Flutter Android启动源码分析（二）

前言

上篇文章Flutter Android启动源码分析（一）中主要对FlutterApplicaiton和FlutterActivity进行了分析，在FlutterActivity源码部分中FlutterActivityAndFragmentDelegate这个类一直被提到，这篇文章我们就主要分析一下它做了哪些事情

Flutter Android启动源码分析（一）

FlutterActivityAndFragmentDelegate

• 作用
  • FlutterActivity和FlutterFragment之间相同的Flutter逻辑的委托
• 为什么要用这个类
  • 假设可以在Activity中放置一个 Fragment，那么在FlutterActivity中使用一个FlutterFragment更有意义。Fragment支持库为应用程序增加了100k二进制大小，而全Flutter应用程序不需要二进制命中。因此，得出的结论是，Flutter必须基于AOSP(Android Open Source Project: Android 开源项目)Activity 提供FlutterActivity，并为应用程序添加开发人员提供独立的 FlutterFragment
  • 大概意思就是FlutterActivity和FlutterFragment是相互独立的，所以需要一个代理委托类去处理它们之间相同的逻辑

成员变量

  // Host实现类对象即FlutterActivity
  @NonNull private Host host;
  // Flutter执行环境
  @Nullable private FlutterEngine flutterEngine;
  // 闪屏View，直到FlutterView显示第一帧
  @Nullable private FlutterSplashView flutterSplashView;
  // Flutter UI
  @Nullable private FlutterView flutterView;
  // 安卓平台的插件实现
  @Nullable private PlatformPlugin platformPlugin;
  // 是否是Host实现类提供FlutterEngine
  private boolean isFlutterEngineFromHost;

构造函数

 FlutterActivityAndFragmentDelegate(@NonNull Host host) {
    this.host = host;
  }

在FlutterActivity的onCreate方法里里实例化FlutterActivityAndFragmentDelegate并传递当前FA上下文对象，

然后调用onAttach、onActivityCreated、onCreateView，下面我们对这三个方法进行逐一分析

 // FlutterActivity  删除部分源码
 @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    delegate.onActivityCreated(savedInstanceState);
    setContentView(createFlutterView());
  }
  
  @NonNull
  private View createFlutterView() {
    return delegate.onCreateView(
        null /* inflater */, null /* container */, null /* savedInstanceState */);
  }

onAttach

 void onAttach(@NonNull Context context) {
    ensureAlive();
    if (flutterEngine == null) {
    	// 1
      setupFlutterEngine();
    }
    // 2
    platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);
    // 3 
    if (host.shouldAttachEngineToActivity()) {
      flutterEngine
          .getActivityControlSurface()
          .attachToActivity(host.getActivity(), host.getLifecycle());
    }
    // 4 
    host.configureFlutterEngine(flutterEngine);
  }

1. 实例化FlutterEngine

   	void setupFlutterEngine() {
       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;
       }
       flutterEngine = host.provideFlutterEngine(host.getContext());
       if (flutterEngine != null) {
         isFlutterEngineFromHost = true;
         return;
       }
       flutterEngine =
           new FlutterEngine(
               host.getContext(),
               host.getFlutterShellArgs().toArray(),
               /*automaticallyRegisterPlugins=*/ false,
               /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
       isFlutterEngineFromHost = false;
     }
   

   首先会判断是否有缓存的EngineId，如果有，去FlutterEngineCache缓存区去取到并实例化，

   如果没有，会去判断Host实现类（FlutterActivity是否有提供Engine），这个的话目前版本是返回null，

   	// FlutterActivity
   	@Nullable
     @Override
     public FlutterEngine provideFlutterEngine(@NonNull Context context) {
       // No-op. Hook for subclasses.
       return null;
     }
   

   最后就会直接去实例化一个新的Engine，然后设置isFlutterEngineFromHost 为false，说明此Engine非Host实现类提供

   我们大概看下FlutterEngine的构造函数

   public FlutterEngine(
       @NonNull Context context,
       @NonNull FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI,
       @NonNull PlatformViewsController platformViewsController,
       @Nullable String[] dartVmArgs,
       boolean automaticallyRegisterPlugins,
       boolean waitForRestorationData) {
     this.dartExecutor = new DartExecutor(flutterJNI, context.getAssets());
     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;
     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 FlutterEnginePluginRegistry(context.getApplicationContext(), this, flutterLoader);
   
     if (automaticallyRegisterPlugins) {
       registerPlugins();
     }
   }
   

   • dartExecutor实例化
   • 一些系统channel初始化
   • FlutterJNI、PlatformViewsController、FlutterRender、FlutterLoader初始化
   • 实例化FlutterEnginePluginRegistry插件注册表

