useState

useState 源码解析

初始化： 构建dispatcher函数和初始值

更新时：

1. 调用dispatcher函数，按序插入update(其实就是一个action)

2. 收集update，调度一次React的更新

3. 在更新的过程中将ReactCurrentDispatcher.current指向负责更新的Dispatcher

4. 执行到函数组件App()时，useState会被重新执行，在resolve dispatcher的阶段拿到了负责更新的dispatcher。

5. useState会拿到Hook对象，Hook.queue中存储了更新队列，依次进行更新后，即可拿到最新的state

6. 函数组件App()执行后返回的nextChild中的count值已经是最新的了。FiberNode中的memorizedState也被设置为最新的state

7. Fiber渲染出真实DOM。更新结束。

const initialState = {count: 0};

// useReducer的方式
function reducer(state, action) {
  switch (action.type) {
    case 'increment':
      return {count: state.count + 1};
    case 'decrement':
      return {count: state.count - 1};
    default:
      throw new Error();
  }
}

// init只在useReducer而非useState时使用，用来初始化 init(initialArg)， 默认为undefined
const [state, dispatch] = useReducer(reducer, initialState, init);

// useState的方式
const [count, setCount] = useState(initialState.count);

useState

ReactFiberHooks.js

function basicStateReducer<S>(state: S, action: BasicStateAction<S>): S {
  // $FlowFixMe: Flow doesn't like mixed types
  return typeof action === 'function' ? action(state) : action;
}

function mountState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  // 创建一个新的 Hook，并返回当前 workInProgressHook
  const hook = mountWorkInProgressHook();
  if (typeof initialState === 'function') {
    // $FlowFixMe: Flow doesn't like mixed types
    initialState = initialState();
  }
  // 赋值初始化的state值
  hook.memoizedState = hook.baseState = initialState;
  // 新建一个队列
  const queue = (hook.queue = {
    pending: null, // 最后一次更新逻辑,  包括 {action，next} 即状态值和下一次 Update
    interleaved: null,
    lanes: NoLanes,
    dispatch: null,
    lastRenderedReducer: basicStateReducer,
    lastRenderedState: (initialState: any), // 最后一次渲染组件时的状态
  });
  // 绑定当前 fiber 和 queue 到 dispatchAction 上:
  const dispatch: Dispatch<
    BasicStateAction<S>,
  > = (queue.dispatch = (dispatchAction.bind(
    null,
    currentlyRenderingFiber,
    queue,
  ): any));
  return [hook.memoizedState, dispatch];
}

function updateState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return updateReducer(basicStateReducer, (initialState: any));
}

function rerenderState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  return rerenderReducer(basicStateReducer, (initialState: any));
}

useReducer

ReactFiberHooks.js

// updateReducer
function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  // 获取当前正在工作中的 hook
  const hook = updateWorkInProgressHook();
  const queue = hook.queue;
  invariant(
    queue !== null,
    'Should have a queue. This is likely a bug in React. Please file an issue.',
  );

  queue.lastRenderedReducer = reducer;

  const current: Hook = (currentHook: any);

  // fiber 渲染后，尚未执行完成 update 更新，存于 currentFiber
  let baseQueue = current.baseQueue;

  // 排队中的 update 更新，存于 workInProgressFiber
  const pendingQueue = queue.pending;
  
 // 将排队中的 update 更新添加到 baseQueue
  if (pendingQueue !== null) {
    // We have new updates that haven't been processed yet.
    // We'll add them to the base queue.
    if (baseQueue !== null) {
      // Merge the pending queue and the base queue.
      const baseFirst = baseQueue.next;
      const pendingFirst = pendingQueue.next;
      baseQueue.next = pendingFirst;
      pendingQueue.next = baseFirst; // pending 队列尾节点添加 baseFirst
    }
    if (__DEV__) {
      if (current.baseQueue !== baseQueue) {
        // Internal invariant that should never happen, but feasibly could in
        // the future if we implement resuming, or some form of that.
        console.error(
          'Internal error: Expected work-in-progress queue to be a clone. ' +
            'This is a bug in React.',
        );
      }
    }
    current.baseQueue = baseQueue = pendingQueue;
    queue.pending = null;
  }

  if (baseQueue !== null) {
    // We have a queue to process.
    const first = baseQueue.next;
    let newState = current.baseState;

