一 Element 概念

这个玩意的概念。到底是什么 ？

官方解释是在树中特定位置的实例。

二 继承关系

element 有 ComponentElement 和 RenderObjectElement  之分

1 ComponentElement

class StatelessElement extends ComponentElement

class StatefulElement extends ComponentElement

三  生命周期

1 framework 通过在将要被用来作为Element的初始配置的widget 上调用其createElement 方法来创建一个element

2 framework 通过调用mount 方法 将一个新创建的element 加入树中给定的父节点的插槽下面。

mount 方法负责注入任何child widgets,并且会在有需要·的时候，会调用attachRenderObject 

将关联的render objects  添加到渲染树中 render tree 中。到这一步的时候，element 会进入active 状态，并且会显示在屏幕上方。

四 方法分析

Element 这个抽象类中有一个方法 叫做 mount 方法 。

  /// Add this element to the tree in the given slot of the given parent.////// The framework calls this function when a newly created element is added to/// the tree for the first time. Use this method to initialize state that/// depends on having a parent. State that is independent of the parent can/// more easily be initialized in the constructor.////// This method transitions the element from the "initial" lifecycle state to/// the "active" lifecycle state.////// Subclasses that override this method are likely to want to also override/// [update], [visitChildren], [RenderObjectElement.insertRenderObjectChild],/// [RenderObjectElement.moveRenderObjectChild], and/// [RenderObjectElement.removeRenderObjectChild].////// Implementations of this method should start with a call to the inherited/// method, as in `super.mount(parent, newSlot)`.@mustCallSupervoid 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) {// Only assign ownership if the parent is non-null. If parent is null// (the root node), the owner should have already been assigned.// See RootRenderObjectElement.assignOwner()._owner = parent.owner;}assert(owner != null);final Key? key = widget.key;if (key is GlobalKey) {owner!._registerGlobalKey(key, this);}_updateInheritance();attachNotificationTree();}

renderObjectElement 的mount 方法

其主要作用 将element相关联的renderObject插入到渲染树中，插入到渲染树后的element就处于“active”状态，处于“active”状态后就可以显示在屏幕上了。

此处可以看出来,RenderObject? _renderObject;  element 是持有renderObject 的引用的

  @overridevoid 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;}