   FlutterRender 就是与提供的RenderSurface协同工作，将Flutter 像素绘制到Android View层次结构中。FlutterRender管理渲染纹理，并通过JNI将一些Java调用转发到本地 Flutter代码

   FlutterLoader就是在应用程序APK中查找Flutter资源，并加载Flutter的本机库

   这两个大家可以自己去看一下，就不多提及了

2. 实例化PlatformPlugin

    // FlutterActivity
    @Nullable
     @Override
     public PlatformPlugin providePlatformPlugin(
         @Nullable Activity activity, @NonNull FlutterEngine flutterEngine) {
       if (activity != null) {
         return new PlatformPlugin(getActivity(), flutterEngine.getPlatformChannel());
       } else {
         return null;
       }
     }
   

   可以看到FlutterActivity重写这个方法并实例化一个新的PlatformPlugin

   这里有一个注意点：如果FlutterFragment 连接到一个新的Activity上的时候，都要重新去创建一个PlatformPlugin

   PlatformPlugin的话大家可以自己去看，就是平台提供的原生插件，如：状态栏操作、剪贴板等Plugin

3. attachToActivity

    // FlutterActivity
    @Override
     public boolean shouldAttachEngineToActivity() {
       return true;
     }
   

   这里话默认是返回true，建立Activity和Engine的关联关系

   attachToActivity方法的作用就是通知当前连接到FlutterEngine的所有插件，将它们附加到Activity

    //  FlutterEnginePluginRegistry 插件注册表
    @Override
     public void attachToActivity(@NonNull Activity activity, @NonNull Lifecycle lifecycle) {
       detachFromAndroidComponent();
       this.activity = activity;
       this.activityPluginBinding = new FlutterEngineActivityPluginBinding(activity, lifecycle);
       flutterEngine
           .getPlatformViewsController()
           .attach(activity, flutterEngine.getRenderer(), flutterEngine.getDartExecutor());
       for (ActivityAware activityAware : activityAwarePlugins.values()) {
         if (isWaitingForActivityReattachment) {
           activityAware.onReattachedToActivityForConfigChanges(activityPluginBinding);
         } else {
           activityAware.onAttachedToActivity(activityPluginBinding);
         }
       }
       isWaitingForActivityReattachment = false;
     }
   

   FlutterEnginePluginRegistry这个类大家有兴趣可以看下，其实就是一个Engine插件注册表，这里不做过多分析

   它是在FlutterEngine的构造函数里进行实例化的

4. configureFlutterEngine

   // FlutterActivity
   @Override
     public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
       registerPlugins(flutterEngine);
     }
   

   注册pubspec.yaml里的原生插件，所以像上篇文章所说，新版本帮我们进行了自动注册。

onActivityCreated

	void onActivityCreated(@Nullable Bundle bundle) {
    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);
    }
  }

这个我们不做过多分析，就是为插件和Activity提供恢复数据

onCreateView

View onCreateView(
      LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    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());
    flutterView.attachToFlutterEngine(flutterEngine);
    return flutterSplashView;
  }

1. 首先会判断渲染模式

   	// FlutterActivity
   	@NonNull
     @Override
     public RenderMode getRenderMode() {
       return getBackgroundMode() == BackgroundMode.opaque ? RenderMode.surface : RenderMode.texture;
     }
     
      @NonNull
     protected BackgroundMode getBackgroundMode() {
       if (getIntent().hasExtra(EXTRA_BACKGROUND_MODE)) {
         return BackgroundMode.valueOf(getIntent().getStringExtra(EXTRA_BACKGROUND_MODE));
       } else {
         return BackgroundMode.opaque;
       }
     }
   

   框架提供了两种渲染模式： RenderMode.surface ， RenderMode.texture，Flutter框架默认使用Surface进行渲染

2. 然后会实例化一个非透明的FlutterSurfaceView，并设置onFlutterSurfaceViewCreated回调

   	public enum BackgroundMode {
       /** Indicates a FlutterActivity with an opaque background. This is the default. */
       opaque,
       /** Indicates a FlutterActivity with a transparent background. */
       transparent
     }
   

