Widget 源码分析

abstract class Widget extends DiagnosticableTree {
  const Widget({ this.key });
  final Key? key;
  @protected
  @factory
  Element createElement();
  @override
  String toStringShort() {
    final String type = objectRuntimeType(this, 'Widget');
    return key == null ? type : '$type-$key';
  }
  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.defaultDiagnosticsTreeStyle = DiagnosticsTreeStyle.dense;
  }
  @override
  @nonVirtual
  bool operator ==(Object other) => super == other;
  @override
  @nonVirtual
  int get hashCode => super.hashCode;
  static bool canUpdate(Widget oldWidget, Widget newWidget) {
    return oldWidget.runtimeType == newWidget.runtimeType
        && oldWidget.key == newWidget.key;
  }
  // 返回特定 `Widget` 具体子类型的数字编码。
  // 这在 `Element.updateChild` 中用于确定热重载是否修改了
  // 已安装元素配置的超类。 每个 `Widget` 的编码
  // 必须匹配 `Element._debugConcreteSubtype` 中对应的 `Element` 编码。
  static int _debugConcreteSubtype(Widget widget) {
    return widget is StatefulWidget ? 1 :
           widget is StatelessWidget ? 2 :
           0;
  }
}

这里 Widget 会创建对应 的 Element，StatelessWidget 会创建 StatelessElement 类型的 Element，而 StatefulWidget 会创建 StatefulElement 类型的 Element。

abstract class StatelessWidget extends Widget {
  const StatelessWidget({ super.key });
  @override
  StatelessElement createElement() => StatelessElement(this);
  @protected
  Widget build(BuildContext context);
}

abstract class StatefulWidget extends Widget {
  const StatefulWidget({ super.key });
  @override
  StatefulElement createElement() => StatefulElement(this);
  @protected
  @factory
  State createState();
}

从这里可以看出有 状态组件 和 无状态 组件就差别在于是否有配置信息。

State 源码分析

abstract class State<T extends StatefulWidget> with Diagnosticable {
  T get widget => _widget!;
  T? _widget;
  /// 此状态对象的生命周期中的当前阶段。
  _StateLifecycle _debugLifecycleState = _StateLifecycle.created;
  /// 为那个特定的 [Widget] 创建。
  bool _debugTypesAreRight(Widget widget) => widget is T;
  ///    当前 Widget 在树中所在的位置
  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;
  //    当前 Widget 是否被挂载到树上
  bool get mounted => _element != null;
  ///    当这个对象被插入到树中时调用。
  ///    将为每个 [State] 对象仅调用一次此方法
  @protected
  @mustCallSuper
  void initState() {
    assert(_debugLifecycleState == _StateLifecycle.created);
  }
  ///    当 此对象 配置发生变化时调用
  @mustCallSuper
  @protected
  void didUpdateWidget(covariant T oldWidget) { }
  ///    调用此方法将 调用 build 方法 ；reassemble 方法并不需要做些什么
  @protected
  @mustCallSuper
  void reassemble() { }
  ///    通知框架这个对象的内部状态已经改变。
  ///    调用 此方法可能会影响用户界面
  ///    子树也会因此父 Widget 调用 SetState 而 build
  @protected
  void setState(VoidCallback fn) {
    assert(fn != null);
    assert(() {
      if (_debugLifecycleState == _StateLifecycle.defunct) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() called after dispose(): $this'),
          ErrorDescription(
            'This error happens if you call setState() on a State object for a widget that '
            'no longer appears in the widget tree (e.g., whose parent widget no longer '
            'includes the widget in its build). This error can occur when code calls '
            'setState() from a timer or an animation callback.',
          ),
          ErrorHint(
            'The preferred solution is '
            'to cancel the timer or stop listening to the animation in the dispose() '
            'callback. Another solution is to check the "mounted" property of this '
            'object before calling setState() to ensure the object is still in the '
            'tree.',
          ),
          ErrorHint(
            'This error might indicate a memory leak if setState() is being called '
            'because another object is retaining a reference to this State object '
            'after it has been removed from the tree. To avoid memory leaks, '
            'consider breaking the reference to this object during dispose().',
          ),
        ]);
      }
      if (_debugLifecycleState == _StateLifecycle.created && !mounted) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() called in constructor: $this'),
          ErrorHint(
            'This happens when you call setState() on a State object for a widget that '
            "hasn't been inserted into the widget tree yet. It is not necessary to call "
            'setState() in the constructor, since the state is already assumed to be dirty '
            'when it is initially created.',
          ),
        ]);
      }
      return true;
    }());
    final Object? result = fn() as dynamic;
    assert(() {
      if (result is Future) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() callback argument returned a Future.'),
          ErrorDescription(
            'The setState() method on $this was called with a closure or method that '
            'returned a Future. Maybe it is marked as "async".',
          ),
          ErrorHint(
            'Instead of performing asynchronous work inside a call to setState(), first '
            'execute the work (without updating the widget state), and then synchronously '
            'update the state inside a call to setState().',
          ),
        ]);
      }
      return true;
    }());
    _element!.markNeedsBuild();
  }
  ///    当这个对象从树中移除时调用。
  @protected
  @mustCallSuper
  void deactivate() { }
  /// 当这个对象被重新插入到树中时调用
  @protected
  @mustCallSuper
  void activate() { }
  /// 当此对象从树中永久删除时调用。
  @protected
  @mustCallSuper
  void dispose() {
    assert(_debugLifecycleState == _StateLifecycle.ready);
    assert(() {
      _debugLifecycleState = _StateLifecycle.defunct;
      return true;
    }());
  }
  /// 描述这个小部件所代表的用户界面部分。
  @protected
  Widget build(BuildContext context);
  /// 当此 [State] 对象的依赖项发生更改时调用。
  ///    在 initState() 后，调用
  @protected
  @mustCallSuper
  void didChangeDependencies() { }
  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    assert(() {
      properties.add(EnumProperty<_StateLifecycle>('lifecycle state', _debugLifecycleState, defaultValue: _StateLifecycle.ready));
      return true;
    }());
    properties.add(ObjectFlagProperty<T>('_widget', _widget, ifNull: 'no widget'));
    properties.add(ObjectFlagProperty<StatefulElement>('_element', _element, ifNull: 'not mounted'));
  }
}

state 在 StatefulWidget 被继承的时候被创建，并在 StatefulElement 构造函数里被再次创建，并被对应的 StatefulElement 持有。State 让 StatelessWidget 和 StatefulWidget 做出本质的区别，Widget 的 配置信息可否复用（在 Widget.runtimeType 和 Widget.key 以及在Element tree中节点位置 均相同的情况下）。因此结合 BuildContext 源码分析得出 State 和 BuildContext 是互补关系，State 是当前 Widget 的配置信息，而 BuildContext 是储存当前 Widget && Element 的 上至 root 下至节点最底层 child 的信息，BuildContext 和 State 在Element 实例化后就被永久绑定，从 Element tree 移除 对应的 Element 后，State 和 BuildContext 的也就解绑。

