Flutter-Android闪屏页(启动页)实现，以及启动流程分析，看这篇文章就够了

文章目录

• 环境
• 启动页配置
• 启动流程
• 源码分析
• • FlutterActivity
  • FlutterActivityAndFragmentDelegate
  • • delegate.onAttach(this);
  • 启动页核心setContentView(createFlutterView());
  • flutterSplashView.displayFlutterViewWithSplash

环境

• Flutter 1.22.4
• Framework • revision 1aafb3a8b9 (4 weeks ago) • 2020-11-13 09:59:28 -0800
• Engine • revision 2c956a31c0
• Dart 2.10.4

启动页配置

• 向 Android 应用中添加闪屏页和启动页

启动流程

分为两个部分，Android应用启动和Flutter启动，FlutterView显示出来才表示Flutter加载完成，从MainActivity到FlutterView显示有一个时间间隔，会出现白屏现象，Flutter框架中为了消除白屏，在创建FlutterView的同时创建了FlutterSplashView，当FlutterView第一帧加载完成，则通知将FlutterSplashView移除。

源码分析

FlutterActivity

public class MainActivity extends FlutterActivity {
	@Override
    public void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }

    @Override
    public SplashScreen provideSplashScreen() {
        return new SimpleSplashScreen();
    }
}

首先看入口，MainActivity就只是继承了FlutterActivity，没有实现其他方法，看onCreate()方法。

@Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
  	// 切换背景为普通主题
    switchLaunchThemeForNormalTheme();

    super.onCreate(savedInstanceState);
	// 发送onCreate的通知
    lifecycle.handleLifecycleEvent(Lifecycle.Event.ON_CREATE);

    delegate = new FlutterActivityAndFragmentDelegate(this);
    // 非常重要的方法
    delegate.onAttach(this);
    delegate.onActivityCreated(savedInstanceState);

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

switchLaunchThemeForNormalTheme

在Flutter构建的项目中src/main/res/values/styles.xml，包含LaunchTheme和NormalTheme主题，LaunchTheme作为Android启动时配置背景使用，NormalTheme则为Android应用启动后，Activity的默认主题。

FlutterActivityAndFragmentDelegate

比较核心的类，实际执行操作的代理类。使用一个代理类能保证FlutterActivity和FlutterFragment实现相同的逻辑。内部接口Host继承

• SplashScreenProvider提供首屏页。
• FlutterEngineProvider提供一个自定义的FlutterEngine, 默认为Null，使用系统自己的FlutterEngine。
• FlutterEngineConfigurator提供一个配置给FlutterEngine，可以在这里初始化插件配置

interface Host
      extends SplashScreenProvider, FlutterEngineProvider, FlutterEngineConfigurator {
      }
      
public interface SplashScreenProvider {
  @Nullable
  SplashScreen provideSplashScreen();
}

public interface FlutterEngineProvider {
  @Nullable
  FlutterEngine provideFlutterEngine(@NonNull Context context);
}

public interface FlutterEngineConfigurator {
  void configureFlutterEngine(@NonNull FlutterEngine flutterEngine);
  void cleanUpFlutterEngine(@NonNull FlutterEngine flutterEngine);
}

delegate.onAttach(this);

这是个初始化Flutter非常核心的方法。主要有以下几个功能

1. 初始化Flutter系统。
2. 获取或创建FlutterEngine。
3. 创建并配置插件PlatformPlugin。
4. 如果有需要，把FlutterEngine与FlutterActivity绑定。
5. 通过configureFlutterEngine来配置FlutterEngine。

void onAttach(@NonNull Context context) {
	// 确认是否实例还在
    ensureAlive();
    if (flutterEngine == null) {
      // 第一步
      setupFlutterEngine();
    }
    // 第二步
    platformPlugin = host.providePlatformPlugin(host.getActivity(), flutterEngine);
	// 第三步
    if (host.shouldAttachEngineToActivity()) {
      flutterEngine
          .getActivityControlSurface()
          .attachToActivity(host.getActivity(), host.getLifecycle());
    }
	// 第四步
    host.configureFlutterEngine(flutterEngine);
}