3. 然后接着创建拥有FlutterSurfaceView的FlutterView，上面有提到这个FlutterView就是用来在安卓设备上显示Flutter UI

    flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
   

   TextureView也是一样，不去分析

4. 然后会添加一个绘制第一帧的监听器

    flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener);
   

5. 接着会创建一个FlutterSplashView，然后调用displayFlutterViewWithSplash 方法在FlutterView上显示闪屏界面，直到绘制了第一帧

     flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());
     
     //FlutterActivity
     @Nullable
     @Override
     public SplashScreen provideSplashScreen() {
       Drawable manifestSplashDrawable = getSplashScreenFromManifest();
       if (manifestSplashDrawable != null) {
         return new DrawableSplashScreen(manifestSplashDrawable);
       } else {
         return null;
       }
     }
   
     @Nullable
     @SuppressWarnings("deprecation")
     private Drawable getSplashScreenFromManifest() {
       try {
         ActivityInfo activityInfo =
             getPackageManager().getActivityInfo(getComponentName(), PackageManager.GET_META_DATA);
         Bundle metadata = activityInfo.metaData;
         int splashScreenId = metadata != null ? metadata.getInt(SPLASH_SCREEN_META_DATA_KEY) : 0;
         return splashScreenId != 0
             ? Build.VERSION.SDK_INT > Build.VERSION_CODES.LOLLIPOP
                 ? getResources().getDrawable(splashScreenId, getTheme())
                 : getResources().getDrawable(splashScreenId)
             : null;
       } catch (PackageManager.NameNotFoundException e) {
         // This is never expected to happen.
         return null;
       }
     }
     
     // FlutterActivityLaunchConfigs
   	static final String SPLASH_SCREEN_META_DATA_KEY ="io.flutter.embedding.android.SplashScreenDrawable";
   	
   	// AndroidManifest.xml
   			<meta-data
                   android:name="io.flutter.embedding.android.SplashScreenDrawable"
                   android:resource="@drawable/launch_background" />
   

   可以看到FlutterActivity重写provideSplashScreen，返回我们在AndroidManifest.xml设置的闪屏drawable

6. flutterView.attachToFlutterEngine(flutterEngine);

   然后将此FlutterView连接到给定的 FlutterEngine。此FlutterView将开始呈现给定FlutterEngine绘制的UI。并且这个FlutterView还将开始将交互事件从此FlutterView转发到给定的FlutterEngine例如，用户触摸事件，可访问性事件，键盘事件等

7. 返回flutterSplashView到FlutterActivity。到此刻为止，FlutterActivityAndFragmentDelegate 的onAttach方法已全部分析完毕

8. 同时在添加SplashScreenDrawable之后，也能解决安卓启动黑屏的问题—>从MainActivity的setContentView直到收到Flutter绘制第一帧的回调时，才会关闭这个splashView

接下来我们分析onStart方法

onStart

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

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

可以看到FlutterActivity.onStart调用了FlutterActivityAndFragmentDelegate.onStart方法，然后其调用了doInitialFlutterViewRun方法

这个方法的话就是首次开始在FlutterView中运行Dart

private void doInitialFlutterViewRun() {
		// 1 
    if (host.getCachedEngineId() != null) {
      return;
    }
    if (flutterEngine.getDartExecutor().isExecutingDart()) {
      return;
    }
    
    // 2 
    if (host.getInitialRoute() != null) {
      flutterEngine.getNavigationChannel().setInitialRoute(host.getInitialRoute());
    }
    
    // 3
    DartExecutor.DartEntrypoint entrypoint =
        new DartExecutor.DartEntrypoint(
            host.getAppBundlePath(), host.getDartEntrypointFunctionName());
    flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
  }

1. 首先判断engine缓存id是否为空，具体实现在FlutterActivity

   	// FlutterActivity
   	@Override
     @Nullable
     public String getCachedEngineId() {
       return getIntent().getStringExtra(EXTRA_CACHED_ENGINE_ID);
     }
   