Element 源码分析

BuildContext 源码分析

BuildContext 对象实际上是 [Element] 对象。

abstract class BuildContext {
  /// The current configuration of the [Element] that is this [BuildContext].
  Widget get widget;
  /// 管理此上下文的渲染管道。
  BuildOwner? get owner;
  ///    当前是否正在更新 Widget 或 渲染树
  bool get debugDoingBuild;
  ///    Widget 的当前 [RenderObject]
  RenderObject? findRenderObject();
  /// [findRenderObject] 返回的 [RenderBox] 的大小。
  Size? get size;
  ///    用 [ancestor] 注册这个构建上下文
  InheritedWidget dependOnInheritedElement(InheritedElement ancestor, { Object aspect });
  /// 获取给定类型`T`的最近的 Widget ，它必须是a的类型
  /// 具体的 [InheritedWidget] 子类，并将此构建上下文注册到
  /// 该 Widget 使得当该 Widget 更改时（或该 Widget 的新 Widget
  /// 类型被引入，或者 Widget 消失），这个构建上下文是
  /// 重建，以便它可以从该 Widget 获取新值。
  T? dependOnInheritedWidgetOfExactType<T extends InheritedWidget>({ Object? aspect });
  /// 获取与给定类型`T`最近的 Widget 对应的 Element，
  /// 必须是具体的 [InheritedWidget] 子类的类型。
  /// 如果没有找到这样的 Element，则返回 null。
  InheritedElement? getElementForInheritedWidgetOfExactType<T extends InheritedWidget>();
  /// 返回给定类型`T`的最近的祖先小部件，它必须是
  /// 具体 [Widget] 子类的类型。
  T? findAncestorWidgetOfExactType<T extends Widget>();
  /// 返回最近的祖先 [StatefulWidget] 小部件的 [State] 对象
  /// 这是给定类型 `T` 的一个实例。
  T? findAncestorStateOfType<T extends State>();
  /// 返回最远祖先 [StatefulWidget] 小部件的 [State] 对象
  /// 这是给定类型 `T` 的一个实例。
  T? findRootAncestorStateOfType<T extends State>();
  /// 返回最近的祖先 [RenderObjectWidget] Widget 的 [RenderObject] 对象
  /// 这是给定类型 `T` 的一个实例。
  T? findAncestorRenderObjectOfType<T extends RenderObject>();
  /// 遍历祖先链，从这个构建上下文的父节点开始
  ///    回调被赋予一个对祖先 Widget 对应的 [Element] 对象的引用。
  void visitAncestorElements(bool Function(Element element) visitor);
  /// 遍历这个 Widget 的子节点。
  void visitChildElements(ElementVisitor visitor);
  /// 在给定的构建上下文中开始冒泡这个通知。
  void dispatchNotification(Notification notification);
  /// 返回与当前构建 context 关联的 [Element] 的描述。
  DiagnosticsNode describeElement(String name, {DiagnosticsTreeStyle style = DiagnosticsTreeStyle.errorProperty});
  /// 返回与当前构建 context 关联的 [Widget] 的描述。
  DiagnosticsNode describeWidget(String name, {DiagnosticsTreeStyle style = DiagnosticsTreeStyle.errorProperty});
  /// 添加当前缺少的特定类型小部件的描述
  List<DiagnosticsNode> describeMissingAncestor({ required Type expectedAncestorType });
  /// 从特定的 [Element] 添加所有权链的描述
  DiagnosticsNode describeOwnershipChain(String name);
}

