深入理解 React 18 并发特性的实现原理，包括 Concurrent Mode、startTransition、useTransition 等

并发特性分析

React 18 引入了并发特性（Concurrent Features），这是 React 架构的一次重大升级。本文将深入分析这些新特性的实现原理。

🎯 核心概念

什么是并发特性？

并发特性允许 React 同时准备多个版本的 UI，并能够中断和恢复渲染工作。这使得 React 能够：

优先处理用户交互：确保用户输入得到及时响应
中断低优先级更新：为高优先级任务让路
并行处理多个更新：提高整体性能

并发模式 vs 传统模式

// 传统模式：同步渲染
function TraditionalComponent() {
  const [count, setCount] = useState(0);
  
  const handleClick = () => {
    setCount(c => c + 1); // 同步更新，可能阻塞 UI
  };
  
  return <button onClick={handleClick}>{count}</button>;
}

// 并发模式：可中断渲染
function ConcurrentComponent() {
  const [count, setCount] = useState(0);
  const [isPending, startTransition] = useTransition();
  
  const handleClick = () => {
    startTransition(() => {
      setCount(c => c + 1); // 可中断的更新
    });
  };
  
  return (
    <button onClick={handleClick}>
      {count} {isPending && '(更新中...)'}
    </button>
  );
}

🔍 源码实现分析

1. 并发模式检测

// packages/react-reconciler/src/ReactFiberWorkLoop.js

function requestUpdateLane(fiber: Fiber): Lane {
  const mode = fiber.mode;
  
  if ((mode & ConcurrentMode) === NoMode) {
    // 非并发模式，使用同步优先级
    return SyncLane;
  }
  
  const eventTime = requestEventTime();
  const schedulerPriority = getCurrentPriorityLevel();
  
  switch (schedulerPriority) {
    case ImmediatePriority:
      return SyncLane;
    case UserBlockingPriority:
      return InputContinuousLane;
    case NormalPriority:
      return DefaultLane;
    case LowPriority:
      return TransitionLane;
    case IdlePriority:
      return IdleLane;
    default:
      return DefaultLane;
  }
}

2. startTransition 实现

// packages/react-reconciler/src/ReactFiberWorkLoop.js

function startTransition(scope: () => void) {
  const prevTransition = ReactCurrentBatchConfig.transition;
  ReactCurrentBatchConfig.transition = 1;
  
  try {
    scope();
  } finally {
    ReactCurrentBatchConfig.transition = prevTransition;
  }
}

// 在调度器中处理 transition 更新
function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
): FiberRoot | null {
  const root = markUpdateLaneFromFiberToRoot(fiber, lane);
  
  if (lane === SyncLane) {
    // 同步更新，立即执行
    ensureRootIsScheduled(root, eventTime);
    schedulePendingInteractions(root, lane);
  } else {
    // 并发更新，可以中断
    if (root === null) {
      return null;
    }
    
    ensureRootIsScheduled(root, eventTime);
    schedulePendingInteractions(root, lane);
  }
  
  return root;
}

3. useTransition Hook 实现

// packages/react-reconciler/src/ReactFiberHooks.js

function useTransition(): [boolean, (callback: () => void) => void] {
  const dispatcher = resolveDispatcher();
  return dispatcher.useTransition();
}

function mountTransition(): [boolean, (callback: () => void) => void] {
  const [isPending, setPending] = mountState(false);
  const start = useCallback((callback: () => void) => {
    setPending(true);
    const prevTransition = ReactCurrentBatchConfig.transition;
    ReactCurrentBatchConfig.transition = 1;
    
    try {
      callback();
    } finally {
      ReactCurrentBatchConfig.transition = prevTransition;
      setPending(false);
    }
  }, []);
  
  return [isPending, start];
}

function updateTransition(): [boolean, (callback: () => void) => void] {
  const [isPending] = updateState(false);
  const start = useCallback((callback: () => void) => {
    setPending(true);
    const prevTransition = ReactCurrentBatchConfig.transition;
    ReactCurrentBatchConfig.transition = 1;
    
    try {
      callback();
    } finally {
      ReactCurrentBatchConfig.transition = prevTransition;
      setPending(false);
    }
  }, []);
  
  return [isPending, start];
}

4. useDeferredValue Hook 实现

// packages/react-reconciler/src/ReactFiberHooks.js

function useDeferredValue<T>(value: T): T {
  const dispatcher = resolveDispatcher();
  return dispatcher.useDeferredValue(value);
}

function mountDeferredValue<T>(value: T): T {
  const hook = mountWorkInProgressHook();
  const [resolvedValue, setResolvedValue] = mountState(value);
  
  hook.memoizedState = value;
  
  useEffect(() => {
    const prevTransition = ReactCurrentBatchConfig.transition;
    ReactCurrentBatchConfig.transition = 1;
    
    try {
      setResolvedValue(value);
    } finally {
      ReactCurrentBatchConfig.transition = prevTransition;
    }
  }, [value]);
  
  return resolvedValue;
}

function updateDeferredValue<T>(value: T): T {
  const hook = updateWorkInProgressHook();
  const [resolvedValue, setResolvedValue] = updateState(hook.memoizedState);
  
  if (!Object.is(hook.memoizedState, value)) {
    // 值发生变化，安排延迟更新
    const prevTransition = ReactCurrentBatchConfig.transition;
    ReactCurrentBatchConfig.transition = 1;
    
    try {
      setResolvedValue(value);
    } finally {
      ReactCurrentBatchConfig.transition = prevTransition;
    }
  }
  
  hook.memoizedState = value;
  return resolvedValue;
}

5. 时间切片实现

// packages/scheduler/src/Scheduler.js

function workLoop(hasTimeRemaining: boolean, initialTime: number) {
  let currentTime = initialTime;
  advanceTimers(currentTime);
  currentTask = peek(taskQueue);
  
  while (currentTask !== null && !isHostCallbackScheduled) {
    if (currentTask.expirationTime > currentTime) {
      if (!hasTimeRemaining) {
        // 时间片用完，让出控制权
        break;
      }
    } else {
      const callback = currentTask.callback;
      if (typeof callback === 'function') {
        currentTask.callback = null;
        currentPriorityLevel = currentTask.priorityLevel;
        const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
        
        const continuationCallback = callback(didUserCallbackTimeout);
        currentTime = getCurrentTime();
        
        if (typeof continuationCallback === 'function') {
          currentTask.callback = continuationCallback;
        } else {
          if (currentTask === peek(taskQueue)) {
            pop(taskQueue);
          }
        }
        advanceTimers(currentTime);
      } else {
        pop(taskQueue);
      }
    }
    currentTask = peek(taskQueue);
  }
  
  if (currentTask !== null) {
    return true; // 还有任务需要处理
  } else {
    const firstTimer = peek(timerQueue);
    if (firstTimer !== null) {
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }
    return false;
  }
}

6. 优先级调度

// packages/react-reconciler/src/ReactFiberWorkLoop.js

function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  const existingCallbackNode = root.callbackNode;
  
  // 标记过期的 lanes
  markStarvedLanesAsExpired(root, currentTime);
  
  // 获取下一个 lanes
  const nextLanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  
  if (nextLanes === NoLanes) {
    if (existingCallbackNode !== null) {
      cancelCallback(existingCallbackNode);
    }
    root.callbackNode = null;
    root.callbackPriority = NoLane;
    return;
  }
  
  // 获取新的回调优先级
  const newCallbackPriority = getHighestPriorityLane(nextLanes);
  const existingCallbackPriority = root.callbackPriority;
  
  if (existingCallbackPriority === newCallbackPriority) {
    return;
  }
  
  // 取消现有的回调
  if (existingCallbackNode != null) {
    cancelCallback(existingCallbackNode);
  }
  
  // 调度新的回调
  let newCallbackNode;
  if (newCallbackPriority === SyncLane) {
    // 同步优先级，立即执行
    scheduleSyncCallback(performSyncWorkOnRoot.bind(null, root));
    scheduleMicrotask(flushSyncCallbackQueue);
    newCallbackNode = null;
  } else {
    // 并发优先级，使用调度器
    const schedulerPriorityLevel = lanesToSchedulerPriority(newCallbackPriority);
    newCallbackNode = scheduleCallback(
      schedulerPriorityLevel,
      performConcurrentWorkOnRoot.bind(null, root),
    );
  }
  
  root.callbackNode = newCallbackNode;
  root.callbackPriority = newCallbackPriority;
}