   这里说明一下，Flutter框架是每次都去实例化一个新的FlutterEngine，上面有提到，所以这里恒为null。具体使用场景以后再说

   然后会判断Dart是否执行

2. 这里会设置执行 Dart代码之前接收Flutter应用程序的初始路径，以确保初始路径能够及时应用。

   //FlutterActivity
   @NonNull
     public String getInitialRoute() {
       if (getIntent().hasExtra(EXTRA_INITIAL_ROUTE)) {
         return getIntent().getStringExtra(EXTRA_INITIAL_ROUTE);
       }
   
       try {
         ActivityInfo activityInfo =
             getPackageManager().getActivityInfo(getComponentName(), PackageManager.GET_META_DATA);
         Bundle metadata = activityInfo.metaData;
         String desiredInitialRoute =
             metadata != null ? metadata.getString(INITIAL_ROUTE_META_DATA_KEY) : null;
         return desiredInitialRoute != null ? desiredInitialRoute : DEFAULT_INITIAL_ROUTE;
       } catch (PackageManager.NameNotFoundException e) {
         return DEFAULT_INITIAL_ROUTE;
       }
     }
   

   设置初始化路径可以通过Intent，还可以在Android manifest.定义指定INITIAL_ROUTE_META_DATA_KEY

   这里我们不多聊，这个最后返回的是DEFAULT_INITIAL_ROUTE，默认路径

   //FlutterActivityLaunchConfigs
   static final String DEFAULT_INITIAL_ROUTE = "/";
   

3. 然后配置Dart代码执行入口点并且去执行它

   host.getAppBundlePath返回的是默认路径

   private static final String DEFAULT_FLUTTER_ASSETS_DIR = "flutter_assets";
   

   host.getDartEntrypointFunctionName()返回的是也是默认启动方法名，

   static final String DEFAULT_DART_ENTRYPOINT = "main";
   

   就是我们main.dart里的这个main方法

   import 'package:flutter/material.dart';
   
   void main() {
     runApp(MyApp());
   }
   

   //DartExecutor 配置，引导并开始执行Dart代码。
   
   public void executeDartEntrypoint(@NonNull DartEntrypoint dartEntrypoint) {
       if (isApplicationRunning) {
         Log.w(TAG, "Attempted to run a DartExecutor that is already running.");
         return;
       }
   
       Log.v(TAG, "Executing Dart entrypoint: " + dartEntrypoint);
   
       flutterJNI.runBundleAndSnapshotFromLibrary(
           dartEntrypoint.pathToBundle, dartEntrypoint.dartEntrypointFunctionName, null, assetManager);
   
       isApplicationRunning = true;
     }
   

   可以看到这里设置 isApplicationRunning = true，代表程序正在运行，

   //DartExecutor 
   public boolean isExecutingDart() {
       return isApplicationRunning;
     }
   

   判断是否正在执行dart代码也是通过此变量

4. FlutterJNI类是Flutter嵌入的Java代码和Flutter引擎的C / C ++代码之间的接口。这块大家自己去看吧

5. 到此刻，一个flutter程序就正常运行起来了，接着就是执行dart代码和我们自己的业务逻辑

接下来，我们看下生命周期Channel LifecycleChannel

 void onResume() {
    Log.v(TAG, "onResume()");
    ensureAlive();
    flutterEngine.getLifecycleChannel().appIsResumed();
  }

它主要是将Android生命周期事件发送到Flutter，onPause、onStop、onDetach也做了同样处理

FlutterActivity onDestroy

  @Override
  protected void onDestroy() {
    super.onDestroy();
    delegate.onDestroyView();
    delegate.onDetach();
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
  }

这里主要是做一些清除引用，销毁资源的一些操作。大家可以自行阅读源码

总结

通过这篇文章我们可以大概的看到

• FlutterActivityAndFragmentDelegate是如何去代理FlutterActivity的工作
• FlutterActivityAndFragmentDelegate是如何将FlutterEngine和FlutterActivity连接在一起，并执行dart代码，启动应用程序
• FlutterRender、FlutterJNI、FlutterLoader大家可以自己去结合源码看一下，基本的启动流程就是这样

下篇文章我会通过一个Android 项目嵌入Flutter Module的混合工程来让大家更直观的看到Flutter在Android上到底是如何去展示的

前言

上篇文章Flutter Android启动源码分析（一）中主要对FlutterApplicaiton和FlutterActivity进行了分析，在FlutterActivity源码部分中FlutterActivityAndFragmentDelegate这个类一直被提到，这篇文章我们就主要分析一下它做了哪些事情