///    节选代码
abstract class Element extends DiagnosticableTree implements BuildContext {
  /// 此 Element 的配置。
  @override
  Widget get widget => _widget!;
  Widget? _widget;
  /// 管理此 Element 生命周期的对象。
  @override
  BuildOwner? get owner => _owner;
  BuildOwner? _owner;
  /// 树中此位置（或下方）的 renderObject。
  RenderObject? get renderObject {
    RenderObject? result;
    void visit(Element element) {
      assert(result == null); // this verifies that there's only one child
      if (element._lifecycleState == _ElementLifecycle.defunct) {
        return;
      } else if (element is RenderObjectElement) {
        result = element.renderObject;
      } else {
        element.visitChildren(visit);
      }
    }
    visit(this);
    return result;
  }
  /// 使用给定的新配置更新给定的子节点。
  Element? updateChild(Element? child, Widget? newWidget, Object? newSlot) {
    if (newWidget == null) {
      if (child != null)
        deactivateChild(child);
      return null;
    }
    final Element newChild;
    if (child != null) {
      bool hasSameSuperclass = true;
      assert(() {
        final int oldElementClass = Element._debugConcreteSubtype(child);
        final int newWidgetClass = Widget._debugConcreteSubtype(newWidget);
        hasSameSuperclass = oldElementClass == newWidgetClass;
        return true;
      }());
      if (hasSameSuperclass && child.widget == newWidget) {
        if (child.slot != newSlot)
          updateSlotForChild(child, newSlot);
        newChild = child;
      } else if (hasSameSuperclass && Widget.canUpdate(child.widget, newWidget)) {
        if (child.slot != newSlot)
          updateSlotForChild(child, newSlot);
        final bool isTimelineTracked = !kReleaseMode && _isProfileBuildsEnabledFor(newWidget);
        if (isTimelineTracked) {
          Map<String, String>? debugTimelineArguments;
          assert(() {
            if (kDebugMode && debugEnhanceBuildTimelineArguments) {
              debugTimelineArguments = newWidget.toDiagnosticsNode().toTimelineArguments();
            }
            return true;
          }());
          Timeline.startSync(
            '${newWidget.runtimeType}',
            arguments: debugTimelineArguments,
          );
        }
        child.update(newWidget);
        if (isTimelineTracked)
          Timeline.finishSync();
        assert(child.widget == newWidget);
        assert(() {
          child.owner!._debugElementWasRebuilt(child);
          return true;
        }());
        newChild = child;
      } else {
        deactivateChild(child);
        assert(child._parent == null);
        newChild = inflateWidget(newWidget, newSlot);
      }
    } else {
      newChild = inflateWidget(newWidget, newSlot);
    }
    assert(() {
      if (child != null)
        _debugRemoveGlobalKeyReservation(child);
      final Key? key = newWidget.key;
      if (key is GlobalKey) {
        assert(owner != null);
        owner!._debugReserveGlobalKeyFor(this, newChild, key);
      }
      return true;
    }());
    return newChild;
  }
  /// 将此元素添加到给定父级的给定槽中的树中。    
  void mount(Element? parent, Object? newSlot) {
    assert(_lifecycleState == _ElementLifecycle.initial);
    assert(widget != null);
    assert(_parent == null);
    assert(parent == null || parent._lifecycleState == _ElementLifecycle.active);
    assert(slot == null);
    _parent = parent;
    _slot = newSlot;
    _lifecycleState = _ElementLifecycle.active;
    _depth = _parent != null ? _parent!.depth + 1 : 1;
    if (parent != null) {
      _owner = parent.owner;
    }
    assert(owner != null);
    final Key? key = widget.key;
    if (key is GlobalKey) {
      owner!._registerGlobalKey(key, this);
    }
    _updateInheritance();
    attachNotificationTree();
  }
  /// 更改用于配置此 Element 的 Widget。
  @mustCallSuper
  void update(covariant Widget newWidget) {
    assert(
      _lifecycleState == _ElementLifecycle.active
        && widget != null
        && newWidget != null
        && newWidget != widget
        && depth != null
        && Widget.canUpdate(widget, newWidget),
    );
    assert(() {
      _debugForgottenChildrenWithGlobalKey?.forEach(_debugRemoveGlobalKeyReservation);
      _debugForgottenChildrenWithGlobalKey?.clear();
      return true;
    }());
    _widget = newWidget;
  }
  /// 将 [renderObject] 添加到render tree中 `newSlot` 指定的位置。
  void attachRenderObject(Object? newSlot) {
    assert(_slot == null);
    visitChildren((Element child) {
      child.attachRenderObject(newSlot);
    });
    _slot = newSlot;
  }
  /// 从渲染树中移除 [renderObject]。
  void detachRenderObject() {
    visitChildren((Element child) {
      child.detachRenderObject();
    });
    _slot = null;
  }
  ///    返回 与此 Element 对应的 renderObject
  RenderObject? findRenderObject() {
    assert(() {
      if (_lifecycleState != _ElementLifecycle.active) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('Cannot get renderObject of inactive element.'),
          ErrorDescription(
            'In order for an element to have a valid renderObject, it must be '
            'active, which means it is part of the tree.\n'
            'Instead, this element is in the $_lifecycleState state.\n'
            'If you called this method from a State object, consider guarding '
            'it with State.mounted.',
          ),
          describeElement('The findRenderObject() method was called for the following element'),
        ]);
      }
      return true;
    }());
    return renderObject;
  }
  /// 当这个 Element 的依赖改变时调用。
  @mustCallSuper
  void didChangeDependencies() {
    assert(_lifecycleState == _ElementLifecycle.active); // otherwise markNeedsBuild is a no-op
    assert(_debugCheckOwnerBuildTargetExists('didChangeDependencies'));
    markNeedsBuild();
  }
  /// 将元素标记为脏并将其添加到 Widget 的全局列表中
  /// 在下一帧重建。
  void markNeedsBuild() {
