上一篇文章中，介绍了顶层对象ReactCompositeComponent[T]是如何构造的，接下来我们看看 batchedMountComponentIntoNode 做了什么事情。

本文将要讲解的调用栈是下面这个样子的：

|=ReactMount.render(nextElement, container, callback) ___
|=ReactMount._renderSubtreeIntoContainer() |
 |-ReactMount._renderNewRootComponent() |
 |-instantiateReactComponent() |
 |~batchedMountComponentIntoNode() upper half
 |~mountComponentIntoNode() (平台无关)
 |-ReactReconciler.mountComponent() |
 |-ReactCompositeComponent.mountComponent() |
 |-ReactCompositeComponent.performInitialMount() |
 |-instantiateReactComponent() _|_
 |-ReactDOMComponent.mountComponent() lower half
 |-_mountImageIntoNode() (HTML DOM 相关)
 _|_

如果看源码，我们会留意到很多transaction相关的代码，我们暂时先将其忽略，会在后续的文章中进行讲解。暂时可以理解为调用transaction.perform时，实际上就是对第一个参数进行函数调用。跳过一些模版代码后，实际上做事情的是 mountComponentIntoNode 这个方法。

// 文件位置：src/renderers/dom/client/ReactMount.js

function mountComponentIntoNode(
 wrapperInstance, // ReactCompositeComponent[T]
 container, // document.getElementById("root")
 transaction,
 shouldReuseMarkup,
 context
) {
 ...
 
 var markup = ReactReconciler.mountComponent(
 wrapperInstance,
 transaction,
 null,
 ReactDOMContainerInfo(wrapperInstance, container),
 context,
 0 /* parentDebugID */
 );

 ...
 
 ReactMount._mountImageIntoNode(
 markup,
 container,
 wrapperInstance,
 shouldReuseMarkup,
 transaction
 );
}

ReactReconciler.mountComponent 用于创建 DOM 元素，而 ReactMount._mountImageIntoNode 则是将刚创建的 DOM 元素挂载到页面。ReactReconciler.mountComponent 会调用 ReactCompositeComponent[T]的 mountComponent 方法。在看 mountComponent 方法前，还需要先准备好 hostContainerInfo，由 ReactDOMContainerInfo 生成：

// 文件位置：src/renderers/shared/stack/reconciler/ReactContainerInfo.js

function ReactDOMContainerInfo(
 topLevelWrapper, // ReactCompositeComponent[T]
 node // document.getElementById("root")
) {
 var info = {
 _topLevelWrapper: topLevelWrapper,
 _idCounter: 1,
 _ownerDocument: node ?
 node.nodeType === DOC_NODE_TYPE ? node : node.ownerDocument : null,
 _node: node,
 _tag: node ? node.nodeName.toLowerCase() : null,
 _namespaceURI: node ? node.namespaceURI : null,
 };
 
 ...
 
 return info;
}

现在各实例间的关系是这样的：

再继续看 mountComponent 方法：

// 文件位置：src/renderers/shared/stack/reconciler/ReactCompositeComponent.js

mountComponent: function (
 transaction,
 hostParent,
 hostContainerInfo,
 context
) {
 ...

 // this._currentElement 为ReactElement[2](TopLevelWrapper)
 var publicProps = this._currentElement.props;
 var publicContext = this._processContext(context);

 // TopLevelWrapper
 var Component = this._currentElement.type;

 ...

 // Initialize the public class
 var doConstruct = shouldConstruct(Component);
 
 // 生成TopLevelWrapper 实例
 var inst = this._constructComponent(
 doConstruct,
 publicProps,
 publicContext,
 updateQueue
 );
 
 ...

 var markup;
 
 ...
 
 markup = this.performInitialMount(renderedElement,
 hostParent, hostContainerInfo, transaction, context

 ...

 return markup;
},

performInitialMount: function (renderedElement, hostParent,
 hostContainerInfo, transaction, context) {
 
 // TopLevelWrapper 实例
 var inst = this._instance;

 ...
 
 // If not a stateless component, we now render
 if (renderedElement === undefined) {
 // 返回值为 ReactElement[1]
 renderedElement = this._renderValidatedComponent();
 }

 // 返回 ReactNodeTypes.HOST
 var nodeType = ReactNodeTypes.getType(renderedElement);
 
 this._renderedNodeType = nodeType;
 
 // instantiateReactComponent.js
 var child = this._instantiateReactComponent(
 renderedElement,
 nodeType !== ReactNodeTypes.EMPTY /* shouldHaveDebugID */
 );
 this._renderedComponent = child;

 var markup = ReactReconciler.mountComponent(
 child,
 transaction,
 hostParent,
 hostContainerInfo,
 this._processChildContext(context),
 debugID
 );

 ...

 return markup;
},

当运行到var child = this._instantiateReactComponent时，就会调用上篇文章说到的instantiateReactComponent文件：

// 文件位置：src/renderers/shared/stack/reconciler/instantiateReactComponent.js

function instantiateReactComponent(node, shouldHaveDebugID) {
 var instance;

 ...
 
 } else if (typeof node === 'object') {
 ...

 // element.type 为 ‘h1’
 if (typeof element.type === 'string') {
 instance = ReactHostComponent.createInternalComponent(element);
 } 

 return instance;
}

ReactDom 会在执行的时候，执行ReactDefaultInjection.inject()将 ReactDOMComponent 注入到 ReactHostComponent 中，ReactHostComponent.createInternalComponent 最终会调用 ReactDOMComponent：

// 文件位置：src/renderers/dom/shared/ReactDomComponent.js

function ReactDOMComponent(element) {
 // h1
 var tag = element.type;
 
 validateDangerousTag(tag);
 
 // ReactElement[1]
 this._currentElement = element;
 
 this._tag = tag.toLowerCase();
 this._namespaceURI = null;
 this._renderedChildren = null;
 this._previousStyle = null;
 this._previousStyleCopy = null;
 this._hostNode = null;
 this._hostParent = null;
 this._rootNodeID = 0;
 this._domID = 0;
 this._hostContainerInfo = null;
 this._wrapperState = null;
 this._topLevelWrapper = null;
 this._flags = 0;
}

我们将返回的实例命名为 ReactDOMComponent[ins]。

ReactReconciler.mountComponent 会调用 ReactDomComponent 的 mountComponent 方法，这就会涉及到 HTML DOM 相关的内容，我们在下一篇进行讲解。

现在我们来看一下各实例间的关系：

目前为止的调用栈：

|=ReactMount.render(nextElement, container, callback) ___
|=ReactMount._renderSubtreeIntoContainer() |
 |-ReactMount._renderNewRootComponent() |
 |-instantiateReactComponent() |
 |~batchedMountComponentIntoNode() upper half
 |~mountComponentIntoNode() (平台无关)
 |-ReactReconciler.mountComponent() |
 |-ReactCompositeComponent.mountComponent() |
 |-ReactCompositeComponent.performInitialMount() |
 |-instantiateReactComponent() _|_
 |-ReactDOMComponent.mountComponent() lower half
 |-_mountImageIntoNode() (HTML DOM 相关下一篇讲解)
 _|_