost = true;
      return;
    }

    Log.v(
        TAG,
        "No preferred FlutterEngine was provided. Creating a new FlutterEngine for"
            + " this FlutterFragment.");
    flutterEngine =
        new FlutterEngine(
            host.getContext(),
            host.getFlutterShellArgs().toArray(),
            /*automaticallyRegisterPlugins=*/ false,
            /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
    isFlutterEngineFromHost = false;
  }

這里一般是第三種情況，直接新建一個Engine：

  public FlutterEngine(
      @NonNull Context context,
      @Nullable FlutterLoader flutterLoader,
      @NonNull FlutterJNI flutterJNI,
      @NonNull PlatformViewsController platformViewsController,
      @Nullable String[] dartVmArgs,
      boolean automaticallyRegisterPlugins,
      boolean waitForRestorationData) {
    AssetManager assetManager;
    try {
      assetManager = context.createPackageContext(context.getPackageName(), 0).getAssets();
    } catch (NameNotFoundException e) {
      assetManager = context.getAssets();
    }
    this.dartExecutor = new DartExecutor(flutterJNI, assetManager);
    this.dartExecutor.onAttachedToJNI();

    accessibilityChannel = new AccessibilityChannel(dartExecutor, flutterJNI);
    keyEventChannel = new KeyEventChannel(dartExecutor);
    lifecycleChannel = new LifecycleChannel(dartExecutor);
    localizationChannel = new LocalizationChannel(dartExecutor);
    mouseCursorChannel = new MouseCursorChannel(dartExecutor);
    navigationChannel = new NavigationChannel(dartExecutor);
    platformChannel = new PlatformChannel(dartExecutor);
    restorationChannel = new RestorationChannel(dartExecutor, waitForRestorationData);
    settingsChannel = new SettingsChannel(dartExecutor);
    systemChannel = new SystemChannel(dartExecutor);
    textInputChannel = new TextInputChannel(dartExecutor);

    this.localizationPlugin = new LocalizationPlugin(context, localizationChannel);

    this.flutterJNI = flutterJNI;
    if (flutterLoader == null) {
      flutterLoader = FlutterInjector.instance().flutterLoader();
    }
    flutterLoader.startInitialization(context.getApplicationContext());
    flutterLoader.ensureInitializationComplete(context, dartVmArgs);

    flutterJNI.addEngineLifecycleListener(engineLifecycleListener);
    flutterJNI.setPlatformViewsController(platformViewsController);
    flutterJNI.setLocalizationPlugin(localizationPlugin);
    attachToJni();

    // TODO(mattcarroll): FlutterRenderer is temporally coupled to attach(). Remove that coupling if
    // possible.
    this.renderer = new FlutterRenderer(flutterJNI);

    this.platformViewsController = platformViewsController;
    this.platformViewsController.onAttachedToJNI();

    this.pluginRegistry =
        new FlutterEngineConnectionRegistry(context.getApplicationContext(), this, flutterLoader);

    if (automaticallyRegisterPlugins) {
      registerPlugins();
    }
  }

主要做了以下幾件事：

1. 新建Dart運行環境DartExecutor

2. 創建各種用于Flutter和Android通信的channel物件

3. 確認engine初始化完成（呼叫Native側的初始化方法，執行Flutter engine的啟動邏輯）

4. 插件注冊

flutterLoader.startInitialization由于之前呼叫過，這里會直接回傳，FlutterLoader#ensureInitializationComplete則負責engine最后的準備作業：

  public void ensureInitializationComplete(
      @NonNull Context applicationContext, @Nullable String[] args) {
    if (initialized) {
      return;
    }
    if (Looper.myLooper() != Looper.getMainLooper()) {
      throw new IllegalStateException(
          "ensureInitializationComplete must be called on the main thread");
    }
    if (settings == null) {
      throw new IllegalStateException(
          "ensureInitializationComplete must be called after startInitialization");
    }
    try {
      InitResult result = initResultFuture.get();

      List<String> shellArgs = new ArrayList<>();
      shellArgs.add("--icu-symbol-prefix=_binary_icudtl_dat");

      shellArgs.add(
          "--icu-native-lib-path="
              + flutterApplicationInfo.nativeLibraryDir
              + File.separator
              + DEFAULT_LIBRARY);
      if (args != null) {
        Collections.addAll(shellArgs, args);
      }

      String kernelPath = null;
      if (BuildConfig.DEBUG || BuildConfig.JIT_RELEASE) {
        String snapshotAssetPath =
            result.dataDirPath + File.separator + flutterApplicationInfo.flutterAssetsDir;
        kernelPath = snapshotAssetPath + File.separator + DEFAULT_KERNEL_BLOB;
        shellArgs.add("--" + SNAPSHOT_ASSET_PATH_KEY + "=" + snapshotAssetPath);
        shellArgs.add("--" + VM_SNAPSHOT_DATA_KEY + "=" + flutterApplicationInfo.vmSnapshotData);
        shellArgs.add(
            "--" + ISOLATE_SNAPSHOT_DATA_KEY + "=" + flutterApplicationInfo.isolateSnapshotData);
      } else {
        shellArgs.add(
            "--" + AOT_SHARED_LIBRARY_NAME + "=" + flutterApplicationInfo.aotSharedLibraryName);