🎯 实际应用示例

1. 使用 startTransition

function SearchResults({ query }) {
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();
  
  useEffect(() => {
    if (query) {
      startTransition(() => {
        // 这个更新可以被中断
        fetchSearchResults(query).then(setResults);
      });
    }
  }, [query]);
  
  return (
    <div>
      {isPending && <div>搜索中...</div>}
      <ul>
        {results.map(result => (
          <li key={result.id}>{result.title}</li>
        ))}
      </ul>
    </div>
  );
}

2. 使用 useDeferredValue

function ExpensiveList({ items }) {
  const deferredItems = useDeferredValue(items);
  const [isPending, startTransition] = useTransition();
  
  return (
    <div>
      {isPending && <div>更新中...</div>}
      <ul>
        {deferredItems.map(item => (
          <ExpensiveItem key={item.id} item={item} />
        ))}
      </ul>
    </div>
  );
}

3. 自动批处理

function BatchExample() {
  const [count, setCount] = useState(0);
  const [flag, setFlag] = useState(false);
  
  const handleClick = () => {
    // React 18 中，这些更新会自动批处理
    setCount(c => c + 1);
    setFlag(f => !f);
    // 只会触发一次重新渲染
  };
  
  return (
    <button onClick={handleClick}>
      Count: {count}, Flag: {flag.toString()}
    </button>
  );
}

4. Suspense 与并发特性

function AsyncComponent() {
  return (
    <Suspense fallback={<div>加载中...</div>}>
      <AsyncDataComponent />
    </Suspense>
  );
}

function AsyncDataComponent() {
  const data = use(fetchData()); // 使用 use Hook
  
  return <div>{data.title}</div>;
}

🚀 性能优化技巧

1. 合理使用优先级

function OptimizedComponent() {
  const [urgentData, setUrgentData] = useState(null);
  const [nonUrgentData, setNonUrgentData] = useState(null);
  const [isPending, startTransition] = useTransition();
  
  const handleUrgentUpdate = () => {
    // 紧急更新，同步执行
    setUrgentData(newData);
  };
  
  const handleNonUrgentUpdate = () => {
    // 非紧急更新，使用 transition
    startTransition(() => {
      setNonUrgentData(newData);
    });
  };
  
  return (
    <div>
      <div>{urgentData}</div>
      <div>{nonUrgentData} {isPending && '(更新中...)'}</div>
    </div>
  );
}

2. 避免过度使用 transition

// ❌ 错误：过度使用 transition
function BadExample() {
  const [count, setCount] = useState(0);
  const [isPending, startTransition] = useTransition();
  
  const handleClick = () => {
    startTransition(() => {
      setCount(c => c + 1); // 简单的状态更新不需要 transition
    });
  };
  
  return <button onClick={handleClick}>{count}</button>;
}

// ✅ 正确：只在必要时使用 transition
function GoodExample() {
  const [count, setCount] = useState(0);
  const [isPending, startTransition] = useTransition();
  
  const handleClick = () => {
    setCount(c => c + 1); // 简单更新直接执行
  };
  
  const handleExpensiveUpdate = () => {
    startTransition(() => {
      // 昂贵的更新使用 transition
      setExpensiveData(processLargeDataset());
    });
  };
  
  return (
    <div>
      <button onClick={handleClick}>{count}</button>
      <button onClick={handleExpensiveUpdate}>
        处理大数据 {isPending && '(处理中...)'}
      </button>
    </div>
  );
}

3. 结合 Suspense 使用

function ConcurrentSuspense() {
  const [query, setQuery] = useState('');
  const [isPending, startTransition] = useTransition();
  
  const handleSearch = (newQuery) => {
    setQuery(newQuery); // 立即更新输入框
    
    startTransition(() => {
      // 延迟更新搜索结果
      setSearchResults(null); // 触发 Suspense
    });
  };
  
  return (
    <div>
      <input 
        value={query} 
        onChange={(e) => handleSearch(e.target.value)} 
      />
      <Suspense fallback={<div>搜索中...</div>}>
        <SearchResults query={query} />
      </Suspense>
    </div>
  );
}

📝 总结

React 18 的并发特性通过以下机制实现：

优先级调度：不同更新有不同的优先级
时间切片：将渲染工作分割成小块
可中断渲染：高优先级任务可以中断低优先级任务
自动批处理：将多个更新合并成一次渲染

这些特性让 React 能够提供更好的用户体验，特别是在处理大量数据或复杂交互时。

下一步：事件系统机制 - 深入分析 React 的事件系统

并发特性分析

On this page