Flutter Element概念简明分析

Flutter 中 Element 作用的是作为中枢来管理和调度Widget和RenderObject,这里我们主要说一下RenderObjectWidget 来主要说一下Element 的生命周期,这里我删除了一些assert 的方法,方便查看

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

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; }

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