Flutter Android启动源码分析（一）

FlutterActivityAndFragmentDelegate

• 作用
  • FlutterActivity和FlutterFragment之间相同的Flutter逻辑的委托
• 为什么要用这个类
  • 假设可以在Activity中放置一个 Fragment，那么在FlutterActivity中使用一个FlutterFragment更有意义。Fragment支持库为应用程序增加了100k二进制大小，而全Flutter应用程序不需要二进制命中。因此，得出的结论是，Flutter必须基于AOSP(Android Open Source Project: Android 开源项目)Activity 提供FlutterActivity，并为应用程序添加开发人员提供独立的 FlutterFragment
  • 大概意思就是FlutterActivity和FlutterFragment是相互独立的，所以需要一个代理委托类去处理它们之间相同的逻辑

成员变量

  // Host实现类对象即FlutterActivity
  @NonNull private Host host;
  // Flutter执行环境
  @Nullable private FlutterEngine flutterEngine;
  // 闪屏View，直到FlutterView显示第一帧
  @Nullable private FlutterSplashView flutterSplashView;
  // Flutter UI
  @Nullable private FlutterView flutterView;
  // 安卓平台的插件实现
  @Nullable private PlatformPlugin platformPlugin;
  // 是否是Host实现类提供FlutterEngine
  private boolean isFlutterEngineFromHost;

构造函数

 FlutterActivityAndFragmentDelegate(@NonNull Host host) {
    this.host = host;
  }

在FlutterActivity的onCreate方法里里实例化FlutterActivityAndFragmentDelegate并传递当前FA上下文对象，

然后调用onAttach、onActivityCreated、onCreateView，下面我们对这三个方法进行逐一分析

 // FlutterActivity  删除部分源码
 @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    delegate = new FlutterActivityAndFragmentDelegate(this);
    delegate.onAttach(this);
    delegate.onActivityCreated(savedInstanceState);
    setContentView(createFlutterView());
  }
  
  @NonNull
  private View createFlutterView() {
    return delegate.onCreateView(
        null /* inflater */, null /* container */, null /* savedInstanceState */);
  }

onAttach

 void onAttach(@NonNull Context context) {
    ensureAlive();
    if (flutterEngine == null) {
    	// 1
      setupFlutterEngine();
    }
    // 2
    platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);
    // 3 
    if (host.shouldAttachEngineToActivity()) {
      flutterEngine
          .getActivityControlSurface()
          .attachToActivity(host.getActivity(), host.getLifecycle());
    }
    // 4 
    host.configureFlutterEngine(flutterEngine);
  }

1. 实例化FlutterEngine

   	void setupFlutterEngine() {
       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;
       }
       flutterEngine = host.provideFlutterEngine(host.getContext());
       if (flutterEngine != null) {
         isFlutterEngineFromHost = true;
         return;
       }
       flutterEngine =
           new FlutterEngine(
               host.getContext(),
               host.getFlutterShellArgs().toArray(),
               /*automaticallyRegisterPlugins=*/ false,
               /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());
       isFlutterEngineFromHost = false;
     }
   

   首先会判断是否有缓存的EngineId，如果有，去FlutterEngineCache缓存区去取到并实例化，

   如果没有，会去判断Host实现类（FlutterActivity是否有提供Engine），这个的话目前版本是返回null，

   	// FlutterActivity
   	@Nullable
     @Override
     public FlutterEngine provideFlutterEngine(@NonNull Context context) {
       // No-op. Hook for subclasses.
       return null;
     }
   

   最后就会直接去实例化一个新的Engine，然后设置isFlutterEngineFromHost 为false，说明此Engine非Host实现类提供

   我们大概看下FlutterEngine的构造函数

   public FlutterEngine(
       @NonNull Context context,
       @NonNull FlutterLoader flutterLoader,
       @NonNull FlutterJNI flutterJNI,
       @NonNull PlatformViewsController platformViewsController,
       @Nullable String[] dartVmArgs,
       boolean automaticallyRegisterPlugins,
       boolean waitForRestorationData) {
     this.dartExecutor = new DartExecutor(flutterJNI, context.getAssets());
     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;
     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 FlutterEnginePluginRegistry(context.getApplicationContext(), this, flutterLoader);
   
     if (automaticallyRegisterPlugins) {
       registerPlugins();
     }
   }
   

   • dartExecutor实例化
   • 一些系统channel初始化
   • FlutterJNI、PlatformViewsController、FlutterRender、FlutterLoader初始化
   • 实例化FlutterEnginePluginRegistry插件注册表

