Flutter生命周期的获取

在Android开发中，系统对Activity、Fragment的生命周期有着很是明显且较于区分的定义，可是在flutter中，因为flutter的生命周期依附在activity或fragment，它的生命周期就不一样以往了，下面就展现如下flutter生命周期的理解。

flutter 生命周期相关函数的调用过程

首先，先上一张图，这张图很简单明了的阐释了一个页面启动所要执行的widget方法流程：

下面解释一下各个方法的做用：

initState

在生命周期中只调用一次，此时没法获取widget对象，能够作一些初始化操做。

didChangeDependencies

当State对象的依赖发生变化时会被调用；例如：在以前build() 中包含了一个InheritedWidget，而后在以后的build() 中InheritedWidget发生了变化，那么此时InheritedWidget的子widget的didChangeDependencies()回调都会被调用。InheritedWidget这个widget能够由父控件向子控件共享数据，案例能够参考 scoped_model开源库。

didUpdateWidget

widget状态改变的时候调用

deactivate

相似于Activity的onResume和onStop，两种状态都会调用

dispose

相似于Android的onDestroy

上面的介绍都比较简单，下面则介绍如下，如何去获取app的生命周期

flutter app的生命周期

flutter提供了一个枚举类来表明了app各个生命周期的状态：

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.
  ///
  /// Android apps in this state should assume that they may enter the
  /// [suspending] state at any time.
  paused,

  /// The application will be suspended momentarily.
  ///
  /// When the application is in this state, the engine will not call the
  /// [Window.onBeginFrame] and [Window.onDrawFrame] callbacks.
  ///
  /// On iOS, this state is currently unused.
  suspending,
}
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resumed

应用程序对用户可见的时候输出

inactive

界面处于不可点击状态，可是可见时候的回调，相似于Android的onpause

paused

app处于不可见的时候，相似于Android的onStop

suspending

ios中这个属性无效，android中表明处于后台

获取方法

class _MyHomePageState extends State<MyHomePage> with WidgetsBindingObserver {
  AppLifecycleState _lastLifecycleState;
 
  void dispose() {
    super.dispose();
    WidgetsBinding.instance.removeObserver(this);
  }
 
  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance.addObserver(this);
  }
 
  @override
  void didChangeAppLifecycleState(AppLifecycleState state) {
    print(state);
  }
 
  ...
}
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注意：第一次进入的时候并不会执行didChangeAppLifecycleState方法。
获取app的生命周期方法很简单，可是注意这并非当前widget的生命周期，那咱们若是获取当前页面的生命周期呢。

获取flutter页面的生命周期

当flutter页面跳转切入后台，flutter并无清楚的给咱们展现flutter页面的各个生命周期状态。若是咱们想要获取某个widget页面的状态，好比可见不可见那该如何操做呢？

flutter页面的onDestroy

这个比较简单，重写State的dispose，这个方法便可理解为页面的onDestroy操做。

flutter页面的onStop、onResume

上面介绍了deactivate相似于activity的onResume、onStop那么咱们能够利用这个函数来本身标志一下生命周期。
由于deactivate这个方法第一次是不执行的，所以咱们能够定义一个默认值isVisible为true来表明是否可见。

class MyState extends State<StatefulWidget>{


  bool isVisible = true;

  @override
  void deactivate() {
    isVisible = !isVisible;
    if(isVisible){
      //onResume
    }else {
      //onStop
    }
    super.deactivate();
  }

  @override
  Widget build(BuildContext context) {
    // TODO: implement build
    return null;
  }

}
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这时候咱们就能够经过isVisible来判断当前页面是否可见了，以此来作一些操做。