ReactDOM-diff节点源码解析

  function reconcileSingleElement(returnFiber, currentFirstChild, element) {
    //新的虚拟DOM的key,也就是唯一标准
    const key = element.key;        // null
    let child = currentFirstChild; //老的FunctionComponent对应的fiber
    while (child !== null) {
      //判断此老fiber对应的key和新的虚拟DOM对象的key是否一样 null===null
      if (child.key === key) {
        //判断老fiber对应的类型和新虚拟DOM元素对应的类型是否相同
        if (child.type === element.type) {// p div
          deleteRemainingChildren(returnFiber, child.sibling);
          //如果key一样，类型也一样，则认为此节点可以复用
          const existing = useFiber(child, element.props);
          existing.ref = element.ref;
          existing.return = returnFiber;
          return existing;
        } else {
          //如果找到一key一样老fiber,但是类型不一样，不能此老fiber,把剩下的全部删除
          deleteRemainingChildren(returnFiber, child);
        }
      } else {
        deleteChild(returnFiber, child);
      }
      child = child.sibling;
    }
    //因为我们现实的初次挂载，老节点currentFirstChild肯定是没有的，所以可以直接根据虚拟DOM创建新的Fiber节点
    const created = createFiberFromElement(element);
    created.ref = element.ref;
    created.return = returnFiber;
    return created;
  }

<div>
  <div key='A'>A</div> 
  <div key='B'>B</div>
</div>
<!-- 变化到 -->
<div>
  <div key='A'>C</div>
</div>

<div>
  <div key='A'>A</div> 
  <div key='B'>B</div>
</div>
<!-- 变化到 -->
<div>
  <div key='C'>C</div>
</div>

<div>
  <div key='A'>A</div> 
  <div key='B'>B</div>
</div>
<!-- 变化到 -->
<div>
  <p key='A'>C</p>
</div>

  function reconcileChildrenArray(returnFiber, currentFirstChild, newChildren) {
    let resultingFirstChild = null; //返回的第一个新儿子
    let previousNewFiber = null; //上一个的一个新的儿fiber
    let newIdx = 0;//用来遍历新的虚拟DOM的索引
    let oldFiber = currentFirstChild;//第一个老fiber
    let nextOldFiber = null;//下一个第fiber
    let lastPlacedIndex = 0;//上一个不需要移动的老节点的索引
    // 开始第一轮循环 如果老fiber有值，新的虚拟DOM也有值
    for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
      //先暂下一个老fiber
      nextOldFiber = oldFiber.sibling;
      //试图更新或者试图复用老的fiber
      const newFiber = updateSlot(returnFiber, oldFiber, newChildren[newIdx]);
      if (newFiber === null) {
        break;
      }
      //如果有老fiber,但是新的fiber并没有成功复用老fiber和老的真实DOM，那就删除老fiber,在提交阶段会删除真实DOM
      if (oldFiber && newFiber.alternate === null) {
        deleteChild(returnFiber, oldFiber);
      }
      //指定新fiber的位置
      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
      if (previousNewFiber === null) {
        resultingFirstChild = newFiber;//li(A).sibling=p(B).sibling=>li(C)
      } else {
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
      oldFiber = nextOldFiber
    }
    //新的虚拟DOM已经循环完毕
    if (newIdx === newChildren.length) {
      //删除剩下的老fiber
      deleteRemainingChildren(returnFiber, oldFiber);
      return resultingFirstChild;
    }
    if (oldFiber === null) {
      //如果老的 fiber已经没有了， 新的虚拟DOM还有，进入插入新节点的逻辑
      for (; newIdx < newChildren.length; newIdx++) {
        const newFiber = createChild(returnFiber, newChildren[newIdx]);
        if (newFiber === null) continue;
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        //如果previousNewFiber为null，说明这是第一个fiber
        if (previousNewFiber === null) {
          resultingFirstChild = newFiber; //这个newFiber就是大儿子
        } else {
          //否则说明不是大儿子，就把这个newFiber添加上一个子节点后面
          previousNewFiber.sibling = newFiber;
        }
        //让newFiber成为最后一个或者说上一个子fiber
        previousNewFiber = newFiber;
      }
    }
    // 开始处理移动的情况
    const existingChildren = mapRemainingChildren(returnFiber, oldFiber);
    //开始遍历剩下的虚拟DOM子节点
    for (; newIdx < newChildren.length; newIdx++) {
      const newFiber = updateFromMap(existingChildren, returnFiber, newIdx, newChildren[newIdx]);
      if (newFiber !== null) {
      //如果要跟踪副作用，并且有老fiber
       if (newFiber.alternate !== null) {
         existingChildren.delete(newFiber.key === null ? newIdx : newFiber.key);
       }
        //指定新的fiber存放位置 ，并且给lastPlacedIndex赋值
        lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
        if (previousNewFiber === null) {
          resultingFirstChild = newFiber; //这个newFiber就是大儿子
        } else {
          //否则说明不是大儿子，就把这个newFiber添加上一个子节点后面
          previousNewFiber.sibling = newFiber;
        }
        //让newFiber成为最后一个或者说上一个子fiber
        previousNewFiber = newFiber;
      }
    }
    //等全部处理完后，删除map中所有剩下的老fiber
    existingChildren.forEach(child => deleteChild(returnFiber, child));
    return resultingFirstChild;
  }

