Question

在Flutter中，有没有办法在用户离开应用程序并立即返回时更新小部件？我的应用程序是基于时间的，尽快更新时间会很有帮助。

Answer 1

您可以收听生命周期事件

class LifecycleEventHandler extends WidgetsBindingObserver {
  LifecycleEventHandler({this.resumeCallBack, this.suspendingCallBack});

  final FutureVoidCallback resumeCallBack;
  final FutureVoidCallback suspendingCallBack;

  @override
  Future<Null> didChangeAppLifecycleState(AppLifecycleState state) async {
    switch (state) {
      case AppLifecycleState.inactive:
      case AppLifecycleState.paused:
      case AppLifecycleState.suspending:
        await suspendingCallBack();
        break;
      case AppLifecycleState.resumed:
        await resumeCallBack();
        break;
    }
  }
}


class AppWidgetState extends State<AppWidget> {
  void initState() {
    super.initState();

    WidgetsBinding.instance.addObserver(
        new LifecycleEventHandler(resumeCallback(() => setState((){}))
    );
  }
  ...
}

Answer 2

使用系统频道：

import 'package:flutter/services.dart';

SystemChannels.lifecycle.setMessageHandler((msg){
  debugPrint('SystemChannels> $msg');
  if(msg==AppLifecycleState.resumed.toString())setState((){});
});

`

Answer 3

import 'package:flutter/material.dart';

abstract class LifecycleWatcherState<T extends StatefulWidget> extends State<T>
    with WidgetsBindingObserver {
  @override
  Widget build(BuildContext context) {
    return null;
  }

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    switch (state) {
      case AppLifecycleState.resumed:
        onResumed();
        break;
      case AppLifecycleState.inactive:
        onPaused();
        break;
      case AppLifecycleState.paused:
        onInactive();
        break;
      case AppLifecycleState.detached:
        onDetached();
        break;
    }
  }

  void onResumed();
  void onPaused();
  void onInactive();
  void onDetached();
}

示例

class ExampleStatefulWidget extends StatefulWidget {
  @override
  _ExampleStatefulWidgetState createState() => _ExampleStatefulWidgetState();
}

class _ExampleStatefulWidgetState
    extends LifecycleWatcherState<ExampleStatefulWidget> {
  @override
  Widget build(BuildContext context) {
    return Container();
  }

  @override
  void onDetached() {

  }

  @override
  void onInactive() {

  }

  @override
  void onPaused() {

  }

  @override
  void onResumed() {

  }
}

Answer 4

简单方法：

import 'package:flutter/services.dart';

handleAppLifecycleState() {
    AppLifecycleState _lastLifecyleState;
    SystemChannels.lifecycle.setMessageHandler((msg) {

     print('SystemChannels> $msg');

        switch (msg) {
          case "AppLifecycleState.paused":
            _lastLifecyleState = AppLifecycleState.paused;
            break;
          case "AppLifecycleState.inactive":
            _lastLifecyleState = AppLifecycleState.inactive;
            break;
          case "AppLifecycleState.resumed":
            _lastLifecyleState = AppLifecycleState.resumed;
            break;
          case "AppLifecycleState.suspending":
            _lastLifecyleState = AppLifecycleState.suspending;
            break;
          default:
        }
    });
  }

只需在您的handleAppLifecycleState()中添加init()

OR

class AppLifecycleReactor extends StatefulWidget {
      const AppLifecycleReactor({ Key key }) : super(key: key);

      @override
      _AppLifecycleReactorState createState() => _AppLifecycleReactorState();
    }

    class _AppLifecycleReactorState extends State<AppLifecycleReactor> with WidgetsBindingObserver {
      @override
      void initState() {
        super.initState();
        WidgetsBinding.instance.addObserver(this);
      }

      @override
      void dispose() {
        WidgetsBinding.instance.removeObserver(this);
        super.dispose();
      }

      AppLifecycleState _notification;

      @override
      void didChangeAppLifecycleState(AppLifecycleState state) {
        setState(() { _notification = state; });
      }

      @override
      Widget build(BuildContext context) {
        return Text('Last notification: $_notification');
      }
    }

有关更多详细信息，请参见documentation

Answer 5

对于深入测试，我认为结果值得一读。如果您对应该使用哪种方法感到好奇，请阅读以下内容：（在Android上经过测试）

LifeCycle解决方案有三种方法。

WidgetsBindingObserver
SystemChannels.lifecycle
flutter-android-lifecycle-plugin

WidgetsBindingObserver和SystemChannels.lifecycle之间的主要区别在于，WidgetsBindingObserver具有更多的功能，如果您有一堆需要监听LifeCycle的小部件。 SystemChannels是较低层，由WidgetsBindingObserver使用。

经过几次测试，如果在SystemChannels之后使用runApp，并且home小部件与WidgetsBindingObserver混合，则home小部件将失败，因为SystemChannels.lifecycle.setMessageHandler会覆盖home方法。

因此，如果要使用全局的单一方法，请使用SystemChannels.lifecycle，其他使用WidgetsBindingObserver。

那第三种方法呢？这仅适用于Android，如果必须在之前 runApp之前绑定方法，这是唯一的方法。

Answer 6

下面是一个如何观察包含活动（Flutter for Android developers）的生命周期状态的示例：

import 'package:flutter/widgets.dart';

class LifecycleWatcher extends StatefulWidget {
  @override
  _LifecycleWatcherState createState() => _LifecycleWatcherState();
}

class _LifecycleWatcherState extends State<LifecycleWatcher> with WidgetsBindingObserver {
  AppLifecycleState _lastLifecycleState;

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    setState(() {
      _lastLifecycleState = state;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (_lastLifecycleState == null)
      return Text('This widget has not observed any lifecycle changes.', textDirection: TextDirection.ltr);

    return Text('The most recent lifecycle state this widget observed was: $_lastLifecycleState.',
        textDirection: TextDirection.ltr);
  }
}

void main() {
  runApp(Center(child: LifecycleWatcher()));
}

Answer 7

使用“WidgetsBindingObserver”检测 onResume 事件的解决方案或“SystemChannels.lifecycle”仅在应用程序完全进入后台时才起作用，例如在锁定屏幕事件期间或在切换到另一个应用程序期间。如果用户在应用程序的屏幕之间导航，它将不起作用。如果即使在同一应用程序的不同屏幕之间切换时也想检测 onResume 事件，请使用这里的visibility_detector 库：https://pub.dev/packages/visibility_detector

  @override
Widget build(BuildContext context) {
  return VisibilityDetector(
    key: Key('my-widget-key'),
    onVisibilityChanged: (visibilityInfo) {
      num visiblePercentage = visibilityInfo.visibleFraction * 100;
      debugPrint(
          'Widget ${visibilityInfo.key} is ${visiblePercentage}% visible');
      if(visiblePercentage == 100){
                debugPrint("Resumed !");
              }
    },
    child: someOtherWidget,
  );
}

颤动：恢复小部件更新小部件？

7 个答案: