_updateChildren: function(nextNestedChildrenElements, transaction, context) {
    var prevChildren = this._renderedChildren;
    var removedNodes = {};
    var mountImages = [];
    // 获取新的子元素数组
    var nextChildren = this._reconcilerUpdateChildren(
      prevChildren,
      nextNestedChildrenElements,
      mountImages,
      removedNodes,
      transaction,
      context
    );
    if (!nextChildren && !prevChildren) {
      return;
    }
    var updates = null;
    var name;
    var nextIndex = 0;
    var lastIndex = 0;
    var nextMountIndex = 0;
    var lastPlacedNode = null;
    for (name in nextChildren) {
      if (!nextChildren.hasOwnProperty(name)) {
        continue;
      }
      var prevChild = prevChildren && prevChildren[name];
      var nextChild = nextChildren[name];
      if (prevChild === nextChild) {
        // 同一个引用，说明是使用的同一个component,所以我们需要做移动的操作
        // 移动已有的子节点
        // NOTICE：这里根据nextIndex, lastIndex决定是否移动
        updates = enqueue(
          updates,
          this.moveChild(prevChild, lastPlacedNode, nextIndex, lastIndex)
        );
        // 更新lastIndex
        lastIndex = Math.max(prevChild._mountIndex, lastIndex);
        // 更新component的.mountIndex属性
        prevChild._mountIndex = nextIndex;
      } else {
        if (prevChild) {
          // 更新lastIndex
          lastIndex = Math.max(prevChild._mountIndex, lastIndex);
        }

        // 添加新的子节点在指定的位置上
        updates = enqueue(
          updates,
          this._mountChildAtIndex(
            nextChild,
            mountImages[nextMountIndex],
            lastPlacedNode,
            nextIndex,
            transaction,
            context
          )
        );
        nextMountIndex++;
      }
      // 更新nextIndex
      nextIndex++;
      lastPlacedNode = ReactReconciler.getHostNode(nextChild);
    }
    // 移除掉不存在的旧子节点，和旧子节点和新子节点不同的旧子节点
    for (name in removedNodes) {
      if (removedNodes.hasOwnProperty(name)) {
        updates = enqueue(
          updates,
          this._unmountChild(prevChildren[name], removedNodes[name])
        );
      }
    }
  }