   FlutterRender 就是与提供的RenderSurface协同工作，将Flutter 像素绘制到Android View层次结构中。FlutterRender管理渲染纹理，并通过JNI将一些Java调用转发到本地 Flutter代码

   FlutterLoader就是在应用程序APK中查找Flutter资源，并加载Flutter的本机库

   这两个大家可以自己去看一下，就不多提及了

2. 实例化PlatformPlugin

    // FlutterActivity
    @Nullable
     @Override
     public PlatformPlugin providePlatformPlugin(
         @Nullable Activity activity, @NonNull FlutterEngine flutterEngine) {
       if (activity != null) {
         return new PlatformPlugin(getActivity(), flutterEngine.getPlatformChannel());
       } else {
         return null;
       }
     }
   

   可以看到FlutterActivity重写这个方法并实例化一个新的PlatformPlugin

   这里有一个注意点：如果FlutterFragment 连接到一个新的Activity上的时候，都要重新去创建一个PlatformPlugin

   PlatformPlugin的话大家可以自己去看，就是平台提供的原生插件，如：状态栏操作、剪贴板等Plugin

3. attachToActivity

    // FlutterActivity
    @Override
     public boolean shouldAttachEngineToActivity() {
       return true;
     }
   

   这里话默认是返回true，建立Activity和Engine的关联关系

   attachToActivity方法的作用就是通知当前连接到FlutterEngine的所有插件，将它们附加到Activity

    //  FlutterEnginePluginRegistry 插件注册表
    @Override
     public void attachToActivity(@NonNull Activity activity, @NonNull Lifecycle lifecycle) {
       detachFromAndroidComponent();
       this.activity = activity;
       this.activityPluginBinding = new FlutterEngineActivityPluginBinding(activity, lifecycle);
       flutterEngine
           .getPlatformViewsController()
           .attach(activity, flutterEngine.getRenderer(), flutterEngine.getDartExecutor());
       for (ActivityAware activityAware : activityAwarePlugins.values()) {
         if (isWaitingForActivityReattachment) {
           activityAware.onReattachedToActivityForConfigChanges(activityPluginBinding);
         } else {
           activityAware.onAttachedToActivity(activityPluginBinding);
         }
       }
       isWaitingForActivityReattachment = false;
     }
   

   FlutterEnginePluginRegistry这个类大家有兴趣可以看下，其实就是一个Engine插件注册表，这里不做过多分析

   它是在FlutterEngine的构造函数里进行实例化的

4. configureFlutterEngine

   // FlutterActivity
   @Override
     public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
       registerPlugins(flutterEngine);
     }
   

   注册pubspec.yaml里的原生插件，所以像上篇文章所说，新版本帮我们进行了自动注册。

onActivityCreated

	void onActivityCreated(@Nullable Bundle bundle) {
    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);
    }
  }

这个我们不做过多分析，就是为插件和Activity提供恢复数据

onCreateView

View onCreateView(
      LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
    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());
    flutterView.attachToFlutterEngine(flutterEngine);
    return flutterSplashView;
  }

1. 首先会判断渲染模式

   	// FlutterActivity
   	@NonNull
     @Override
     public RenderMode getRenderMode() {
       return getBackgroundMode() == BackgroundMode.opaque ? RenderMode.surface : RenderMode.texture;
     }
     
      @NonNull
     protected BackgroundMode getBackgroundMode() {
       if (getIntent().hasExtra(EXTRA_BACKGROUND_MODE)) {
         return BackgroundMode.valueOf(getIntent().getStringExtra(EXTRA_BACKGROUND_MODE));
       } else {
         return BackgroundMode.opaque;
       }
     }
   

   框架提供了两种渲染模式： RenderMode.surface ， RenderMode.texture，Flutter框架默认使用Surface进行渲染

2. 然后会实例化一个非透明的FlutterSurfaceView，并设置onFlutterSurfaceViewCreated回调

   	public enum BackgroundMode {
       /** Indicates a FlutterActivity with an opaque background. This is the default. */
       opaque,
       /** Indicates a FlutterActivity with a transparent background. */
       transparent
     }
   

3. 然后接着创建拥有FlutterSurfaceView的FlutterView，上面有提到这个FlutterView就是用来在安卓设备上显示Flutter UI

    flutterView = new FlutterView(host.getActivity(), flutterSurfaceView);
   

