本次整理一下widget 搭建过程中遇到的一些类和对象。
主要有：Widget，Element，State，RenderObject

1、widget

根Widget构造方法：

RenderObjectToWidgetAdapter<RenderBox>(
     container: renderView,
     debugShortDescription: '[root]',
     child: rootWidget,
   )

普通Widget 构造方法：

runApp(const MyApp());

Widget 有createElement 方法

StatelessElement createElement() => StatelessElement(this);
StatefulElement createElement() => StatefulElement(this);
InheritedElement createElement() => InheritedElement(this);
LeafRenderObjectElement createElement() => LeafRenderObjectElement(this);
SingleChildRenderObjectElement createElement() => SingleChildRenderObjectElement(this);
MultiChildRenderObjectElement createElement() => MultiChildRenderObjectElement(this);
ParentDataElement<T> createElement() => ParentDataElement<T>(this);

RenderObjectElement element类型：

abstract class RootRenderObjectElement extends RenderObjectElement 
class LeafRenderObjectElement extends RenderObjectElement 
class SingleChildRenderObjectElement extends RenderObjectElement
class MultiChildRenderObjectElement extends RenderObjectElement 

StatelessWidget 有build方法
ProxyWidget有child
statefull widget有createState 方法

2、state

State持有element， widget
State有initState方法，build子widget方法

3、element

  StatefulElement(StatefulWidget widget)
      : _state = widget.createState(),
        super(widget) {
 
    state._element = this;
    state._widget = widget;
  }

element持有widget

  Element(Widget widget)
    : assert(widget != null),
      _widget = widget;

  Element? _parent;

element持有state

class StatefulElement extends ComponentElement {
  StatefulElement(StatefulWidget widget)
      : _state = widget.createState(),
        super(widget) {
}
  State<StatefulWidget>? _state;
}

element持有子element

abstract class ComponentElement extends Element {
  ComponentElement(Widget widget) : super(widget);
  Element? _child;

}

element持有renderObject

abstract class RenderObjectElement extends Element {
 RenderObjectElement(RenderObjectWidget widget) : super(widget);
 @override
 RenderObject get renderObject {
   return _renderObject!;
 }
 RenderObject? _renderObject;
}

visit 方法递归调用

abstract class Element extends DiagnosticableTree implements BuildContext {
  Element(Widget widget)
    : assert(widget != null),
      _widget = widget;

  RenderObject? get renderObject {
    RenderObject? result;
    void visit(Element element) {
      if (element._lifecycleState == _ElementLifecycle.defunct) {
        return;
      } else if (element is RenderObjectElement) {
        result = element.renderObject;
      } else {
        element.visitChildren(visit);
      }
    }
    visit(this);
    return result;
  }

}

ComponentElement 实现visitChildren

  void visitChildren(ElementVisitor visitor) {
    if (_child != null)
      visitor(_child!);
  }

class SingleChildRenderObjectElement extends RenderObjectElement 实现：

  void visitChildren(ElementVisitor visitor) {
    if (_child != null)
      visitor(_child!);
  }

class MultiChildRenderObjectElement extends RenderObjectElement实现：

  void visitChildren(ElementVisitor visitor) {
    for (final Element child in _children) {
      if (!_forgottenChildren.contains(child))
        visitor(child);
    }
  }

element 的build方法

  Widget build() => (widget as StatelessWidget).build(this);
  Widget build() => state.build(this);
  Widget build() => (widget as ProxyWidget).child;

widget持有子widget

ProxyWidget

abstract class ProxyWidget extends Widget {
  const ProxyWidget({ Key? key, required this.child }) : super(key: key);
  final Widget child;
}

ProxyWidget子类

abstract class ParentDataWidget<T extends ParentData> extends ProxyWidget
abstract class InheritedWidget extends ProxyWidget 
class NotificationListener<T extends Notification> extends ProxyWidget 

4、RenderObject

abstract class RenderObjectElement extends Element

RenderObjectElement类：

  void mount(Element? parent, Object? newSlot) {
    super.mount(parent, newSlot);
 
    _renderObject = (widget as RenderObjectWidget).createRenderObject(this);
    
    attachRenderObject(newSlot);
    _dirty = false;
  }