React Diff算法的核心原理和源码简版实现

React Diff算法的核心原理和源码简版实现。 由于react源码实在太重了，大佬们先看源码，后面我理清楚再一一解析：

/**
 * React Diff算法的核心实现
 */

// 表示差异类型的常量
const PLACEMENT = 'PLACEMENT';     // 新增
const DELETION = 'DELETION';       // 删除
const UPDATE = 'UPDATE';          // 更新

// 表示工作单元类型
const TAG_ROOT = 'TAG_ROOT';           // 根节点
const TAG_HOST = 'TAG_HOST';           // 原生DOM节点
const TAG_TEXT = 'TAG_TEXT';           // 文本节点
const TAG_FUNCTION = 'TAG_FUNCTION';   // 函数组件

/**
 * Fiber节点的数据结构
 */
class FiberNode {
  constructor(tag, pendingProps, key) {
    this.tag = tag;                // 标记节点类型
    this.key = key;                // 节点的key属性
    this.type = null;              // 节点的类型
    this.stateNode = null;         // 对应的真实DOM节点

    // Fiber树相关
    this.return = null;            // 指向父Fiber节点
    this.child = null;             // 指向第一个子Fiber节点
    this.sibling = null;           // 指向下一个兄弟Fiber节点
    this.index = 0;                // 在父节点下的索引

    this.pendingProps = pendingProps;  // 新的属性
    this.memoizedProps = null;         // 旧的属性
    this.updateQueue = null;           // 更新队列
    this.memoizedState = null;         // 旧的状态

    // 副作用相关
    this.flags = 0;                    // 标记节点的副作用
    this.subtreeFlags = 0;             // 子树的副作用标记
    this.deletions = null;             // 需要删除的节点
    this.alternate = null;             // 指向旧树的对应节点
  }
}

/**
 * 创建Fiber节点
 */
function createFiber(tag, pendingProps, key) {
  return new FiberNode(tag, pendingProps, key);
}

/**
 * 创建工作进行中的Fiber节点
 */
function createWorkInProgress(current, pendingProps) {
  let workInProgress = current.alternate;
  
  if (workInProgress === null) {
    workInProgress = createFiber(
      current.tag,
      pendingProps,
      current.key
    );
    
    workInProgress.type = current.type;
    workInProgress.stateNode = current.stateNode;
    workInProgress.alternate = current;
    current.alternate = workInProgress;
  } else {
    workInProgress.pendingProps = pendingProps;
    workInProgress.type = current.type;
    workInProgress.flags = 0;
    workInProgress.subtreeFlags = 0;
    workInProgress.deletions = null;
  }

  workInProgress.child = current.child;
  workInProgress.memoizedProps = current.memoizedProps;
  workInProgress.memoizedState = current.memoizedState;
  workInProgress.updateQueue = current.updateQueue;
  workInProgress.sibling = current.sibling;
  workInProgress.index = current.index;

  return workInProgress;
}

/**
 * 协调子节点
 * @param {FiberNode} returnFiber 父Fiber节点
 * @param {FiberNode} currentFirstChild 当前第一个子节点
 * @param {Array} newChildren 新的子节点数组
 */
function reconcileChildrenArray(returnFiber, currentFirstChild, newChildren) {
  let resultingFirstChild = null;    // 返回的第一个子节点
  let previousNewFiber = null;       // 上一个新建的Fiber节点
  let oldFiber = currentFirstChild;  // 当前正在处理的旧Fiber节点
  let lastPlacedIndex = 0;          // 最后一个可复用节点的位置
  let newIdx = 0;                   // 新子节点的索引
  let nextOldFiber = null;          // 下一个旧Fiber节点