<ul key="container">
  <li key="A">A</li>
  <li key="B">B</li>
  <li key="C">C</li>
  <li key="D">D</li>
  <li key="E">E</li>
  <li key="F">F</li>
</ul>
<!-- 变化到 -->
<ul key="container">
  <li key="A">A2</li>
  <li key="C">C2</li>
  <li key="E">E2</li>
  <li key="B">B2</li>
  <li key="G">G</li>
  <li key="D">D2</li>
</ul>

for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
      //先暂下一个老fiber
      nextOldFiber = oldFiber.sibling;
      //试图更新或者试图复用老的fiber
      const newFiber = updateSlot(returnFiber, oldFiber, newChildren[newIdx]);
      if (newFiber === null) {
        break;
      }
      if (shouldTrackSideEffects) {
        //如果有老fiber,但是新的fiber并没有成功复用老fiber和老的真实DOM，那就删除老fiber,在提交阶段会删除真实DOM
        if (oldFiber && newFiber.alternate === null) {
          deleteChild(returnFiber, oldFiber);
        }
      }
      //指定新fiber的位置
      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
      if (previousNewFiber === null) {
        resultingFirstChild = newFiber;//li(A).sibling=p(B).sibling=>li(C)
      } else {
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
      oldFiber = nextOldFiber
    }

if (oldFiber === null) {
  //如果老的 fiber已经没有了， 新的虚拟DOM还有，进入插入新节点的逻辑
  for (; newIdx < newChildren.length; newIdx++) {
    const newFiber = createChild(returnFiber, newChildren[newIdx]);
    if (newFiber === null) continue;
    lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
    //如果previousNewFiber为null，说明这是第一个fiber
    if (previousNewFiber === null) {
      resultingFirstChild = newFiber; //这个newFiber就是大儿子
    } else {
      //否则说明不是大儿子，就把这个newFiber添加上一个子节点后面
      previousNewFiber.sibling = newFiber;
    }
    //让newFiber成为最后一个或者说上一个子fiber
    previousNewFiber = newFiber;
  }
}

// 开始处理移动的情况
const existingChildren = mapRemainingChildren(returnFiber, oldFiber);
//开始遍历剩下的虚拟DOM子节点
for (; newIdx < newChildren.length; newIdx++) {
  const newFiber = updateFromMap(
    existingChildren,
    returnFiber,
    newIdx,
    newChildren[newIdx],
  );
  if (newFiber !== null) {
  //如果要跟踪副作用，并且有老fiber
  if (newFiber.alternate !== null) {
    existingChildren.delete(newFiber.key === null ? newIdx : newFiber.key);
  }
    //指定新的fiber存放位置 ，并且给lastPlacedIndex赋值
    lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);
    if (previousNewFiber === null) {
      resultingFirstChild = newFiber; //这个newFiber就是大儿子
    } else {
      //否则说明不是大儿子，就把这个newFiber添加上一个子节点后面
      previousNewFiber.sibling = newFiber;
    }
    //让newFiber成为最后一个或者说上一个子fiber
    previousNewFiber = newFiber;
  }
}
function mapRemainingChildren(returnFiber, currentFirstChild) {
  const existingChildren = new Map();
  let existingChild = currentFirstChild;
  while (existingChild != null) {
    //如果有key用key,如果没有key使用索引
    if (existingChild.key !== null) {
      existingChildren.set(existingChild.key, existingChild);
    } else {
      existingChildren.set(existingChild.index, existingChild);
    }
    existingChild = existingChild.sibling;
  }
  return existingChildren;
}