介绍

Flutter应用是由平台来创建、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);  }

按上面方法的注释来看，是初始化 native system（即C++）的，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（框架）代码    Callable initTask =        new Callable() {          @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 */);  }

这里插一句，可以看一下这篇文章：

Flutter&Android 启动页（闪屏页）的加载流程和优化方案

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FlutterActivityAndFragmentDelegateactivity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。

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介绍