    assert(_lifecycleState != _ElementLifecycle.defunct);
    if (_lifecycleState != _ElementLifecycle.active)
      return;
    assert(owner != null);
    assert(_lifecycleState == _ElementLifecycle.active);
    assert(() {
      if (owner!._debugBuilding) {
        assert(owner!._debugCurrentBuildTarget != null);
        assert(owner!._debugStateLocked);
        if (_debugIsInScope(owner!._debugCurrentBuildTarget!))
          return true;
        if (!_debugAllowIgnoredCallsToMarkNeedsBuild) {
          final List<DiagnosticsNode> information = <DiagnosticsNode>[
            ErrorSummary('setState() or markNeedsBuild() called during build.'),
            ErrorDescription(
              'This ${widget.runtimeType} widget cannot be marked as needing to build because the framework '
              'is already in the process of building widgets. A widget can be marked as '
              'needing to be built during the build phase only if one of its ancestors '
              'is currently building. This exception is allowed because the framework '
              'builds parent widgets before children, which means a dirty descendant '
              'will always be built. Otherwise, the framework might not visit this '
              'widget during this build phase.',
            ),
            describeElement('The widget on which setState() or markNeedsBuild() was called was'),
          ];
          if (owner!._debugCurrentBuildTarget != null)
            information.add(owner!._debugCurrentBuildTarget!.describeWidget('The widget which was currently being built when the offending call was made was'));
          throw FlutterError.fromParts(information);
        }
        assert(dirty); // can only get here if we're not in scope, but ignored calls are allowed, and our call would somehow be ignored (since we're already dirty)
      } else if (owner!._debugStateLocked) {
        assert(!_debugAllowIgnoredCallsToMarkNeedsBuild);
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('setState() or markNeedsBuild() called when widget tree was locked.'),
          ErrorDescription(
            'This ${widget.runtimeType} widget cannot be marked as needing to build '
            'because the framework is locked.',
          ),
          describeElement('The widget on which setState() or markNeedsBuild() was called was'),
        ]);
      }
      return true;
    }());
    if (dirty)
      return;
    _dirty = true;
    owner!.scheduleBuildFor(this);
  }
  /// 使 Widget 自行更新。
  void rebuild() {
    assert(_lifecycleState != _ElementLifecycle.initial);
    if (_lifecycleState != _ElementLifecycle.active || !_dirty)
      return;
    assert(() {
      debugOnRebuildDirtyWidget?.call(this, _debugBuiltOnce);
      if (debugPrintRebuildDirtyWidgets) {
        if (!_debugBuiltOnce) {
          debugPrint('Building $this');
          _debugBuiltOnce = true;
        } else {
          debugPrint('Rebuilding $this');
        }
      }
      return true;
    }());
    assert(_lifecycleState == _ElementLifecycle.active);
    assert(owner!._debugStateLocked);
    Element? debugPreviousBuildTarget;
    assert(() {
      debugPreviousBuildTarget = owner!._debugCurrentBuildTarget;
      owner!._debugCurrentBuildTarget = this;
      return true;
    }());
    performRebuild();
    assert(() {
      assert(owner!._debugCurrentBuildTarget == this);
      owner!._debugCurrentBuildTarget = debugPreviousBuildTarget;
      return true;
    }());
    assert(!_dirty);
  }
  ///    检测是否更新Element
  @protected
  @pragma('vm:prefer-inline')
  Element? updateChild(Element? child, Widget? newWidget, Object? newSlot) {
    if (newWidget == null) {
      if (child != null)
        deactivateChild(child);
      return null;
    }
    final Element newChild;
    if (child != null) {
      bool hasSameSuperclass = true;
      assert(() {
        final int oldElementClass = Element._debugConcreteSubtype(child);
        final int newWidgetClass = Widget._debugConcreteSubtype(newWidget);
        hasSameSuperclass = oldElementClass == newWidgetClass;
        return true;
      }());
      if (hasSameSuperclass && child.widget == newWidget) {
        if (child.slot != newSlot)
          updateSlotForChild(child, newSlot);
        newChild = child;
      } else if (hasSameSuperclass && Widget.canUpdate(child.widget, newWidget)) {
        if (child.slot != newSlot)
          updateSlotForChild(child, newSlot);
        final bool isTimelineTracked = !kReleaseMode && _isProfileBuildsEnabledFor(newWidget);
        if (isTimelineTracked) {
          Map<String, String>? debugTimelineArguments;
          assert(() {
            if (kDebugMode && debugEnhanceBuildTimelineArguments) {
              debugTimelineArguments = newWidget.toDiagnosticsNode().toTimelineArguments();
            }
            return true;
          }());
          Timeline.startSync(
            '${newWidget.runtimeType}',
            arguments: debugTimelineArguments,
          );
        }
        child.update(newWidget);
        if (isTimelineTracked)
          Timeline.finishSync();
        assert(child.widget == newWidget);
        assert(() {
          child.owner!._debugElementWasRebuilt(child);
          return true;
        }());
        newChild = child;
      } else {
        deactivateChild(child);
        assert(child._parent == null);
        newChild = inflateWidget(newWidget, newSlot);
      }
    } else {
      newChild = inflateWidget(newWidget, newSlot);
    }
    assert(() {
      if (child != null)
        _debugRemoveGlobalKeyReservation(child);
      final Key? key = newWidget.key;
      if (key is GlobalKey) {
        assert(owner != null);
        owner!._debugReserveGlobalKeyFor(this, newChild, key);
      }
      return true;
    }());
    return newChild;
  }
}