   TextureView也是一样，不去分析

4. 然后会添加一个绘制第一帧的监听器

    flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener);
   

5. 接着会创建一个FlutterSplashView，然后调用displayFlutterViewWithSplash 方法在FlutterView上显示闪屏界面，直到绘制了第一帧

     flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());
     
     //FlutterActivity
     @Nullable
     @Override
     public SplashScreen provideSplashScreen() {
       Drawable manifestSplashDrawable = getSplashScreenFromManifest();
       if (manifestSplashDrawable != null) {
         return new DrawableSplashScreen(manifestSplashDrawable);
       } else {
         return null;
       }
     }
   
     @Nullable
     @SuppressWarnings("deprecation")
     private Drawable getSplashScreenFromManifest() {
       try {
         ActivityInfo activityInfo =
             getPackageManager().getActivityInfo(getComponentName(), PackageManager.GET_META_DATA);
         Bundle metadata = activityInfo.metaData;
         int splashScreenId = metadata != null ? metadata.getInt(SPLASH_SCREEN_META_DATA_KEY) : 0;
         return splashScreenId != 0
             ? Build.VERSION.SDK_INT > Build.VERSION_CODES.LOLLIPOP
                 ? getResources().getDrawable(splashScreenId, getTheme())
                 : getResources().getDrawable(splashScreenId)
             : null;
       } catch (PackageManager.NameNotFoundException e) {
         // This is never expected to happen.
         return null;
       }
     }
     
     // FlutterActivityLaunchConfigs
   	static final String SPLASH_SCREEN_META_DATA_KEY ="io.flutter.embedding.android.SplashScreenDrawable";
   	
   	// AndroidManifest.xml
   			<meta-data
                   android:name="io.flutter.embedding.android.SplashScreenDrawable"
                   android:resource="@drawable/launch_background" />
   

   可以看到FlutterActivity重写provideSplashScreen，返回我们在AndroidManifest.xml设置的闪屏drawable

6. flutterView.attachToFlutterEngine(flutterEngine);

   然后将此FlutterView连接到给定的 FlutterEngine。此FlutterView将开始呈现给定FlutterEngine绘制的UI。并且这个FlutterView还将开始将交互事件从此FlutterView转发到给定的FlutterEngine例如，用户触摸事件，可访问性事件，键盘事件等

7. 返回flutterSplashView到FlutterActivity。到此刻为止，FlutterActivityAndFragmentDelegate 的onAttach方法已全部分析完毕

接下来我们分析onStart方法

onStart

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

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

可以看到FlutterActivity.onStart调用了FlutterActivityAndFragmentDelegate.onStart方法，然后其调用了doInitialFlutterViewRun方法

这个方法的话就是首次开始在FlutterView中运行Dart

private void doInitialFlutterViewRun() {
		// 1 
    if (host.getCachedEngineId() != null) {
      return;
    }
    if (flutterEngine.getDartExecutor().isExecutingDart()) {
      return;
    }
    
    // 2 
    if (host.getInitialRoute() != null) {
      flutterEngine.getNavigationChannel().setInitialRoute(host.getInitialRoute());
    }
    
    // 3
    DartExecutor.DartEntrypoint entrypoint =
        new DartExecutor.DartEntrypoint(
            host.getAppBundlePath(), host.getDartEntrypointFunctionName());
    flutterEngine.getDartExecutor().executeDartEntrypoint(entrypoint);
  }

1. 首先判断engine缓存id是否为空，具体实现在FlutterActivity

   	// FlutterActivity
   	@Override
     @Nullable
     public String getCachedEngineId() {
       return getIntent().getStringExtra(EXTRA_CACHED_ENGINE_ID);
     }
   

