Flutter——在Android平台上的启动流程浅析

介绍

Flutter应用是由平台(及native)来创建、初始化并启动的，这里我们以android为例，对启动过程做一个走马观花式的了解,旨在对平台端的工作有个大致了解。

Android端的启动流程

启动流程实际上还涉及了很多native 层的工作，但是宥于篇幅，暂且只看Android端。

FlutterApplication

flutter应用下，原生的启动流程并没有什么变化，我们来看Application的onCreate函数。

  @Override
  @CallSuper
  public void onCreate() {
    super.onCreate();
    FlutterMain.startInitialization(this);
  }

很简单，继续往里走

  public static void startInitialization(@NonNull Context applicationContext) {
    if (isRunningInRobolectricTest) {
      return;
    }
    FlutterLoader.getInstance().startInitialization(applicationContext);
  }

上面的方法用于初始化 native system（即C++），并最终会调用下面的方法：

我将说明以注释的形式写在下面

  public void startInitialization(@NonNull Context applicationContext, @NonNull Settings settings) {

    if (this.settings != null) {
      return;
    }
    ///确保运行在 主线程
    if (Looper.myLooper() != Looper.getMainLooper()) {
      throw new IllegalStateException("startInitialization must be called on the main thread");
    }

    // Ensure that the context is actually the application context.
    final Context appContext = applicationContext.getApplicationContext();

    this.settings = settings;

    initStartTimestampMillis = SystemClock.uptimeMillis();

    ///配置 aotSharedLibraryName、flutterAssetsDir、
    ///      vmSnapshotData、isolateSnapshotData
    ///等参数

    initConfig(appContext);
    ///初始化VsyncWaiter，并设置回调
    /// 当vsync信号到来时，就调用我们设置的回调，最终会触发页面的刷新
    VsyncWaiter.getInstance((WindowManager) appContext.getSystemService(Context.WINDOW_SERVICE))
        .init();

    // 子线程

    ///这里主要是抽取资源文件，
    ///加载 flutter（libflutter.so）代码
    Callable<InitResult> initTask =
        new Callable<InitResult>() {
          @Override
          public InitResult call() {
            ResourceExtractor resourceExtractor = initResources(appContext);

            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);
  }

至此FlutterApplication 的相关流程就走完了。

另外，虽然上面的代码中使用了子线程，但是最终在这些任务没有完成前，是不会进入flutter侧的，我们接着走FlutterActivity。

FlutterActivity & onCreate

开始的地方依然是 onCreate()方法：

  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    ///切换主题
    switchLaunchThemeForNormalTheme();

    super.onCreate(savedInstanceState);
    ///通知生命周期
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);

    ///初始化delete，这个很重要，
    ///所有的工作都是由它来完成的
    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    ///是否需要恢复（包括通知插件）一些状态
    delegate.onActivityCreated(savedInstanceState);
    ///配置窗口
    configureWindowForTransparency();
    ///创建flutterView
    setContentView(createFlutterView());
    configureStatusBarForFullscreenFlutterExperience();
  }

这里面比较重的代码是这几行:

    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    ...
    setContentView(createFlutterView());

我们一步一步来,首先创建了FlutterActivityAndFragmentDelegate 并调用了它的attact(this)方法。

FlutterActivityAndFragmentDelegate

void onAttach(@NonNull Context context) {
    ensureAlive();
    ///初始化engine
    if (flutterEngine == null) {
    ///这里面会对已有的engine进行复用
      setupFlutterEngine();
    }

    ///初始化平台插件
    ///本质上，是将engine的 channel回调与平台的系统服务进行绑定
    ///如：震动、复制粘贴、声音播放等...
    platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);

    if (host.shouldAttachEngineToActivity()) {

      Log.v(TAG, "Attaching FlutterEngine to the Activity that owns this Fragment.");
      /// 激活 原生viewController
      /// 并通知相关插件
      /// PlatformViewsController 这个类你应该很熟悉（如果你接入过原生view的话）
      flutterEngine
          .getActivityControlSurface()
          .attachToActivity(host.getActivity(), host.getLifecycle());
    }
    ///注册插件
    ///通过反射调用 “io.flutter.plugins.GeneratedPluginRegistrant”
    ///的 “registerWith”方法，这个过程走完了，你的插件基本就能用了
    host.configureFlutterEngine(flutterEngine);
  }

通过上面，我们大致了解了，在flutter端使用的平台功能是什么时候装配的了。

我们回到FlutterActivity，继续重要的第二步:

setContentView(createFlutterView());

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

最终会调用 delete的onCreateView :

