Flutter之万物皆Widget（一种你没见过的方式来深入Widget）

class TestWidget extends Widget{
  @override
  Element createElement() {
    // TODO: implement createElement
    throw UnimplementedError();
  }
}

class TestElement extends Element{
  TestElement(Widget widget) : super(widget);
  @override
  bool get debugDoingBuild => throw UnimplementedError();
  @override
  void performRebuild() {
  }
}

abstract class Element extends DiagnosticableTree implements BuildContext 
abstract class BuildContext {
  /// Whether the [widget] is currently updating the widget or render tree.
  ///
  /// For [StatefulWidget]s and [StatelessWidget]s this flag is true while
  /// their respective build methods are executing.
  /// [RenderObjectWidget]s set this to true while creating or configuring their
  /// associated [RenderObject]s.
  /// Other [Widget] types may set this to true for conceptually similar phases
  /// of their lifecycle.
  ///
  /// When this is true, it is safe for [widget] to establish a dependency to an
  /// [InheritedWidget] by calling [dependOnInheritedElement] or
  /// [dependOnInheritedWidgetOfExactType].
  ///
  /// Accessing this flag in release mode is not valid.
  bool get debugDoingBuild;

void rebuild() {
     if (!_active || !_dirty)
      return;
    performRebuild();
  }
    @override
  void update(ProxyWidget newWidget) {
    rebuild();
  }

class TestElement extends Element {
  TestElement(Widget widget) : super(widget);
  @override
  bool get debugDoingBuild => throw UnimplementedError();
  @override
  void performRebuild() {
  }
  @override
  void update(Widget newWidget) {
    super.update(newWidget);
    print("TestWidget update");
    performRebuild();
  }
  @override
  TestWidget get widget => super.widget as TestWidget;
  Widget build() => widget.build(this);
}

class TestWidget extends Widget {
  @override
  Element createElement() {
    /// 将自己传递进去，让Element调用下面的build函数
    return TestElement(this);
  }
   /// 这个context其实就是Element
  Widget build(BuildContext context) {
    print("TestWidget build");
    return Text("TestWidget");
  }
}
class TestElement extends Element {
  Element _child;
  TestElement(Widget widget) : super(widget);
  @override
  bool get debugDoingBuild => throw UnimplementedError();
  @override
  void performRebuild() {
    ///调用build函数
    var _build = build();
    ///更新子视图
   _child =  updateChild(_child, _build, slot);
  }
  @override
  void update(Widget newWidget) {
    super.update(newWidget);
    print("TestWidget update");
    ///更新
    performRebuild();
  }
  ///将widget强转成TestWidget
  @override
  TestWidget get widget => super.widget as TestWidget;
  /// 调用TestWidget的build函数
  Widget build() => widget.build(this);
}

/// 创建LifecycleElement 实现生命周期函数
class LifecycleElement extends TestElement{
  LifecycleElement(Widget widget) : super(widget);
  @override
  void mount(Element parent, newSlot) {
    print("LifecycleElement mount");
    super.mount(parent, newSlot);
  }
  @override
  void unmount() {
    print("LifecycleElement unmount");
    super.unmount();
  }
  @override
  void activate() {
    print("LifecycleElement activate");
    super.activate();
  }
  @override
  void rebuild() {
    print("LifecycleElement rebuild");
    super.rebuild();
  }
  @override
  void deactivate() {
    print("LifecycleElement deactivate");
    super.deactivate();
  }
  @override
  void didChangeDependencies() {
    print("LifecycleElement didChangeDependencies");
    super.didChangeDependencies();
  }
  @override
  void update(Widget newWidget) {
    print("LifecycleElement update");
    super.update(newWidget);
  }
  @override
  Element updateChild(Element child, Widget newWidget, newSlot) {
    print("LifecycleElement updateChild");
    return super.updateChild(child, newWidget, newSlot);
  }
  @override
  void deactivateChild(Element child) {
    print("LifecycleElement deactivateChild");
    super.deactivateChild(child);
  }
}
class TestWidget extends Widget {
  @override
  Element createElement() {
    /// 将自己传递进去，让Element调用下面的build函数
    /// 更新TestElement为LifecycleElement
    return LifecycleElement(this);
  }
  /// 这个context其实就是Element
  Widget build(BuildContext context) {
    return Text("TestWidget");
  }
}

///添加变量
  bool isShow = true;
/// 加入变量控制
  isShow ? TestWidget() : Container(),
/// 将floatingActionButton改为这样的实现
 onPressed: () {
          setState(() {
            isShow = !isShow;
          });
        },

/// widget 基类其实有一个canUpdate函数，我们猜测肯定是这里导致的，加入日志如下
  static bool canUpdate(Widget oldWidget, Widget newWidget) {
    if(oldWidget.toString()=="TestWidget") {
      print("canUpdate${oldWidget.runtimeType == newWidget.runtimeType
          && oldWidget.key == newWidget.key}");
    }
    return oldWidget.runtimeType == newWidget.runtimeType
        && oldWidget.key == newWidget.key;
  }

@mustCallSuper
  void update(covariant Widget newWidget) {
     if (newWidget.toString() == "TestWidget") {
      print("TestWidget update start");
    }
    assert(_debugLifecycleState == _ElementLifecycle.active
        && widget != null
        && newWidget != null
        && newWidget != widget
        && depth != null
        && _active
        && Widget.canUpdate(widget, newWidget));
    assert(() {
      _debugForgottenChildrenWithGlobalKey.forEach(_debugRemoveGlobalKeyReservation);
      _debugForgottenChildrenWithGlobalKey.clear();
      return true;
    }());
      if (newWidget.toString() == "TestWidget") {
      print("TestWidget:${newWidget.hashCode}");
    }
    _widget = newWidget;
  }

@override
  void mount(Element parent, newSlot) {
    print("LifecycleElement mount");
    super.mount(parent, newSlot);
    assert(_child == null);
    print("LifecycleElement firstBuild");
    performRebuild();
  }

@override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: isShow ? TestWidget() : Container(),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            isShow = !isShow;
          });
        },
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }

/// 改成const
const TestWidget()
/// 加入当前widget hashcode输出，用来判断两次是否一致
  @override
  void mount(Element parent, newSlot) {
    print("LifecycleElement  widget hashcode${widget.hashCode}");
    print("LifecycleElement hashcode${this.hashCode}");
    print("LifecycleElement mount");
    super.mount(parent, newSlot);
    assert(_child == null);
    print("LifecycleElement firstBuild");
    performRebuild();
  }