当前 Element 所依赖的配置 发生变化时，会把当前 Element 的 _dirty 标致为 true ，加入待渲染管道，在下一帧渲染。

StatelessElement 源码分析

class StatelessElement extends ComponentElement {
  StatelessElement(StatelessWidget super.widget);
  @override
  Widget build() => (widget as StatelessWidget).build(this);
  @override
  void update(StatelessWidget newWidget) {
    super.update(newWidget);
    assert(widget == newWidget);
    _dirty = true;
    rebuild();
  }
}

根据 StatelessElement 源码得知，在 StatelessWidget 的配置信息发生变化时，当前 StatelessElement 的 脏点 会被标记为 true ，在下一帧会被重新刷新，以及刷新当前 Widget 的子组件。因此尽量在Widget tree中变化少的节点使用 StatelessWidget 。

StatefulElement 源码分析

class StatefulElement extends ComponentElement {
  StatefulElement(StatefulWidget widget)
      : _state = widget.createState(),
        super(widget) {
    assert(() {
      if (!state._debugTypesAreRight(widget)) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('StatefulWidget.createState must return a subtype of State<${widget.runtimeType}>'),
          ErrorDescription(
            'The createState function for ${widget.runtimeType} returned a state '
            'of type ${state.runtimeType}, which is not a subtype of '
            'State<${widget.runtimeType}>, violating the contract for createState.',
          ),
        ]);
      }
      return true;
    }());
    assert(state._element == null);
    state._element = this;
    assert(
      state._widget == null,
      'The createState function for $widget returned an old or invalid state '
      'instance: ${state._widget}, which is not null, violating the contract '
      'for createState.',
    );
    state._widget = widget;
    assert(state._debugLifecycleState == _StateLifecycle.created);
  }
  ///    创建 State
  @override
  Widget build() => state.build(this);
  /// The [State] instance associated with this location in the tree.
  ///
  /// There is a one-to-one relationship between [State] objects and the
  /// [StatefulElement] objects that hold them. The [State] objects are created
  /// by [StatefulElement] in [mount].
  State<StatefulWidget> get state => _state!;
  State<StatefulWidget>? _state;
  ///    热重载
  @override
  void reassemble() {
    if (_debugShouldReassemble(_debugReassembleConfig, _widget)) {
      state.reassemble();
    }
    super.reassemble();
  }
  @override
  void _firstBuild() {
    assert(state._debugLifecycleState == _StateLifecycle.created);
    try {
      _debugSetAllowIgnoredCallsToMarkNeedsBuild(true);
      final Object? debugCheckForReturnedFuture = state.initState() as dynamic;
      assert(() {
        if (debugCheckForReturnedFuture is Future) {
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary('${state.runtimeType}.initState() returned a Future.'),
            ErrorDescription('State.initState() must be a void method without an `async` keyword.'),
            ErrorHint(
              'Rather than awaiting on asynchronous work directly inside of initState, '
              'call a separate method to do this work without awaiting it.',
            ),
          ]);
        }
        return true;
      }());
    } finally {
      _debugSetAllowIgnoredCallsToMarkNeedsBuild(false);
    }
    assert(() {
      state._debugLifecycleState = _StateLifecycle.initialized;
      return true;
    }());
    state.didChangeDependencies();
    assert(() {
      state._debugLifecycleState = _StateLifecycle.ready;
      return true;
    }());
    super._firstBuild();
  }
  ///     更改配置信息，重建 Element
  @override
  void performRebuild() {
    if (_didChangeDependencies) {
      state.didChangeDependencies();
      _didChangeDependencies = false;
    }
    super.performRebuild();
  }
  ///    更新 当前 Element
  @override
  void update(StatefulWidget newWidget) {
    super.update(newWidget);
    assert(widget == newWidget);
    final StatefulWidget oldWidget = state._widget!;
    _dirty = true;
    state._widget = widget as StatefulWidget;
    try {
      _debugSetAllowIgnoredCallsToMarkNeedsBuild(true);
      final Object? debugCheckForReturnedFuture = state.didUpdateWidget(oldWidget) as dynamic;
      assert(() {
        if (debugCheckForReturnedFuture is Future) {
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary('${state.runtimeType}.didUpdateWidget() returned a Future.'),
            ErrorDescription( 'State.didUpdateWidget() must be a void method without an `async` keyword.'),
            ErrorHint(
              'Rather than awaiting on asynchronous work directly inside of didUpdateWidget, '
              'call a separate method to do this work without awaiting it.',
            ),
          ]);
        }
        return true;
      }());
    } finally {
      _debugSetAllowIgnoredCallsToMarkNeedsBuild(false);
    }
    rebuild();
  }
 ///    激活 Element
 @override
  void activate() {
    super.activate();
    state.activate();
    // Since the State could have observed the deactivate() and thus disposed of
    // resources allocated in the build method, we have to rebuild the widget
    // so that its State can reallocate its resources.
    assert(_lifecycleState == _ElementLifecycle.active); // otherwise markNeedsBuild is a no-op
    markNeedsBuild();
  }
  ///    Element 消活
  @override
  void deactivate() {
    state.deactivate();
    super.deactivate();
  }
  ///    把 Element 从 tree 中取消挂载
  @override
  void unmount() {
    super.unmount();
    state.dispose();
    assert(() {
      if (state._debugLifecycleState == _StateLifecycle.defunct)
        return true;
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('${state.runtimeType}.dispose failed to call super.dispose.'),
        ErrorDescription(
          'dispose() implementations must always call their superclass dispose() method, to ensure '
          'that all the resources used by the widget are fully released.',
        ),
      ]);
    }());
    state._element = null;
    // Release resources to reduce the severity of memory leaks caused by
    // defunct, but accidentally retained Elements.
    _state = null;
  }
  @override
  InheritedWidget dependOnInheritedElement(Element ancestor, { Object? aspect }) {
    assert(ancestor != null);
    assert(() {
      final Type targetType = ancestor.widget.runtimeType;
      if (state._debugLifecycleState == _StateLifecycle.created) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('dependOnInheritedWidgetOfExactType<$targetType>() or dependOnInheritedElement() was called before ${state.runtimeType}.initState() completed.'),
          ErrorDescription(
            'When an inherited widget changes, for example if the value of Theme.of() changes, '
            "its dependent widgets are rebuilt. If the dependent widget's reference to "
            'the inherited widget is in a constructor or an initState() method, '
            'then the rebuilt dependent widget will not reflect the changes in the '
            'inherited widget.',
          ),
          ErrorHint(
            'Typically references to inherited widgets should occur in widget build() methods. Alternatively, '
            'initialization based on inherited widgets can be placed in the didChangeDependencies method, which '
            'is called after initState and whenever the dependencies change thereafter.',
          ),
        ]);
      }
      if (state._debugLifecycleState == _StateLifecycle.defunct) {
        throw FlutterError.fromParts(<DiagnosticsNode>[
          ErrorSummary('dependOnInheritedWidgetOfExactType<$targetType>() or dependOnInheritedElement() was called after dispose(): $this'),
          ErrorDescription(
            'This error happens if you call dependOnInheritedWidgetOfExactType() on the '
            'BuildContext for a widget that no longer appears in the widget tree '
            '(e.g., whose parent widget no longer includes the widget in its '
            'build). This error can occur when code calls '
            'dependOnInheritedWidgetOfExactType() from a timer or an animation callback.',
          ),
          ErrorHint(
            'The preferred solution is to cancel the timer or stop listening to the '
            'animation in the dispose() callback. Another solution is to check the '
            '"mounted" property of this object before calling '
            'dependOnInheritedWidgetOfExactType() to ensure the object is still in the '
            'tree.',
          ),
          ErrorHint(
            'This error might indicate a memory leak if '
            'dependOnInheritedWidgetOfExactType() is being called because another object '
            'is retaining a reference to this State object after it has been '
            'removed from the tree. To avoid memory leaks, consider breaking the '
            'reference to this object during dispose().',
          ),
        ]);
      }
      return true;
    }());
    return super.dependOnInheritedElement(ancestor as InheritedElement, aspect: aspect);
  }
  bool _didChangeDependencies = false;
  ///    复用 配置信息
  @override
  void didChangeDependencies() {
    super.didChangeDependencies();
    _didChangeDependencies = true;
  }
  @override
  DiagnosticsNode toDiagnosticsNode({ String? name, DiagnosticsTreeStyle? style }) {
    return _ElementDiagnosticableTreeNode(
      name: name,
      value: this,
      style: style,
      stateful: true,
    );
  }
  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<State<StatefulWidget>>('state', _state, defaultValue: null));
  }
}

StatefulElement 在 首次构建的时候，State.didChangeDependencies( )就被调用，以及在消活时，调用 state.deactivate( )，再被从Element tree 移除挂载时，调用 state.dispose( )，永久删除 Widget，释放资源。因此得出 StatefulWidget 的生命周期为 ：

