React源碼解析-首次渲染（純DOM元素）II

上一篇文章中，介紹了頂層對象ReactCompositeComponent[T]是如何構造的，接下來我們看看 batchedMountComponentIntoNode 做了什麼事情。

本文將要講解的調用棧是下面這個樣子的：

|=ReactMount.render(nextElement, container, callback)     ___
|=ReactMount._renderSubtreeIntoContainer()                 |
  |-ReactMount._renderNewRootComponent()                   |
    |-instantiateReactComponent()                          |
    |~batchedMountComponentIntoNode()                  upper half
      |~mountComponentIntoNode()                       (平臺無關)
        |-ReactReconciler.mountComponent()                 |
          |-ReactCompositeComponent.mountComponent()       |
          |-ReactCompositeComponent.performInitialMount()  |
            |-instantiateReactComponent()                 _|_
            |-ReactDOMComponent.mountComponent()       lower half
        |-_mountImageIntoNode()                      (HTML DOM 相關)
                                                          _|_

如果看源碼，我們會留意到很多transaction相關的代碼，我們暫時先將其忽略，會在後續的文章中進行講解。暫時可以理解爲調用transaction.perform時，實際上就是對第一個參數進行函數調用。跳過一些模版代碼後，實際上做事情的是 mountComponentIntoNode 這個方法。

// 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 用於創建 DOM 元素，而 ReactMount._mountImageIntoNode 則是將剛創建的 DOM 元素掛載到頁面。ReactReconciler.mountComponent 會調用 ReactCompositeComponent[T]的 mountComponent 方法。在看 mountComponent 方法前，還需要先準備好 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;
}

現在各實例間的關係是這樣的：

再繼續看 mountComponent 方法：

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

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

    // this._currentElement 爲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 實例
    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 實例
    var inst = this._instance;

    ...
    
    // If not a stateless component, we now render
    if (renderedElement === undefined) {
        // 返回值爲 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;
},

當運行到var child = this._instantiateReactComponent時，就會調用上篇文章說到的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 爲 ‘h1’
        if (typeof element.type === 'string') {
            instance = ReactHostComponent.createInternalComponent(element);
        } 

    return instance;
}

ReactDom 會在執行的時候，執行ReactDefaultInjection.inject()將 ReactDOMComponent 注入到 ReactHostComponent 中，ReactHostComponent.createInternalComponent 最終會調用 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;
}

我們將返回的實例命名爲 ReactDOMComponent[ins]。

ReactReconciler.mountComponent 會調用 ReactDomComponent 的 mountComponent 方法，這就會涉及到 HTML DOM 相關的內容，我們在下一篇進行講解。

現在我們來看一下各實例間的關係：

目前爲止的調用棧：

|=ReactMount.render(nextElement, container, callback)     ___
|=ReactMount._renderSubtreeIntoContainer()                 |
  |-ReactMount._renderNewRootComponent()                   |
    |-instantiateReactComponent()                          |
    |~batchedMountComponentIntoNode()                  upper half
      |~mountComponentIntoNode()                       (平臺無關)
        |-ReactReconciler.mountComponent()                 |
          |-ReactCompositeComponent.mountComponent()       |
          |-ReactCompositeComponent.performInitialMount()  |
            |-instantiateReactComponent()                 _|_
            |-ReactDOMComponent.mountComponent()       lower half
        |-_mountImageIntoNode()                 (HTML DOM 相關下一篇講解)
                                                          _|_