第一步 初始化FlutterEngine

void setupFlutterEngine() {

    // 首先
    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;
}

1. 首先，从缓存里面读cachedEngineId，如果存在则通过id获取FlutterEngine。
2. 如果没有cachedEngineId，则通过前面说的接口host.provideFlutterEngine获取自定义的FlutterEngine，但默认返回null。
3. 所以最后通过，Flutter框架自己提供的FlutterEngine类初始化。

FlutterEngine构造函数简略介绍

flutterEngine =
        new FlutterEngine(
            host.getContext(),
            host.getFlutterShellArgs().toArray(),
            /*automaticallyRegisterPlugins=*/ false, 
            /*willProvideRestorationData=*/ host.shouldRestoreAndSaveState());

// ...省略
// 以下是注册插件，
this.pluginRegistry =
        new FlutterEnginePluginRegistry(context.getApplicationContext(), this, flutterLoader);

	// 由于automaticallyRegisterPlugins为false，这里其实并没有关联插件
    if (automaticallyRegisterPlugins) {
      registerPlugins();
    }

1. dartExecutor实例化，开启Dart 中的Isolate线程，并开始事件循环。
2. 创建FlutterJNI用于与C++层进行交。
3. 绑定系统默认的通道。
4. 自动装载插件，调用registerPlugins()方法，绑定第三方插件，将第三方插件存入FlutterEnginePluginRegistry，由FlutterEnginePluginRegistry管理插件，而FlutterEnginePluginRegistry由FlutterEngine管理 。

第二步 初始化PlatformPlugin系统平台插件

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

public PlatformPlugin(Activity activity, PlatformChannel platformChannel) {
    this.activity = activity;
    this.platformChannel = platformChannel;
    this.platformChannel.setPlatformMessageHandler(mPlatformMessageHandler);

    mEnabledOverlays = DEFAULT_SYSTEM_UI;
  }

提供了原生的plugin方法，通过PlatformChannel调用，如：更改状态栏、剪贴板等。

第三步 将插件与activity绑定

if (host.shouldAttachEngineToActivity()) {

      flutterEngine
          .getActivityControlSurface()
          .attachToActivity(host.getActivity(), host.getLifecycle());
}

1. host.shouldAttachEngineToActivity()，默认返回的true，建立Activity和Engine的关联关系。
2. 实例化PlatformViewsChannel，以便能使用WebView、视频播放器等原生就实现的组件。
3. 最终调用的FlutterEnginePluginRegistry.attachToActivity方法，与Activity关联。

for (ActivityAware activityAware : activityAwarePlugins.values()) {
      if (isWaitingForActivityReattachment) {
        activityAware.onReattachedToActivityForConfigChanges(activityPluginBinding);
      } else {
        activityAware.onAttachedToActivity(activityPluginBinding);
      }
    }

如果开启了自动装配automaticallyRegisterPlugins = true，则第一步中FlutterEnginePluginRegistry.registerPlugins()注册的第三方插件，保存在了activityAwarePlugins中。FlutterEnginePluginRegistry.attachToActivity中将activityAwarePlugins方法遍历出来与Activity关联。
但其实Flutter默认实现是把automaticallyRegisterPlugins设置为false，所以这里并没有绑定操作。

第四步 注册第三方插件

@Override
  public void configureFlutterEngine(@NonNull FlutterEngine flutterEngine) {
    GeneratedPluginRegister.registerGeneratedPlugins(flutterEngine);
  }

注册pubspec.yaml里的原生插件，会在这里调用。
以下是我引入了网络环境判断的插件

public final class GeneratedPluginRegistrant {
  public static void registerWith(@NonNull FlutterEngine flutterEngine) {
    flutterEngine.getPlugins().add(new io.flutter.plugins.connectivity.ConnectivityPlugin());
  }
}

最终会调用FlutterEngin -> FlutterEnginePluginRegistry.add方法，并且将这个Flugin与Activity关联起来。

@Override
  public void add(@NonNull FlutterPlugin plugin) {
     
    plugins.put(plugin.getClass(), plugin);
    plugin.onAttachedToEngine(pluginBinding);
    if (plugin instanceof ActivityAware) {
      ActivityAware activityAware = (ActivityAware) plugin;
      activityAwarePlugins.put(plugin.getClass(), activityAware);

      if (isAttachedToActivity()) {
        activityAware.onAttachedToActivity(activityPluginBinding);
      }
    }