   这里说明一下，Flutter框架是每次都去实例化一个新的FlutterEngine，上面有提到，所以这里恒为null。具体使用场景以后再说

   然后会判断Dart是否执行

2. 这里会设置执行 Dart代码之前接收Flutter应用程序的初始路径，以确保初始路径能够及时应用。

   //FlutterActivity
   @NonNull
     public String getInitialRoute() {
       if (getIntent().hasExtra(EXTRA_INITIAL_ROUTE)) {
         return getIntent().getStringExtra(EXTRA_INITIAL_ROUTE);
       }
   
       try {
         ActivityInfo activityInfo =
             getPackageManager().getActivityInfo(getComponentName(), PackageManager.GET_META_DATA);
         Bundle metadata = activityInfo.metaData;
         String desiredInitialRoute =
             metadata != null ? metadata.getString(INITIAL_ROUTE_META_DATA_KEY) : null;
         return desiredInitialRoute != null ? desiredInitialRoute : DEFAULT_INITIAL_ROUTE;
       } catch (PackageManager.NameNotFoundException e) {
         return DEFAULT_INITIAL_ROUTE;
       }
     }
   

   设置初始化路径可以通过Intent，还可以在Android manifest.定义指定INITIAL_ROUTE_META_DATA_KEY

   这里我们不多聊，这个最后返回的是DEFAULT_INITIAL_ROUTE，默认路径

   //FlutterActivityLaunchConfigs
   static final String DEFAULT_INITIAL_ROUTE = "/";
   

3. 然后配置Dart代码执行入口点并且去执行它

   host.getAppBundlePath返回的是默认路径

   private static final String DEFAULT_FLUTTER_ASSETS_DIR = "flutter_assets";
   

   host.getDartEntrypointFunctionName()返回的是也是默认启动方法名，

   static final String DEFAULT_DART_ENTRYPOINT = "main";
   

   就是我们main.dart里的这个main方法

   import 'package:flutter/material.dart';
   
   void main() {
     runApp(MyApp());
   }
   

   //DartExecutor 配置，引导并开始执行Dart代码。
   
   public void executeDartEntrypoint(@NonNull DartEntrypoint dartEntrypoint) {
       if (isApplicationRunning) {
         Log.w(TAG, "Attempted to run a DartExecutor that is already running.");
         return;
       }
   
       Log.v(TAG, "Executing Dart entrypoint: " + dartEntrypoint);
   
       flutterJNI.runBundleAndSnapshotFromLibrary(
           dartEntrypoint.pathToBundle, dartEntrypoint.dartEntrypointFunctionName, null, assetManager);
   
       isApplicationRunning = true;
     }
   

   可以看到这里设置 isApplicationRunning = true，代表程序正在运行，

   //DartExecutor 
   public boolean isExecutingDart() {
       return isApplicationRunning;
     }
   

   判断是否正在执行dart代码也是通过此变量

4. FlutterJNI类是Flutter嵌入的Java代码和Flutter引擎的C / C ++代码之间的接口。这块大家自己去看吧

5. 到此刻，一个flutter程序就正常运行起来了，接着就是执行dart代码和我们自己的业务逻辑

接下来，我们看下生命周期Channel LifecycleChannel

 void onResume() {
    Log.v(TAG, "onResume()");
    ensureAlive();
    flutterEngine.getLifecycleChannel().appIsResumed();
  }

它主要是将Android生命周期事件发送到Flutter，onPause、onStop、onDetach也做了同样处理

FlutterActivity onDestroy

  @Override
  protected void onDestroy() {
    super.onDestroy();
    delegate.onDestroyView();
    delegate.onDetach();
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_DESTROY);
  }

这里主要是做一些清除引用，销毁资源的一些操作。大家可以自行阅读源码

总结

通过这篇文章我们可以大概的看到

• FlutterActivityAndFragmentDelegate是如何去代理FlutterActivity的工作
• FlutterActivityAndFragmentDelegate是如何将FlutterEngine和FlutterActivity连接在一起，并执行dart代码，启动应用程序
• FlutterRender、FlutterJNI、FlutterLoader大家可以自己去结合源码看一下，基本的启动流程就是这样
• 在Android上，Flutter默认是作为一个Activity加载到嵌入器中。视图由FlutterView控制，它根据Flutter内容的构成和z-排序要求，将Flutter内容渲染为视图或纹理。
• Flutter架构综述

下篇文章我会通过一个Android 项目嵌入Flutter Module的混合工程来让大家更直观的看到Flutter在Android上到底是如何去展示的

本人技术水平有限，太深入的分析就不去涉及了。如有错误，请及时指正。当然也欢迎大家能给我一些指导性意见，我们一起进步
本人技术水平有限，太深入的分析就不去涉及了。如有错误，请及时指正。当然也欢迎大家能给我一些指导性意见，我们一起进步

本文地址：https://blog.csdn.net/fighting_android/article/details/108723708