        // Most devices can load the AOT shared library based on the library name
        // with no directory path.  Provide a fully qualified path to the library
        // as a workaround for devices where that fails.
        shellArgs.add(
            "--"
                + AOT_SHARED_LIBRARY_NAME
                + "="
                + flutterApplicationInfo.nativeLibraryDir
                + File.separator
                + flutterApplicationInfo.aotSharedLibraryName);
      }

      shellArgs.add("--cache-dir-path=" + result.engineCachesPath);
      if (!flutterApplicationInfo.clearTextPermitted) {
        shellArgs.add("--disallow-insecure-connections");
      }
      if (flutterApplicationInfo.domainNetworkPolicy != null) {
        shellArgs.add("--domain-network-policy=" + flutterApplicationInfo.domainNetworkPolicy);
      }
      if (settings.getLogTag() != null) {
        shellArgs.add("--log-tag=" + settings.getLogTag());
      }

      long initTimeMillis = SystemClock.uptimeMillis() - initStartTimestampMillis;

      if (FlutterInjector.instance().shouldLoadNative()) {
        FlutterJNI.nativeInit(
            applicationContext,
            shellArgs.toArray(new String[0]),
            kernelPath,
            result.appStoragePath,
            result.engineCachesPath,
            initTimeMillis);
      }

      initialized = true;
    } catch (Exception e) {
      Log.e(TAG, "Flutter initialization failed.", e);
      throw new RuntimeException(e);
    }
  }

主要是拼接各種引數，最終呼叫了一個native方法，讓engine執行自己的初始化邏輯，

至此，FlutterActivityAndFragmentDelegate#onAttach的邏輯終于走完了，

FlutterActivityAndFragmentDelegate#onActivityCreated

  void onActivityCreated(@Nullable Bundle bundle) {
    Log.v(TAG, "onActivityCreated. Giving framework and plugins an opportunity to restore state.");
    ensureAlive();

    Bundle pluginState = null;
    byte[] frameworkState = null;
    if (bundle != null) {
      pluginState = bundle.getBundle(PLUGINS_RESTORATION_BUNDLE_KEY);
      frameworkState = bundle.getByteArray(FRAMEWORK_RESTORATION_BUNDLE_KEY);
    }

    if (host.shouldRestoreAndSaveState()) {
      flutterEngine.getRestorationChannel().setRestorationData(frameworkState);
    }

    if (host.shouldAttachEngineToActivity()) {
      flutterEngine.getActivityControlSurface().onRestoreInstanceState(pluginState);
    }
  }

狀態恢復相關，一般無需關注，

FlutterActivity#createFlutterView

  @NonNull
  private View createFlutterView() {
    return delegate.onCreateView(
        null /* inflater */, null /* container */, null /* savedInstanceState */);
  }

繼續：

  View onCreateView(
      LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    Log.v(TAG, "Creating FlutterView.");
    ensureAlive();

    if (host.getRenderMode() == RenderMode.surface) {
      FlutterSurfaceView flutterSurfaceView =
          new FlutterSurfaceView(
              host.getActivity(), host.getTransparencyMode() == TransparencyMode.transparent);

      host.onFlutterSurfaceViewCreated(flutterSurfaceView);

      flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
    } else {
      FlutterTextureView flutterTextureView = new FlutterTextureView(host.getActivity());

      host.onFlutterTextureViewCreated(flutterTextureView);

      flutterView = new FlutterView(host.getActivity(), flutterTextureView);
    }

    flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener);

    flutterSplashView = new FlutterSplashView(host.getContext());
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
      flutterSplashView.setId(View.generateViewId());
    } else {
      flutterSplashView.setId(486947586);
    }
    flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());

    Log.v(TAG, "Attaching FlutterEngine to FlutterView.");
    flutterView.attachToFlutterEngine(flutterEngine);

    return flutterSplashView;
  }

這里會根據不同mode，創建不同型別的FlutterView，這個View承載了Flutter的UI在Android端的顯示，

此外，會創建一個FlutterSplashView作為啟動階段的閃屏，

最后，FlutterView#attachToFlutterEngine會把FlutterView和FlutterEngine關聯起來：

  public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {
    Log.v(TAG, "Attaching to a FlutterEngine: " + flutterEngine);
    if (isAttachedToFlutterEngine()) {
      if (flutterEngine == this.flutterEngine) {
        Log.v(TAG, "Already attached to this engine. Doing nothing.");
        return;
      }
      Log.v(
          TAG,
          "Currently attached to a different engine. Detaching and then attaching"
              + " to new engine.");
      detachFromFlutterEngine();
    }

    this.flutterEngine = flutterEngine;

    FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer();
    isFlutterUiDisplayed = flutterRenderer.isDisplayingFlutterUi();
    renderSurface.attachToRenderer(flutterRenderer);
    flutterRenderer.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);

    // in a way that Flutter understands.
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
      mouseCursorPlugin = new MouseCursorPlugin(this, this.flutterEngine.getMouseCursorChannel());
    }
    textInputPlugin =
        new TextInputPlugin(
            this,
            this.flutterEngine.getTextInputChannel(),
            this.flutterEngine.getPlatformViewsController());
    localizationPlugin = this.flutterEngine.getLocalizationPlugin();
    androidKeyProcessor =
        new AndroidKeyProcessor(this, this.flutterEngine.getKeyEventChannel(), textInputPlugin);
    androidTouchProcessor =
        new AndroidTouchProcessor(this.flutterEngine.getRenderer(), /*trackMotionEvents=*/ false);
    accessibilityBridge =
        new AccessibilityBridge(
            this,
            flutterEngine.getAccessibilityChannel(),
            (AccessibilityManager) getContext().getSystemService(Context.ACCESSIBILITY_SERVICE),
            getContext().getContentResolver(),
            this.flutterEngine.getPlatformViewsController());
    accessibilityBridge.setOnAccessibilityChangeListener(onAccessibilityChangeListener);
    resetWillNotDraw(
        accessibilityBridge.isAccessibilityEnabled(),
        accessibilityBridge.isTouchExplorationEnabled());

    this.flutterEngine.getPlatformViewsController().attachAccessibilityBridge(accessibilityBridge);
    this.flutterEngine
        .getPlatformViewsController()
        .attachToFlutterRenderer(this.flutterEngine.getRenderer());

    textInputPlugin.getInputMethodManager().restartInput(this);

    // Push View and Context related information from Android to Flutter.
    sendUserSettingsToFlutter();
    localizationPlugin.sendLocalesToFlutter(getResources().getConfiguration());
    sendViewportMetricsToFlutter();

    flutterEngine.getPlatformViewsController().attachToView(this);

    // Notify engine attachment listeners of the attachment.
    for (FlutterEngineAttachmentListener listener : flutterEngineAttachmentListeners) {
      listener.onFlutterEngineAttachedToFlutterView(flutterEngine);
    }

    if (isFlutterUiDisplayed) {
      flutterUiDisplayListener.onFlutterUiDisplayed();
    }
  }

這里主要是關聯兩個物件的欄位，同時對一些必要的Plugin進行初始化，

FlutterActivity#onStart

  @Override
  protected void onStart() {
    super.onStart();
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_START);
    delegate.onStart();
  }

核心邏輯在delegate里面：

  void onStart() {
    Log.v(TAG, "onStart()");
    ensureAlive();
    doInitialFlutterViewRun();
  }


  private void doInitialFlutterViewRun() {
    if (host.getCachedEngineId() != null) {
      return;
    }

    if (flutterEngine.getDartExecutor().isExecutingDart()) {
      return;
    }
    String initialRoute = host.getInitialRoute();
    if (initialRoute == null) {
      initialRoute = getInitialRouteFromIntent(host.getActivity().getIntent());
    }
    Log.v(
        TAG,
        "Executing Dart entrypoint: "
            + host.getDartEntrypointFunctionName()
            + ", and sending initial route: "
            + initialRoute);

    if (initialRoute != null) {
      flutterEngine.getNavigationChannel().setInitialRoute(initialRoute);
    }

    String appBundlePathOverride = host.getAppBundlePath();
    if (appBundlePathOverride == null || appBundlePathOverride.isEmpty()) {
      appBundlePathOverride = FlutterInjector.instance().flutterLoader().findAppBundlePath();
    }

    DartExecutor.DartEntrypoint entrypoint =
        new DartExecutor.DartEntrypoint(
            appBundlePathOverride, host.getDartEntrypointFunctionName());
    flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
  }

可見其核心行為是啟動dart虛擬機，

FlutterActivity#onResume

  @Override
  protected void onResume() {
    super.onResume();
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_RESUME);
    delegate.onResume();
  }

同樣看delegate的代碼：

// io/flutter/embedding/android/FlutterActivityAndFragmentDelegate.java
  void onResume() {
    Log.v(TAG, "onResume()");
    ensureAlive();
    flutterEngine.getLifecycleChannel().appIsResumed();
  }

這里向Flutter發送了一個通知，宿主Activity處于onResume狀態了，

以上，就是Android Embedder層的啟動流程，