【Flutter】Flutter 应用生命周期 ( 前台状态 resumed | 后台状态 paused | 非活动状态 inactive | 组件分离状态 detached )（一）

/// States that an application can be in.
///
/// The values below describe notifications from the operating system.
/// Applications should not expect to always receive all possible
/// notifications. For example, if the users pulls out the battery from the
/// device, no notification will be sent before the application is suddenly
/// terminated, along with the rest of the operating system.
///
/// See also:
///
///  * [WidgetsBindingObserver], for a mechanism to observe the lifecycle state
///    from the widgets layer.
enum AppLifecycleState {
  /// The application is visible and responding to user input.
  resumed,
  /// The application is in an inactive state and is not receiving user input.
  ///
  /// On iOS, this state corresponds to an app or the Flutter host view running
  /// in the foreground inactive state. Apps transition to this state when in
  /// a phone call, responding to a TouchID request, when entering the app
  /// switcher or the control center, or when the UIViewController hosting the
  /// Flutter app is transitioning.
  ///
  /// On Android, this corresponds to an app or the Flutter host view running
  /// in the foreground inactive state.  Apps transition to this state when
  /// another activity is focused, such as a split-screen app, a phone call,
  /// a picture-in-picture app, a system dialog, or another window.
  ///
  /// Apps in this state should assume that they may be [paused] at any time.
  inactive,
  /// The application is not currently visible to the user, not responding to
  /// user input, and running in the background.
  ///
  /// When the application is in this state, the engine will not call the
  /// [Window.onBeginFrame] and [Window.onDrawFrame] callbacks.
  paused,
  /// The application is still hosted on a flutter engine but is detached from
  /// any host views.
  ///
  /// When the application is in this state, the engine is running without
  /// a view. It can either be in the progress of attaching a view when engine
  /// was first initializes, or after the view being destroyed due to a Navigator
  /// pop.
  detached,
}

@override
  void initState() {
    super.initState();
    /// 如果想要监听应用生命周期 , 要先绑定观察者 ,
    /// 绑定完成后 , 如果应用生命周期发生了变化 ,
    /// 就会回调 didChangeAppLifecycleState 方法 ;
    WidgetsBinding.instance.addObserver(this);
  }

/// 当应用生命周期发生变化时 , 会回调该方法
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    super.didChangeAppLifecycleState(state);
    print("当前的应用生命周期状态 : ${state}");
    if(state == AppLifecycleState.paused){
      print("应用进入后台 paused");
    }else if(state == AppLifecycleState.resumed){
      print("应用进入前台 resumed");
    }else if(state == AppLifecycleState.inactive){
      // 应用进入非活动状态 , 如来了个电话 , 电话应用进入前台
      // 本应用进入该状态
      print("应用进入非活动状态 inactive");
    }else if(state == AppLifecycleState.detached){
      // 应用程序仍然在 Flutter 引擎上运行 , 但是与宿主 View 组件分离
      print("应用进入 detached 状态 detached");
    }
  }

/// 移出组件中注册的观察者
  @override
  void dispose() {
    super.dispose();
    WidgetsBinding.instance.removeObserver(this);
  }