Flutter开发进阶之瞧瞧State

书接上回
上回说到StatefulWidget会将自身作为参数创建StatefulElement，然后StatefulElement的创建过程首先要调用_state = widget.createState()，会创建并持有一个state通过对其生命周期的管理去对Widget进行刷新，创建state后还会将自身交给state持有state._element = this;。
接下来会通过解析State的构成去理解State。

首先瞧瞧State里有些什么？

@optionalTypeArgs
abstract class State<T extends StatefulWidget> with Diagnosticable {
   
  
  T get widget => _widget!;
  T? _widget;

  _StateLifecycle _debugLifecycleState = _StateLifecycle.created;

  bool _debugTypesAreRight(Widget widget) => widget is T;

  BuildContext get context {
   
    assert(() {
   
      if (_element == null) {
   
        throw FlutterError(
          'This widget has been unmounted, so the State no longer has a context (and should be considered defunct). \n'
          'Consider canceling any active work during "dispose" or using the "mounted" getter to determine if the State is still active.',
        );
      }
      return true;
    }());
    return _element!;
  }
  StatefulElement? _element;

  bool get mounted => _element != null;

  @protected
  @mustCallSuper
  void initState() {
   
    assert(_debugLifecycleState == _StateLifecycle.created);
    if (kFlutterMemoryAllocationsEnabled) {
   
      MemoryAllocations.instance.dispatchObjectCreated(
        library: _flutterWidgetsLibrary,
        className: '$State',
        object: this,
      );
    }
  }

  @mustCallSuper
  @protected
  void didUpdateWidget(covariant T oldWidget) {
    }

  @protected
  @mustCallSuper
  void reassemble() {
    }

  @protected
  void setState(VoidCallback fn) {
   
    assert(() {
   
      if (_debugLifecycleState == _StateLi