}

启动页核心setContentView(createFlutterView());

@NonNull
  View onCreateView(
      LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {

    // 第一步
    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);
    }

    // 第二步
    flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener);

    // 第三步
    flutterSplashView = new FlutterSplashView(host.getContext());
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
      flutterSplashView.setId(View.generateViewId());
      flutterSplashView.setId(486947586);
    }
    flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());

    // 第四步
    Log.v(TAG, "Attaching FlutterEngine to FlutterView.");
    flutterView.attachToFlutterEngine(flutterEngine);

    // 第五步
    return flutterSplashView;
}

第一步，根据渲染模式加载不同FlutterView

Flutter提供了两种渲染模式， RenderMode.surface ， RenderMode.texture，默认使用了Surface方式。

Android中TextureView与SurfaceView的区别。

第二步，FlutterView增加第一帧显示回调

当Flutter引擎、绘制工具等准备完成，并完成了FlutterView的第一帧绘制则会回调，这个回调后面就有很大用处。

第三步，初始化FlutterSplashView

这里开始就是启动页的初始化。以下是启动页的核心，后面一起讲。
由于flutterView是作为flutterSplashView的一个参数，所以flutterSplashView是持有了flutterView。

flutterSplashView.displayFlutterViewWithSplash(flutterView, host.provideSplashScreen());

第四步，FlutterView与FlutterEngine绑定在一起

此时，Flutter的框架Embedder和Engine层就关联在一起，FlutterEngine将绘制的ui反映到FlutterView上。

第五步，返回一个flutterSplashView

将flutterSplashView返回给setContentView，作为显示内容，也就是屏幕上看到的界面。

flutterSplashView.displayFlutterViewWithSplash

final class FlutterSplashView extends FrameLayout {
}

首先可以看到，FlutterSplashView就是一个FrameLayout。

public void displayFlutterViewWithSplash(
      @NonNull FlutterView flutterView, @Nullable SplashScreen splashScreen) {
    
    // 第一步
    this.flutterView = flutterView;
    addView(flutterView);

    this.splashScreen = splashScreen;

    // 第二步
    if (splashScreen != null) {
      if (isSplashScreenNeededNow()) {
        Log.v(TAG, "Showing splash screen UI.");
        
        // 第三步
        splashScreenView = splashScreen.createSplashView(getContext(), splashScreenState);
        addView(this.splashScreenView);
        // 第四步
        flutterView.addOnFirstFrameRenderedListener(flutterUiDisplayListener);
      } 
    }
  }
splashScreen的参数，是从host.provideSplashScreen()得到的，而host其实就是FlutterActivity实现的一个接口，最终是由FlutterActivity中实现了provideSplashScreen。

第一步，添加FlutterView

前面说了，FlutterSplashView是一个FrameLayout。首先在FlutterSplashView中保存了FlutterView的引用，然后将FlutterView添加到FrameLayout中。

第二步，判断splashScreen是否为空

判断splashScreen是否为空，默认Flutter框架提供了实现View，默认是一个白屏页。

第三步，实现splashScreenView

通过splashScreen.createSplashView得到splashScreenView，splashScreen的参数，是从host.provideSplashScreen()得到的，而host其实就是FlutterActivity实现的一个接口，最终是由FlutterActivity中实现了provideSplashScreen。

第四步，flutterView监听第一帧绘制完成回调

回调函数，onTransitionComplete其实是一个Runnable方法，内部调用了removeView(splashScreenView)，将splashScreenView从上层移除，这样FlutterView就显示在出来了。

移除splashScreenView的方法也有了，只要在splashScreen.transitionToFlutter的实现方法中调用onTransitionComplete.run()就能移除splashScreenView。

private void transitionToFlutter() {
    splashScreen.transitionToFlutter(onTransitionComplete);
  }

 private final Runnable onTransitionComplete =
      new Runnable() {
        @Override
        public void run() {
          removeView(splashScreenView);
          previousCompletedSplashIsolate = transitioningIsolateId;
        }
      };

本文地址：https://blog.csdn.net/ZZB_Bin/article/details/110930371