一.Activity中点击返回键

1、Activity响应返回事件，由onBackPressed方法处理

  @Override
  public void onBackPressed() {
    if (stillAttachedForEvent("onBackPressed")) {
        //delegate为FlutterActivityAndFragmentDelegate实例，实际负责与Flutter交互的类
      delegate.onBackPressed();
    }
  }

delegate是FlutterActivityAndFragmentDelegate的实例，代理Android与Flutter交互的所有事件。

2、FlutterActivityAndFragmentDelegate.onBackPressed()

  void onBackPressed() {
    ...
    if (flutterEngine != null) {
      ...
      //getNavigationChannel为NavigationChannel的实例，把事件传递到Flutter端
      flutterEngine.getNavigationChannel().popRoute();
    } else {
     ...
    }
  }

3、NavigationChannel.popRoute()

  public NavigationChannel(@NonNull DartExecutor dartExecutor) {
    this.channel = new MethodChannel(dartExecutor, "flutter/navigation", JSONMethodCodec.INSTANCE);
  }

  public void popRoute() {
    ...
    //将返回事件分发到Flutter
    channel.invokeMethod("popRoute", null);
  }

二.Flutter注册Channel

FlutterApp在启动时会注册与Native交互的Channel

//Flutter App的启动入口
void runApp(Widget app) {
  WidgetsFlutterBinding.ensureInitialized()
    ..scheduleAttachRootWidget(app)
    ..scheduleWarmUpFrame();
}

class WidgetsFlutterBinding extends BindingBase with GestureBinding, SchedulerBinding, ServicesBinding, PaintingBinding, SemanticsBinding, RendererBinding, WidgetsBinding {}

mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureBinding, RendererBinding, SemanticsBinding {
  @override
  void initInstances() {
    super.initInstances();
    ...
    //加载Native NavigationChannel，并处理Navtive传递过来的事件
    SystemChannels.navigation.setMethodCallHandler(_handleNavigationInvocation);
    ...
  }
}  

Flutter返回键处理，这里popRoute对应Native上点击返回按钮

  Future<dynamic> _handleNavigationInvocation(MethodCall methodCall) {
    switch (methodCall.method) {
      case 'popRoute':
        //处理返回事件
        return handlePopRoute();
      case 'pushRoute':
        return handlePushRoute(methodCall.arguments as String);
      case 'pushRouteInformation':
        return _handlePushRouteInformation(methodCall.arguments as Map<dynamic, dynamic>);
    }
    return Future<dynamic>.value();
  }

三.Flutter事件分发

事件优先有Flutter消耗，如果Flutter没有消耗，最终把事件传回Native。

Future<void> handlePopRoute() async {
    //Flutter内部分发返回事件
  for (final WidgetsBindingObserver observer in List<WidgetsBindingObserver>.from(_observers)) {
    if (await observer.didPopRoute())
      return;
  }
  //如果Flutter不处理，交由Native处理
  SystemNavigator.pop();
}

四.Flutter内部事件分发

  for (final WidgetsBindingObserver observer in List<WidgetsBindingObserver>.from(_observers)) {
    if (await observer.didPopRoute())
      return;
  }

1、_observers是一个List<WidgetsBindingObserver>类型的对象

WidgetsBindingObserver代码如下：

abstract class WidgetsBindingObserver {
 
    //在Android中点击返回键时会触发这个函数
    //如果返回true表示消费这个事件
  Future<bool> didPopRoute() => Future<bool>.value(false);
  ...
}

2、WidgetsBindingObserver子类：_WidgetsAppState - WidgetsApp

class WidgetsApp extends StatefulWidget {

    //一个好多参数的构造方法
  WidgetsApp({
  ...,
  backButtonDispatcher = null,
  })
  
  //一个命名构造函数
  WidgetsApp.router({
  ...,
  backButtonDispatcher = backButtonDispatcher ?? RootBackButtonDispatcher(),
  })
  
  State<WidgetsApp> createState() => _WidgetsAppState();
}

class _WidgetsAppState extends State<WidgetsApp> with WidgetsBindingObserver {
  @override
  void initState() {
    ...
    //把自身注册到_observers
    WidgetsBinding.instance!.addObserver(this);
  }  
 
  // On Android: the user has pressed the back button.
  //每个页面的返回事件由这边处理
  @override
  Future<bool> didPopRoute() async {
    // The back button dispatcher should handle the pop route if we use a router.
    //使用MaterialApp.router这个方法才会走到这里
    if (_usesRouter)
      return false;
    //大部分情况下都会走到这~
    final NavigatorState? navigator = _navigator?.currentState;
    if (navigator == null)
      return false;
    return navigator.maybePop();
  }
}

3、WidgetsApp被使用的时机

在使用MaterialApp时，内部会创建WidgetsApp对象来处理返回事件，并注册到_observers

class MaterialApp extends StatefulWidget {

    const MaterialApp({...})
    
    const MaterialApp.router({...})
  ...
  @override
  State<MaterialApp> createState() => _MaterialAppState();  
}

class _MaterialAppState extends State<MaterialApp> {
  ...
  @override
  Widget build(BuildContext context) {
    Widget result = _buildWidgetApp(context);
    ...
    return ScrollConfiguration(
      ...
      child: HeroControllerScope(
       ...
        child: result,
      ),
    );
  }  
  
  Widget _buildWidgetApp(BuildContext context) {
    ...
    if (_usesRouter) {
      return WidgetsApp.router(...);
    }  
    return WidgetsApp(...);
  }  
}

4、NavigatorState.maybePop()处理返回事件，WillPopScope也会在这个方法进行处理

  Future<bool> maybePop<T extends Object?>([ T? result ]) async {
    final _RouteEntry? lastEntry = _history.cast<_RouteEntry?>().lastWhere(
      (_RouteEntry? e) => e != null && _RouteEntry.isPresentPredicate(e),
      orElse: () => null,
    );
    if (lastEntry == null)
      return false;
    //WillPopScope会被注册到这个地方处理
    final RoutePopDisposition disposition = await lastEntry.route.willPop(); // this is asynchronous
    if (!mounted)
      return true; // forget about this pop, we were disposed in the meantime
    final _RouteEntry? newLastEntry = _history.cast<_RouteEntry?>().lastWhere(
      (_RouteEntry? e) => e != null && _RouteEntry.isPresentPredicate(e),
      orElse: () => null,
    );
    if (lastEntry != newLastEntry)
      return true; // forget about this pop, something happened to our history in the meantime
    switch (disposition) {
      case RoutePopDisposition.bubble:
        return false;
      case RoutePopDisposition.pop:
        pop(result);
        return true;
      case RoutePopDisposition.doNotPop:
        return true;
    }
  }

五.事件交由Native处理

  //如果Flutter不处理，交由Native处理
  SystemNavigator.pop();
  
  //把事件分发到Native
  static Future<void> pop({bool? animated}) async {
    await SystemChannels.platform.invokeMethod<void>('SystemNavigator.pop', animated);
  }

Native接收到Flutter发送的SystemNavigator.pop

  //最终由Android-PlatformPlugin处理
  private void popSystemNavigator() {
    //FlutterActivity中platformPluginDelegate=null，所以不会走到此处
    if (platformPluginDelegate != null && platformPluginDelegate.popSystemNavigator()) {
      // A custom behavior was executed by the delegate. Don't execute default behavior.
      return;
    }
    //最后看到的现象就是Activity退出了~
    if (activity instanceof OnBackPressedDispatcherOwner) {
      ((OnBackPressedDispatcherOwner) activity).getOnBackPressedDispatcher().onBackPressed();
    } else {
      activity.finish();
    }
  }