  // 第一轮遍历：处理更新的节点
  for (; oldFiber !== null && newIdx < newChildren.length; newIdx++) {
    nextOldFiber = oldFiber.sibling;

    const newFiber = updateSlot(
      returnFiber,
      oldFiber,
      newChildren[newIdx]
    );

    if (newFiber === null) {
      break;
    }

    if (oldFiber && newFiber.alternate === null) {
      deleteChild(returnFiber, oldFiber);
    }

    lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);

    if (previousNewFiber === null) {
      resultingFirstChild = newFiber;
    } else {
      previousNewFiber.sibling = newFiber;
    }
    previousNewFiber = newFiber;
    oldFiber = nextOldFiber;
  }

  // 处理剩余的新子节点
  if (newIdx === newChildren.length) {
    deleteRemainingChildren(returnFiber, oldFiber);
    return resultingFirstChild;
  }

  // 处理剩余的旧子节点
  if (oldFiber === null) {
    for (; newIdx < newChildren.length; newIdx++) {
      const newFiber = createChild(
        returnFiber,
        newChildren[newIdx]
      );

      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);

      if (previousNewFiber === null) {
        resultingFirstChild = newFiber;
      } else {
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
    }
    return resultingFirstChild;
  }

  // 将剩余的旧节点放入Map中
  const existingChildren = mapRemainingChildren(returnFiber, oldFiber);

  // 遍历剩余的新子节点，尝试复用旧节点
  for (; newIdx < newChildren.length; newIdx++) {
    const newFiber = updateFromMap(
      existingChildren,
      returnFiber,
      newIdx,
      newChildren[newIdx]
    );

    if (newFiber !== null) {
      if (newFiber.alternate !== null) {
        existingChildren.delete(
          newFiber.key === null ? newIdx : newFiber.key
        );
      }

      lastPlacedIndex = placeChild(newFiber, lastPlacedIndex, newIdx);

      if (previousNewFiber === null) {
        resultingFirstChild = newFiber;
      } else {
        previousNewFiber.sibling = newFiber;
      }
      previousNewFiber = newFiber;
    }
  }

  // 删除Map中剩余的旧节点
  existingChildren.forEach(child => deleteChild(returnFiber, child));

  return resultingFirstChild;
}

/**
 * 更新节点
 */
function updateSlot(returnFiber, oldFiber, newChild) {
  const key = oldFiber !== null ? oldFiber.key : null;

  if (typeof newChild === 'object' && newChild !== null) {
    switch (newChild.$$typeof) {
      case REACT_ELEMENT_TYPE: {
        if (newChild.key === key) {
          return updateElement(returnFiber, oldFiber, newChild);
        }
        return null;
      }
      default:
        return null;
    }
  }

  return null;
}

/**
 * 更新元素节点
 */
function updateElement(returnFiber, current, element) {
  const elementType = element.type;

  if (current !== null) {
    if (current.type === elementType) {
      // 可以复用当前节点
      const existing = useFiber(current, element.props);
      existing.return = returnFiber;
      return existing;
    }
  }

  // 创建新的Fiber节点
  const created = createFiberFromElement(element, returnFiber.mode);
  created.return = returnFiber;
  return created;
}

/**
 * 复用Fiber节点
 */
function useFiber(fiber, pendingProps) {
  const clone = createWorkInProgress(fiber, pendingProps);
  clone.index = 0;
  clone.sibling = null;
  return clone;
}

/**
 * 从元素创建Fiber节点
 */
function createFiberFromElement(element, mode) {
  const fiber = createFiberFromTypeAndProps(
    element.type,
    element.key,
    element.props,
    mode
  );
  return fiber;
}

/**
 * 放置子节点
 */
function placeChild(newFiber, lastPlacedIndex, newIndex) {
  newFiber.index = newIndex;

  if (newFiber.alternate !== null) {
    const oldIndex = newFiber.alternate.index;
    if (oldIndex < lastPlacedIndex) {
      // 需要移动
      newFiber.flags |= Placement;
      return lastPlacedIndex;
    } else {
      return oldIndex;
    }
  } else {
    // 新节点，需要插入
    newFiber.flags |= Placement;
    return lastPlacedIndex;
  }
}

