React 源码深度解读（五）：首次自定义组件渲染 - Part 2

• 前言

React 是一个十分庞大的库，由于要同时考虑 ReactDom 和 ReactNative ，还有服务器渲染等，导致其代码抽象化程度很高，嵌套层级非常深，阅读其源码是一个非常艰辛的过程。在学习 React 源码的过程中，给我帮助最大的就是这个系列文章，于是决定基于这个系列文章谈一下自己的理解。本文会大量用到原文中的例子，想体会原汁原味的感觉，推荐阅读原文。

本系列文章基于 React 15.4.2 ，以下是本系列其它文章的传送门：
React 源码深度解读（一）：首次 DOM 元素渲染 - Part 1
React 源码深度解读（二）：首次 DOM 元素渲染 - Part 2
React 源码深度解读（三）：首次 DOM 元素渲染 - Part 3
React 源码深度解读（四）：首次自定义组件渲染 - Part 1
React 源码深度解读（五）：首次自定义组件渲染 - Part 2
React 源码深度解读（六）：依赖注入
React 源码深度解读（七）：事务 - Part 1
React 源码深度解读（八）：事务 - Part 2
React 源码深度解读（九）：单个元素更新
React 源码深度解读（十）：Diff 算法详解

• 正文

上一篇文章中，我们讲解到ReactCompositeComponent[ins]被初始化后，App[ins]的 render 方法被调用，生成 ReactElement 树，然后对应的ReactDOMComponent[6]被返回。下面我们来看看这个ReactDOMComponent[6]是如何转化为 DOM 树的。

performInitialMount: function (renderedElement, hostParent,
    hostContainerInfo, transaction, context) {
    ...
    
    // 这里会调用 App 实例的 render 方法，而 render 的返回值是 React.createElement 的嵌套调用。
    if (renderedElement === undefined) {
        renderedElement = this._renderValidatedComponent();
    }

    ...
    
    // 上回讲到这里
    // 返回 ReactDOMComponent[6]
    var child = this._instantiateReactComponent(
        renderedElement,
        nodeType !== ReactNodeTypes.EMPTY /* shouldHaveDebugID */
    );
    
    this._renderedComponent = child;

    // 今天讲这部分
    var markup = ReactReconciler.mountComponent(
        child,
        transaction,
        hostParent,
        hostContainerInfo,
        this._processChildContext(context),
        debugID
    );

    return markup;
},

• ReactDOMComponent[6].mountComponent

ReactReconciler.mountComponent 会触发ReactDOMComponent[6]的 mountComponent 方法，调用栈如下：

...
|~mountComponentIntoNode()                                    |
  |-ReactReconciler.mountComponent()                          |
    |-ReactCompositeComponent[T].mountComponent()             |
      |-ReactCompositeComponent[T].performInitialMount()  upper half
        |-ReactReconciler.mountComponent()                    |
          |-ReactCompositeComponent[ins].mountComponent()     |
            |-this.performInitialMount()                      |
              |-this._renderValidatedComponent()              |
              |-instantiateReactComponent()                  _|_ 
                (we are here)                                 |
              |-ReactDOMComponent[6].mountComponent(          |
                  transaction, // scr: -----> not of interest |
                  hostParent,  // scr: -----> null            |
                  hostContainerInfo,// scr:---------------------> ReactDOMContainerInfo[ins]                                lower half
                  context      // scr: -----> not of interest |
                )                                             |
...

mountComponent: function (
    transaction,
    hostParent,
    hostContainerInfo,
    context
) {
    ...
    
    var mountImage;
    if (transaction.useCreateElement) {
        var ownerDocument = hostContainerInfo._ownerDocument;
        
        ...
        
        // 创建 div 元素
        el = ownerDocument.createElement(this._currentElement.type);
 
        ...
        
        // 设置 attributes
        if (!this._hostParent) {
            DOMPropertyOperations.setAttributeForRoot(el);
        }
        
        // 设置 properties
        this._updateDOMProperties(null, props, transaction);
        
        // 构造 DOM 树
        var lazyTree = DOMLazyTree(el);
        
        // 遍历子节点并创建 DOM 结点
        this._createInitialChildren(transaction, props, context, lazyTree);
        
        mountImage = lazyTree;
    }
    
    ...

    return mountImage;
}

这里主要做的事情有3部分：

1. 创建 DOM 元素
2. 设置 attributes 和 properties
3. 遍历子元素并重复上述过程

这时候的数据结构如下：

流程图：

• _createInitialChildren 遍历子节点并创建 DOM 结点

下面来看一下 _createInitialChildren 的细节：

_createInitialChildren: function (transaction, props, context, lazyTree) {
    // Intentional use of != to avoid catching zero/false.
    var innerHTML = props.dangerouslySetInnerHTML;
    if (innerHTML != null) {
        if (innerHTML.__html != null) {
            DOMLazyTree.queueHTML(lazyTree, innerHTML.__html);
        }
    } else {
        // 如果是 string 或者 number，返回 true
        var contentToUse =
            CONTENT_TYPES[typeof props.children] ? props.children :
            null;
        var childrenToUse = contentToUse != null ? null : props.children;