RenderObject 源码分析

//    节选代码
abstract class RenderObject extends AbstractNode with 
               DiagnosticableTreeMixin implements HitTestTarget {
  RenderObject() {
    _needsCompositing = isRepaintBoundary || alwaysNeedsCompositing;
  }
  ///     热重载
  void reassemble() {
    markNeedsLayout();
    markNeedsCompositingBitsUpdate();
    markNeedsPaint();
    markNeedsSemanticsUpdate();
    visitChildren((RenderObject child) {
      child.reassemble();
    });
  }
  ///     将 RenderObject 添加到 RenderObject tree 上
  @override
  void attach(PipelineOwner owner) {
    assert(!_debugDisposed);
    super.attach(owner);
    if (_needsLayout && _relayoutBoundary != null) {
      // Don't enter this block if we've never laid out at all;
      // scheduleInitialLayout() will handle it
      _needsLayout = false;
      markNeedsLayout();
    }
    if (_needsCompositingBitsUpdate) {
      _needsCompositingBitsUpdate = false;
      markNeedsCompositingBitsUpdate();
    }
    if (_needsPaint && _layerHandle.layer != null) {
      // Don't enter this block if we've never painted at all;
      // scheduleInitialPaint() will handle it
      _needsPaint = false;
      markNeedsPaint();
    }
    if (_needsSemanticsUpdate && _semanticsConfiguration.isSemanticBoundary) {
      // Don't enter this block if we've never updated semantics at all;
      // scheduleInitialSemantics() will handle it
      _needsSemanticsUpdate = false;
      markNeedsSemanticsUpdate();
    }
  }
  ///     获取 约束
  @protected
  Constraints get constraints {
    if (_constraints == null)
      throw StateError('A RenderObject does not have any constraints before it has been laid out.');
    return _constraints!;
  }
  ///        标记脏点，布局
  void markNeedsLayout() {
    assert(_debugCanPerformMutations);
    if (_needsLayout) {
      assert(_debugSubtreeRelayoutRootAlreadyMarkedNeedsLayout());
      return;
    }
    if (_relayoutBoundary == null) {
      _needsLayout = true;
      if (parent != null) {
        markParentNeedsLayout();
      }
      return;
    }
    if (_relayoutBoundary != this) {
      markParentNeedsLayout();
    } else {
      _needsLayout = true;
      if (owner != null) {
        assert(() {
          if (debugPrintMarkNeedsLayoutStacks)
            debugPrintStack(label: 'markNeedsLayout() called for $this');
          return true;
        }());
        owner!._nodesNeedingLayout.add(this);
        owner!.requestVisualUpdate();
      }
    }
  }
  //    合成
  void markNeedsCompositingBitsUpdate() {
    assert(!_debugDisposed);
    if (_needsCompositingBitsUpdate)
      return;
    _needsCompositingBitsUpdate = true;
    if (parent is RenderObject) {
      final RenderObject parent = this.parent! as RenderObject;
      if (parent._needsCompositingBitsUpdate)
        return;
      if (!isRepaintBoundary && !parent.isRepaintBoundary) {
        parent.markNeedsCompositingBitsUpdate();
        return;
      }
    }
    assert(() {
      final AbstractNode? parent = this.parent;
      if (parent is RenderObject)
        return parent._needsCompositing;
      return true;
    }());
    // parent is fine (or there isn't one), but we are dirty
    if (owner != null)
      owner!._nodesNeedingCompositingBitsUpdate.add(this);
  }
  ///    绘制
  void markNeedsPaint() {
    assert(!_debugDisposed);
    assert(owner == null || !owner!.debugDoingPaint);
    if (_needsPaint)
      return;
    _needsPaint = true;
    if (isRepaintBoundary) {
      assert(() {
        if (debugPrintMarkNeedsPaintStacks)
          debugPrintStack(label: 'markNeedsPaint() called for $this');
        return true;
      }());
      // If we always have our own layer, then we can just repaint
      // ourselves without involving any other nodes.
      assert(_layerHandle.layer is OffsetLayer);
      if (owner != null) {
        owner!._nodesNeedingPaint.add(this);
        owner!.requestVisualUpdate();
      }
    } else if (parent is RenderObject) {
      final RenderObject parent = this.parent! as RenderObject;
      parent.markNeedsPaint();
      assert(parent == this.parent);
    } else {
      assert(() {
        if (debugPrintMarkNeedsPaintStacks)
          debugPrintStack(label: 'markNeedsPaint() called for $this (root of render tree)');
        return true;
      }());
      if (owner != null)
        owner!.requestVisualUpdate();
    }
  }
}

根据 RenderObject 的 markNeedsLayout 、markNeedsCompositingBitsUpdate 、markNeedsPaint 得出 isRepaintBoundary 就是判断是否继续寻找父级，为 false，则向上寻找父级，直到 isRepaintBoundary = true 为止 。

RenderObjectWidget 源码分析

abstract class RenderObjectWidget extends Widget {
  const RenderObjectWidget({ super.key });
  @override
  @factory
  RenderObjectElement createElement();
  @protected
  @factory
  RenderObject createRenderObject(BuildContext context);
  @protected
  void updateRenderObject(BuildContext context, covariant RenderObject renderObject) { }
  @protected
  void didUnmountRenderObject(covariant RenderObject renderObject) { }
}

RenderObjectWidget 是 RenderObjectElement 的配置信息，操作 RenderObject 在 RenderObject tree 上的更新以及 取消挂载 。

