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 _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);
}
return Future.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 handlePopRoute() async {
//Flutter内部分发返回事件
for (final WidgetsBindingObserver observer in List.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.from(_observers)) {
if (await observer.didPopRoute())
return;
}
1、_observers是一个List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 didPopRoute() => Future.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 createState() => _WidgetsAppState();
}
class _WidgetsAppState extends State 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 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 createState() => _MaterialAppState();
}
class _MaterialAppState extends State {
...
@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 maybePop([ 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 pop({bool? animated}) async {
await SystemChannels.platform.invokeMethod('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();
}
}