        // 直接渲染字符串
        if (contentToUse != null) {
            // Avoid setting textContent when the text is empty. In IE11 setting
            // textContent on a text area will cause the placeholder to not
            // show within the textarea until it has been focused and blurred again.
            // https://github.com/facebook/react/issues/6731#issuecomment-254874553
            if (contentToUse !== '') {
                DOMLazyTree.queueText(lazyTree, contentToUse);
            }
        } 
        // 渲染子节点
        else if (childrenToUse != null) {
            var mountImages = this.mountChildren(
                childrenToUse,
                transaction,
                context
            );
            for (var i = 0; i < mountImages.length; i++) {
                DOMLazyTree.queueChild(lazyTree, mountImages[i]);
            }
        }
    }
},

这部分代码十分好懂，就 3 条分支：

1. 设置了 dangerouslySetInnerHTML 属性，直接渲染 HTML
2. 子节点类型为 string 或 number，渲染字符
3. 其它情况就需要将 ReactElement 转换成 ReactDOMComponent 或 ReactCompositeComponent 作进一步的渲染。

DOMLazyTree 的 queueText 和 queueChild 真正有效的都各只有一行代码：

function queueText(tree, text) {
  if (enableLazy) { // scr: NO, I mean, false
    ...
  } else {
    setTextContent(tree.node, text);
  }
}

var setTextContent = function (node, text) {
  if (text) {
    var firstChild = node.firstChild;

  if (firstChild && firstChild === node.lastChild && firstChild.nodeType === 3) { // scr: false
    ...
    }
  }
  node.textContent = text; // scr: the only effective line
};

function queueChild(parentTree, childTree) {
  if (enableLazy) { // scr: again, false
    ...
  } else {
    parentTree.node.appendChild(childTree.node);
  }
}

mountChildren 的调用栈如下：

ReactDOMComponent[6].mountComponent()    <-------------------------|
    (we are here)                                                  |
  |-this._createInitialChildren()                                  |
  ?{1}                                                             |
    |-DOMLazyTree.queueText()                                      |
  ?{2}                                                             |
    |-this.mountChildren()        // scr: ---------------> 1)(a)   |
      |-this._reconcilerInstantiateChildren()                      |
        |-ReactChildReconciler.instantiateChildren()               |
          |-traverseAllChildren()                                  |
            |-traverseAllChildrenImpl()  <------|inner             |
              |↻traverseAllChildrenImpl() ------|recursion         |
                |-instantiateChild()                               |
                  |-instantiateReactComponent()                    |
      |↻ReactDOMComponent.mountComponent()      // scr: -> 1)(b)---|
    |↻DOMLazyTree.queueChild()    // scr: ---------------> 2)

这中间的函数调用逻辑很清晰，最终会走到 traverseAllChildrenImpl 这里：

function traverseAllChildrenImpl(
    children,
    nameSoFar,
    callback,
    traverseContext
) {
    var type = typeof children;

    if (type === 'undefined' || type === 'boolean') {
        // All of the above are perceived as null.
        children = null;
    }

    if (children === null ||
        type === 'string' ||
        type === 'number' ||
        // The following is inlined from ReactElement. This means we can optimize
        // some checks. React Fiber also inlines this logic for similar purposes.
        (type === 'object' && children.$$typeof === REACT_ELEMENT_TYPE)) {
        callback(
            traverseContext,
            children,
            // If it's the only child, treat the name as if it was wrapped in an array
            // so that it's consistent if the number of children grows.
            nameSoFar === '' ? SEPARATOR + getComponentKey(children, 0) :
            nameSoFar
        );
        return 1;
    }

    var child;
    var nextName;
    var subtreeCount = 0; // Count of children found in the current subtree.
    var nextNamePrefix = nameSoFar === '' ? SEPARATOR : nameSoFar +
        SUBSEPARATOR;

    if (Array.isArray(children)) {
        for (var i = 0; i < children.length; i++) {
            child = children[i];
            nextName = nextNamePrefix + getComponentKey(child, i);
            subtreeCount += traverseAllChildrenImpl(
                child,
                nextName,
                callback,
                traverseContext
            );
        }
    } else {
       ...
    }

    return subtreeCount;
}

这里的逻辑很简单，如果 children 不是数组，则调用回调函数；如果是数组，则继续调用自身，相当于深度优先遍历。这里的回调函数就是 ReactChildReconciler 中的 instantiateChild：

function instantiateChild(childInstances, child, name, selfDebugID) {
    ...
    
    if (child != null && keyUnique) {
        childInstances[name] = instantiateReactComponent(child, true);
    }
}

这里直接调用 instantiateReactComponent，创建ReactDOMComponent。所有的ReactDOMComponent的创建顺序如下：

ReactDOMComponent[6].mountComponent()
  |-this._createInitialChildren()
    |-this.mountChildren() 
...           |↻instantiateReactComponent()[4,5]
      |-ReactDOMComponent[5].mountComponent()
        |-this._createInitialChildren()
          |-node.textContent = text; // scr: [5] done
      |-ReactDOMComponent[4].mountComponent()
        |-this._createInitialChildren()
          |-this.mountChildren() 
...                 |↻instantiateReactComponent()[2,3]
          |-ReactDOMComponent[2].mountComponent() // scr: [2] done
          |-ReactDOMComponent[3].mountComponent()
            |-this._createInitialChildren()
              |-node.textContent = text; // scr: [3] done
        |↻node[4].appendChild()[2,3] // scr: [4] done

    |↻node[6].appendChild()[4,5] // scr: [6] done

完成的流程图：