RenderObjectElement 源码分析

abstract class RenderObjectElement extends Element {
  /// Creates an element that uses the given widget as its configuration.
  RenderObjectElement(RenderObjectWidget super.widget);
  /// The underlying [RenderObject] for this element.
  ///
  /// If this element has been [unmount]ed, this getter will throw.
  @override
  RenderObject get renderObject {
    assert(_renderObject != null, '$runtimeType unmounted');
    return _renderObject!;
  }
  RenderObject? _renderObject;
  bool _debugDoingBuild = false;
  @override
  bool get debugDoingBuild => _debugDoingBuild;
  RenderObjectElement? _ancestorRenderObjectElement;
  RenderObjectElement? _findAncestorRenderObjectElement() {
    Element? ancestor = _parent;
    while (ancestor != null && ancestor is! RenderObjectElement)
      ancestor = ancestor._parent;
    return ancestor as RenderObjectElement?;
  }
  ParentDataElement<ParentData>? _findAncestorParentDataElement() {
    Element? ancestor = _parent;
    ParentDataElement<ParentData>? result;
    while (ancestor != null && ancestor is! RenderObjectElement) {
      if (ancestor is ParentDataElement<ParentData>) {
        result = ancestor;
        break;
      }
      ancestor = ancestor._parent;
    }
    assert(() {
      if (result == null || ancestor == null) {
        return true;
      }
      // Check that no other ParentDataWidgets want to provide parent data.
      final List<ParentDataElement<ParentData>> badAncestors = <ParentDataElement<ParentData>>[];
      ancestor = ancestor!._parent;
      while (ancestor != null && ancestor is! RenderObjectElement) {
        if (ancestor is ParentDataElement<ParentData>) {
          badAncestors.add(ancestor! as ParentDataElement<ParentData>);
        }
        ancestor = ancestor!._parent;
      }
      if (badAncestors.isNotEmpty) {
        badAncestors.insert(0, result);
        try {
          // We explicitly throw here (even though we immediately redirect the
          // exception elsewhere) so that debuggers will notice it when they
          // have "break on exception" enabled.
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary('Incorrect use of ParentDataWidget.'),
            ErrorDescription('The following ParentDataWidgets are providing parent data to the same RenderObject:'),
            for (final ParentDataElement<ParentData> ancestor in badAncestors)
              ErrorDescription('- ${ancestor.widget} (typically placed directly inside a ${(ancestor.widget as ParentDataWidget<ParentData>).debugTypicalAncestorWidgetClass} widget)'),
            ErrorDescription('However, a RenderObject can only receive parent data from at most one ParentDataWidget.'),
            ErrorHint('Usually, this indicates that at least one of the offending ParentDataWidgets listed above is not placed directly inside a compatible ancestor widget.'),
            ErrorDescription('The ownership chain for the RenderObject that received the parent data was:\n  ${debugGetCreatorChain(10)}'),
          ]);
        } on FlutterError catch (e) {
          _debugReportException(ErrorSummary('while looking for parent data.'), e, e.stackTrace);
        }
      }
      return true;
    }());
    return result;
  }
  @override
  void mount(Element? parent, Object? newSlot) {
    super.mount(parent, newSlot);
    assert(() {
      _debugDoingBuild = true;
      return true;
    }());
    _renderObject = (widget as RenderObjectWidget).createRenderObject(this);
    assert(!_renderObject!.debugDisposed!);
    assert(() {
      _debugDoingBuild = false;
      return true;
    }());
    assert(() {
      _debugUpdateRenderObjectOwner();
      return true;
    }());
    assert(_slot == newSlot);
    attachRenderObject(newSlot);
    _dirty = false;
  }
  @override
  void update(covariant RenderObjectWidget newWidget) {
    super.update(newWidget);
    assert(widget == newWidget);
    assert(() {
      _debugUpdateRenderObjectOwner();
      return true;
    }());
    _performRebuild(); // calls widget.updateRenderObject()
  }
  void _debugUpdateRenderObjectOwner() {
    assert(() {
      renderObject.debugCreator = DebugCreator(this);
      return true;
    }());
  }
  @override
  void performRebuild() {
    _performRebuild(); // calls widget.updateRenderObject()
  }
  @pragma('vm:prefer-inline')
  void _performRebuild() {
    assert(() {
      _debugDoingBuild = true;
      return true;
    }());
    (widget as RenderObjectWidget).updateRenderObject(this, renderObject);
    assert(() {
      _debugDoingBuild = false;
      return true;
    }());
    _dirty = false;
  }
  @protected
  List<Element> updateChildren(List<Element> oldChildren, List<Widget> newWidgets, { Set<Element>? forgottenChildren, List<Object?>? slots }) {
    assert(oldChildren != null);
    assert(newWidgets != null);
    assert(slots == null || newWidgets.length == slots.length);
    Element? replaceWithNullIfForgotten(Element child) {
      return forgottenChildren != null && forgottenChildren.contains(child) ? null : child;
    }
    Object? slotFor(int newChildIndex, Element? previousChild) {
      return slots != null
        ? slots[newChildIndex]
        : IndexedSlot<Element?>(newChildIndex, previousChild);
    }
    int newChildrenTop = 0;
    int oldChildrenTop = 0;
    int newChildrenBottom = newWidgets.length - 1;
    int oldChildrenBottom = oldChildren.length - 1;
    final List<Element> newChildren = oldChildren.length == newWidgets.length ?
        oldChildren : List<Element>.filled(newWidgets.length, _NullElement.instance);
    Element? previousChild;
    // Update the top of the list.
    while ((oldChildrenTop <= oldChildrenBottom) && (newChildrenTop <= newChildrenBottom)) {
      final Element? oldChild = replaceWithNullIfForgotten(oldChildren[oldChildrenTop]);
      final Widget newWidget = newWidgets[newChildrenTop];
      assert(oldChild == null || oldChild._lifecycleState == _ElementLifecycle.active);
      if (oldChild == null || !Widget.canUpdate(oldChild.widget, newWidget))
        break;
      final Element newChild = updateChild(oldChild, newWidget, slotFor(newChildrenTop, previousChild))!;
      assert(newChild._lifecycleState == _ElementLifecycle.active);
      newChildren[newChildrenTop] = newChild;
      previousChild = newChild;
      newChildrenTop += 1;
      oldChildrenTop += 1;
    }
    // Scan the bottom of the list.
    while ((oldChildrenTop <= oldChildrenBottom) && (newChildrenTop <= newChildrenBottom)) {
      final Element? oldChild = replaceWithNullIfForgotten(oldChildren[oldChildrenBottom]);
      final Widget newWidget = newWidgets[newChildrenBottom];
      assert(oldChild == null || oldChild._lifecycleState == _ElementLifecycle.active);
      if (oldChild == null || !Widget.canUpdate(oldChild.widget, newWidget))
        break;
      oldChildrenBottom -= 1;
      newChildrenBottom -= 1;
    }
    // Scan the old children in the middle of the list.
    final bool haveOldChildren = oldChildrenTop <= oldChildrenBottom;
    Map<Key, Element>? oldKeyedChildren;
    if (haveOldChildren) {
      oldKeyedChildren = <Key, Element>{};
      while (oldChildrenTop <= oldChildrenBottom) {
        final Element? oldChild = replaceWithNullIfForgotten(oldChildren[oldChildrenTop]);
        assert(oldChild == null || oldChild._lifecycleState == _ElementLifecycle.active);
        if (oldChild != null) {
          if (oldChild.widget.key != null)
            oldKeyedChildren[oldChild.widget.key!] = oldChild;
          else
            deactivateChild(oldChild);
        }
        oldChildrenTop += 1;
      }
    }
    // Update the middle of the list.
    while (newChildrenTop <= newChildrenBottom) {
      Element? oldChild;
      final Widget newWidget = newWidgets[newChildrenTop];
      if (haveOldChildren) {
        final Key? key = newWidget.key;
        if (key != null) {
          oldChild = oldKeyedChildren![key];
          if (oldChild != null) {
            if (Widget.canUpdate(oldChild.widget, newWidget)) {
              // we found a match!
              // remove it from oldKeyedChildren so we don't unsync it later
              oldKeyedChildren.remove(key);
            } else {
              // Not a match, let's pretend we didn't see it for now.
              oldChild = null;
            }
          }
        }
      }
      assert(oldChild == null || Widget.canUpdate(oldChild.widget, newWidget));
