Flutter 了解 Element

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)`.
  @mustCallSuper
  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) {
      // 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

  @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;
  }