/**
 * 将剩余的子节点映射到Map中
 */
function mapRemainingChildren(returnFiber, currentFirstChild) {
  const existingChildren = new Map();

  let existingChild = currentFirstChild;
  while (existingChild !== null) {
    if (existingChild.key !== null) {
      existingChildren.set(existingChild.key, existingChild);
    } else {
      existingChildren.set(existingChild.index, existingChild);
    }
    existingChild = existingChild.sibling;
  }

  return existingChildren;
}

/**
 * 从Map中更新节点
 */
function updateFromMap(
  existingChildren,
  returnFiber,
  newIdx,
  newChild
) {
  if (typeof newChild === 'object' && newChild !== null) {
    switch (newChild.$$typeof) {
      case REACT_ELEMENT_TYPE: {
        const matchedFiber = existingChildren.get(
          newChild.key === null ? newIdx : newChild.key
        );
        return updateElement(returnFiber, matchedFiber, newChild);
      }
    }
  }

  return null;
}

/**
 * 删除子节点
 */
function deleteChild(returnFiber, childToDelete) {
  if (!returnFiber.deletions) {
    returnFiber.deletions = [childToDelete];
    returnFiber.flags |= ChildDeletion;
  } else {
    returnFiber.deletions.push(childToDelete);
  }
}

/**
 * 删除剩余的子节点
 */
function deleteRemainingChildren(returnFiber, currentFirstChild) {
  let childToDelete = currentFirstChild;
  while (childToDelete !== null) {
    deleteChild(returnFiber, childToDelete);
    childToDelete = childToDelete.sibling;
  }
}

/**
 * 标记节点删除
 */
function markChildDeletion(returnFiber, childToDelete) {
  childToDelete.flags |= Deletion;
  returnFiber.flags |= ChildDeletion;
}

/**
 * 完成工作单元
 */
function completeWork(current, workInProgress) {
  const newProps = workInProgress.pendingProps;
  
  switch (workInProgress.tag) {
    case HostComponent: {
      // 处理DOM节点
      if (current !== null && workInProgress.stateNode != null) {
        // 更新DOM节点
        updateHostComponent(current, workInProgress, newProps);
      } else {
        // 创建DOM节点
        const instance = createInstance(workInProgress.type, newProps);
        appendAllChildren(instance, workInProgress);
        workInProgress.stateNode = instance;
      }
      break;
    }
    case HostText: {
      // 处理文本节点
      if (current !== null && workInProgress.stateNode != null) {
        // 更新文本内容
        updateHostText(current, workInProgress);
      } else {
        // 创建文本节点
        workInProgress.stateNode = createTextInstance(newProps);
      }
      break;
    }
  }

  bubbleProperties(workInProgress);
}

/**
 * 冒泡属性
 */
function bubbleProperties(completedWork) {
  let subtreeFlags = NoFlags;
  let child = completedWork.child;

  while (child !== null) {
    subtreeFlags |= child.subtreeFlags;
    subtreeFlags |= child.flags;
    child.return = completedWork;
    child = child.sibling;
  }

  completedWork.subtreeFlags |= subtreeFlags;
}

以上代码实现了React Diff算法的核心功能，主要包括：

1. Fiber节点的数据结构定义
2. 子节点的协调过程
3. 节点的更新、创建和删除
4. 节点属性的处理
5. 完成阶段的处理

主要的Diff策略包括：

1. 同级比较：只对同一层级的节点进行比较
2. 类型判断：不同类型的节点会直接替换
3. key值比较：通过key值来判断节点是否可以复用
4. 位置移动：通过索引比较来确定节点是否需要移动

这个实现涵盖了React Diff算法的主要部分，包括单节点Diff和多节点Diff的处理逻辑。代码中包含详细的中文注释，方便理解每个部分的功能。

需要注意的是，这只是React Diff算法的核心实现，实际的React源码中还有更多的细节处理和优化。比如：

1. 优先级调度
2. 中断和恢复
3. 副作用的收集和处理
4. 特殊节点的处理（如Portal、Fragment等）
5. 错误边界的处理

这些都是构成完整的React协调过程的重要部分。