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

    if (host.getRenderMode() == RenderMode.surface) {

        ///一般flutter应用是 RenderMode.surface，所以会进入到这里
        ///创建FlutterSurfaceView
      FlutterSurfaceView flutterSurfaceView =
          new FlutterSurfaceView(
              host.getActivity(), host.getTransparencyMode() == TransparencyMode.transparent);

      // Allow our host to customize FlutterSurfaceView, if desired.
      host.onFlutterSurfaceViewCreated(flutterSurfaceView);

      // flutterView 创建完成后，便会调用addView
      //将 flutterSurfaceView 显示出来，只不过啥都没有而已
      flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
    } else {
      ...省略代码...
    }

    // Add listener to be notified when Flutter renders its first frame.
    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);
    }
    ///这里显示闪屏页 默认是个白屏
    ///即，AndroidMainfest.xml 的<metadata>所设置
    flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());

    ///将flutterview 绑定到 engine上
    flutterView.attachToFlutterEngine(flutterEngine);

    return flutterSplashView;
  }

flutterView 内部持有flutterSurfaceView （一个Surface），并最终通过attachToFlutterEngine绑定到engine上，我们来看一下其内部实现：

  public void attachToFlutterEngine(@NonNull FlutterEngine flutterEngine) {

   ...省略部分代码...

    this.flutterEngine = flutterEngine;

    ///通过engine的 getRenderer，
    ///可以将flutter的纹理绘制到android 上。
    FlutterRenderer flutterRenderer = this.flutterEngine.getRenderer();
    isFlutterUiDisplayed = flutterRenderer.isDisplayingFlutterUi();
    renderSurface.attachToRenderer(flutterRenderer);
    flutterRenderer.addIsDisplayingFlutterUiListener(flutterUiDisplayListener);

    ...省略部分代码...

    ///输入插件
    textInputPlugin =
        new TextInputPlugin(
            this,
            this.flutterEngine.getTextInputChannel(),
            this.flutterEngine.getPlatformViewsController());
    ///国际化插件
    localizationPlugin = this.flutterEngine.getLocalizationPlugin();
    ///与上面的textInputPlugin相关联
    androidKeyProcessor =
        new AndroidKeyProcessor(this.flutterEngine.getKeyEventChannel(), textInputPlugin);

     /// 触摸事件的初始化
     /// 相关触摸数据会发送到flutter端
    androidTouchProcessor = new AndroidTouchProcessor(this.flutterEngine.getRenderer());
    ///辅助功能
    accessibilityBridge =
        new AccessibilityBridge(
            this,
            flutterEngine.getAccessibilityChannel(),
            (AccessibilityManager) getContext().getSystemService(Context.ACCESSIBILITY_SERVICE),
            getContext().getContentResolver(),
            this.flutterEngine.getPlatformViewsController());

   ...省略部分代码...

    ///通过上面的初始化，将用户相关的设置发送到flutter端
    sendUserSettingsToFlutter();
    localizationPlugin.sendLocalesToFlutter(getResources().getConfiguration());
    sendViewportMetricsToFlutter();

    ///将当前flutter view 绑定到 PlatformViewsController
    flutterEngine.getPlatformViewsController().attachToView(this);

    ...省略部分代码...
  }

相关初始化工作完成，activity的生命周期也从onCreate来到了onStart()

FlutterActivity & onStart()

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

delegate.onStart()此方法 最终会调用doInitialFlutterViewRun（）方法：

  private void doInitialFlutterViewRun() {

    ...省略部分代码...

    // 这里就是获取我们打包所得的 libapp.so路径
    // 即，我们所写的dart代码，并执行它
    DartExecutor.DartEntrypoint entrypoint =
        new DartExecutor.DartEntrypoint(
            host.getAppBundlePath(), host.getDartEntrypointFunctionName());
    flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
  }

至此整个android端的启动流程就走完了，这里再回顾总结一下。

总结

在flutterApplication中：

初始化一些资源路径，配置相关参数
抽取资源并加载(assets)
加载flutter.so关键库
    - 最终走JNI_OnLoad 进入native进行相关工作
    - 如绑定flutter jni
初始化vsyncWaiter

在flutterActivity中：

会初始化重要类FlutterActivityAndFragmentDelegate
activity端的生命周期，也会触发delegate来对应回调
对平台的系统功能（震动、剪贴板）进行绑定
初始化platformViewController以及系统channel
创建flutterView（内部持有一个surfaceView）
    - 会最终进入native进行engine的初始化工作
在onStart生命周期中加载咱们的dart代码，开始执行

在这整个过程中，会穿插进行native层的工作，并最终通过native层的调用，转到flutter端的main()函数，由于这里的内容很多，将会在后面的文章中介绍。

最后，谢谢大家的阅读，如果有不对的地方，还请指出。