      final Element newChild = updateChild(oldChild, newWidget, slotFor(newChildrenTop, previousChild))!;
      assert(newChild._lifecycleState == _ElementLifecycle.active);
      assert(oldChild == newChild || oldChild == null || oldChild._lifecycleState != _ElementLifecycle.active);
      newChildren[newChildrenTop] = newChild;
      previousChild = newChild;
      newChildrenTop += 1;
    }
    // We've scanned the whole list.
    assert(oldChildrenTop == oldChildrenBottom + 1);
    assert(newChildrenTop == newChildrenBottom + 1);
    assert(newWidgets.length - newChildrenTop == oldChildren.length - oldChildrenTop);
    newChildrenBottom = newWidgets.length - 1;
    oldChildrenBottom = oldChildren.length - 1;
    // Update the bottom of the list.
    while ((oldChildrenTop <= oldChildrenBottom) && (newChildrenTop <= newChildrenBottom)) {
      final Element oldChild = oldChildren[oldChildrenTop];
      assert(replaceWithNullIfForgotten(oldChild) != null);
      assert(oldChild._lifecycleState == _ElementLifecycle.active);
      final Widget newWidget = newWidgets[newChildrenTop];
      assert(Widget.canUpdate(oldChild.widget, newWidget));
      final Element newChild = updateChild(oldChild, newWidget, slotFor(newChildrenTop, previousChild))!;
      assert(newChild._lifecycleState == _ElementLifecycle.active);
      assert(oldChild == newChild || oldChild == null || oldChild._lifecycleState != _ElementLifecycle.active);
      newChildren[newChildrenTop] = newChild;
      previousChild = newChild;
      newChildrenTop += 1;
      oldChildrenTop += 1;
    }
    // Clean up any of the remaining middle nodes from the old list.
    if (haveOldChildren && oldKeyedChildren!.isNotEmpty) {
      for (final Element oldChild in oldKeyedChildren.values) {
        if (forgottenChildren == null || !forgottenChildren.contains(oldChild))
          deactivateChild(oldChild);
      }
    }
    assert(newChildren.every((Element element) => element is! _NullElement));
    return newChildren;
  }
  @override
  void deactivate() {
    super.deactivate();
    assert(
      !renderObject.attached,
      'A RenderObject was still attached when attempting to deactivate its '
      'RenderObjectElement: $renderObject',
    );
  }
  @override
  void unmount() {
    assert(
      !renderObject.debugDisposed!,
      'A RenderObject was disposed prior to its owning element being unmounted: '
      '$renderObject',
    );
    final RenderObjectWidget oldWidget = widget as RenderObjectWidget;
    super.unmount();
    assert(
      !renderObject.attached,
      'A RenderObject was still attached when attempting to unmount its '
      'RenderObjectElement: $renderObject',
    );
    oldWidget.didUnmountRenderObject(renderObject);
    _renderObject!.dispose();
    _renderObject = null;
  }
  void _updateParentData(ParentDataWidget<ParentData> parentDataWidget) {
    bool applyParentData = true;
    assert(() {
      try {
        if (!parentDataWidget.debugIsValidRenderObject(renderObject)) {
          applyParentData = false;
          throw FlutterError.fromParts(<DiagnosticsNode>[
            ErrorSummary('Incorrect use of ParentDataWidget.'),
            ...parentDataWidget._debugDescribeIncorrectParentDataType(
              parentData: renderObject.parentData,
              parentDataCreator: _ancestorRenderObjectElement!.widget as RenderObjectWidget,
              ownershipChain: ErrorDescription(debugGetCreatorChain(10)),
            ),
          ]);
        }
      } on FlutterError catch (e) {
        _debugReportException(ErrorSummary('while applying parent data.'), e, e.stackTrace);
      }
      return true;
    }());
    if (applyParentData)
      parentDataWidget.applyParentData(renderObject);
  }
  @override
  void _updateSlot(Object? newSlot) {
    final Object? oldSlot = slot;
    assert(oldSlot != newSlot);
    super._updateSlot(newSlot);
    assert(slot == newSlot);
    _ancestorRenderObjectElement!.moveRenderObjectChild(renderObject, oldSlot, slot);
  }
  @override
  void attachRenderObject(Object? newSlot) {
    assert(_ancestorRenderObjectElement == null);
    _slot = newSlot;
    _ancestorRenderObjectElement = _findAncestorRenderObjectElement();
    _ancestorRenderObjectElement?.insertRenderObjectChild(renderObject, newSlot);
    final ParentDataElement<ParentData>? parentDataElement = _findAncestorParentDataElement();
    if (parentDataElement != null)
      _updateParentData(parentDataElement.widget as ParentDataWidget<ParentData>);
  }
  @override
  void detachRenderObject() {
    if (_ancestorRenderObjectElement != null) {
      _ancestorRenderObjectElement!.removeRenderObjectChild(renderObject, slot);
      _ancestorRenderObjectElement = null;
    }
    _slot = null;
  }
  @protected
  void insertRenderObjectChild(covariant RenderObject child, covariant Object? slot) {
    assert(() {
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('RenderObjectElement.insertChildRenderObject() is deprecated.'),
        toDiagnosticsNode(
          name: 'insertChildRenderObject() was called on this Element',
          style: DiagnosticsTreeStyle.shallow,
        ),
        ErrorDescription(
          'insertChildRenderObject() has been deprecated in favor of '
          'insertRenderObjectChild(). See https://github.com/flutter/flutter/issues/63269 '
          'for details.',
        ),
        ErrorHint(
          'Rather than overriding insertChildRenderObject() in your '
          'RenderObjectElement subclass, override insertRenderObjectChild() instead, '
          "and DON'T call super.insertRenderObjectChild(). If you're implementing a "
          'new RenderObjectElement, you should override/implement '
          'insertRenderObjectChild(), moveRenderObjectChild(), and '
          'removeRenderObjectChild().',
        ),
      ]);
    }());
  }
  @protected
  void moveRenderObjectChild(covariant RenderObject child, covariant Object? oldSlot, covariant Object? newSlot) {
    assert(() {
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('RenderObjectElement.moveChildRenderObject() is deprecated.'),
        toDiagnosticsNode(
          name: 'super.moveChildRenderObject() was called on this Element',
          style: DiagnosticsTreeStyle.shallow,
        ),
        ErrorDescription(
          'moveChildRenderObject() has been deprecated in favor of '
          'moveRenderObjectChild(). See https://github.com/flutter/flutter/issues/63269 '
          'for details.',
        ),
        ErrorHint(
          'Rather than overriding moveChildRenderObject() in your '
          'RenderObjectElement subclass, override moveRenderObjectChild() instead, '
          "and DON'T call super.moveRenderObjectChild(). If you're implementing a "
          'new RenderObjectElement, you should override/implement '
          'insertRenderObjectChild(), moveRenderObjectChild(), and '
          'removeRenderObjectChild().',
        ),
      ]);
    }());
  }
  /// Remove the given child from [renderObject].
  ///
  /// The given child is guaranteed to have been inserted at the given `slot`
  /// and have [renderObject] as its parent.
  @protected
  void removeRenderObjectChild(covariant RenderObject child, covariant Object? slot) {
    assert(() {
      throw FlutterError.fromParts(<DiagnosticsNode>[
        ErrorSummary('RenderObjectElement.removeChildRenderObject() is deprecated.'),
        toDiagnosticsNode(
          name: 'super.removeChildRenderObject() was called on this Element',
          style: DiagnosticsTreeStyle.shallow,
        ),
        ErrorDescription(
          'removeChildRenderObject() has been deprecated in favor of '
          'removeRenderObjectChild(). See https://github.com/flutter/flutter/issues/63269 '
          'for details.',
        ),
        ErrorHint(
          'Rather than overriding removeChildRenderObject() in your '
          'RenderObjectElement subclass, override removeRenderObjectChild() instead, '
          "and DON'T call super.removeRenderObjectChild(). If you're implementing a "
          'new RenderObjectElement, you should override/implement '
          'insertRenderObjectChild(), moveRenderObjectChild(), and '
          'removeRenderObjectChild().',
        ),
      ]);
    }());
  }
  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<RenderObject>('renderObject', _renderObject, defaultValue: null));
  }
}