    let newBaseState = null;
    let newBaseQueueFirst = null;
    let newBaseQueueLast = null;
    let update = first;
    do {
      const updateLane = update.lane;
      // update 优先级不足，将其存入 newBaseQueueList 等待更新
      if (!isSubsetOfLanes(renderLanes, updateLane)) {
        // Priority is insufficient. Skip this update. If this is the first
        // skipped update, the previous update/state is the new base
        // update/state.
        const clone: Update<S, A> = {
          lane: updateLane,
          action: update.action,
          eagerReducer: update.eagerReducer, // eagerReducer 缓存上一个 reducer
          eagerState: update.eagerState, // eagerState 缓存上一个状态
          next: (null: any),
        };
        if (newBaseQueueLast === null) {
          newBaseQueueFirst = newBaseQueueLast = clone;
          newBaseState = newState;
        } else {
          newBaseQueueLast = newBaseQueueLast.next = clone;
        }
        // Update the remaining priority in the queue.
        // TODO: Don't need to accumulate this. Instead, we can remove
        // renderLanes from the original lanes.
        currentlyRenderingFiber.lanes = mergeLanes(
          currentlyRenderingFiber.lanes,
          updateLane,
        );
        markSkippedUpdateLanes(updateLane);
      } else {
        // This update does have sufficient priority.
        // 同 UpdateQueue，当优先级高的 update 任务在优先级低的 update 任务后
        // 添加到 newBaseQueueList 中，更新处理会执行多次

        if (newBaseQueueLast !== null) {
          const clone: Update<S, A> = {
            // This update is going to be committed so we never want uncommit
            // it. Using NoLane works because 0 is a subset of all bitmasks, so
            // this will never be skipped by the check above.
            lane: NoLane,
            action: update.action,
            eagerReducer: update.eagerReducer,
            eagerState: update.eagerState,
            next: (null: any),
          };
          newBaseQueueLast = newBaseQueueLast.next = clone;
        }

        // hook.dispatch 变更状态后再执行组件的渲染函数
        // 若 reducer 未变更，复用之前的状态 update.eagerState
        // 若 reducer 变更，使用 reducer 计算最新的状态
        if (update.eagerReducer === reducer) {
          // If this update was processed eagerly, and its reducer matches the
          // current reducer, we can use the eagerly computed state.
          newState = ((update.eagerState: any): S);
        } else {
          const action = update.action;
          newState = reducer(newState, action);
        }
      }
      update = update.next;
    } while (update !== null && update !== first);

    if (newBaseQueueLast === null) {
      newBaseState = newState;
    } else {
      newBaseQueueLast.next = (newBaseQueueFirst: any);
    }

    // Mark that the fiber performed work, but only if the new state is
    // different from the current state.
    if (!is(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }

    hook.memoizedState = newState;
    hook.baseState = newBaseState;
    hook.baseQueue = newBaseQueueLast;

    queue.lastRenderedState = newState;
  }

  // 多次的更改
  // Interleaved updates are stored on a separate queue. We aren't going to
  // process them during this render, but we do need to track which lanes
  // are remaining.
  const lastInterleaved = queue.interleaved;
  if (lastInterleaved !== null) {
    let interleaved = lastInterleaved;
    do {
      const interleavedLane = interleaved.lane;
      currentlyRenderingFiber.lanes = mergeLanes(
        currentlyRenderingFiber.lanes,
        interleavedLane,
      );
      markSkippedUpdateLanes(interleavedLane);
      interleaved = ((interleaved: any).next: Update<S, A>);
    } while (interleaved !== lastInterleaved);
  } else if (baseQueue === null) {
    // `queue.lanes` is used for entangling transitions. We can set it back to
    // zero once the queue is empty.
    queue.lanes = NoLanes;
  }

  const dispatch: Dispatch<A> = (queue.dispatch: any);
  return [hook.memoizedState, dispatch];
}

// rerenderReducer
function rerenderReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = updateWorkInProgressHook();
  const queue = hook.queue;
  invariant(
    queue !== null,
    'Should have a queue. This is likely a bug in React. Please file an issue.',
  );

  queue.lastRenderedReducer = reducer;

  // This is a re-render. Apply the new render phase updates to the previous
  // work-in-progress hook.
  const dispatch: Dispatch<A> = (queue.dispatch: any);
  const lastRenderPhaseUpdate = queue.pending;
  let newState = hook.memoizedState;
  if (lastRenderPhaseUpdate !== null) {
    // The queue doesn't persist past this render pass.
    queue.pending = null;

    const firstRenderPhaseUpdate = lastRenderPhaseUpdate.next;
    let update = firstRenderPhaseUpdate;
    do {
      // Process this render phase update. We don't have to check the
      // priority because it will always be the same as the current
      // render's.
      const action = update.action;
      newState = reducer(newState, action);
      update = update.next;
    } while (update !== firstRenderPhaseUpdate);

    // Mark that the fiber performed work, but only if the new state is
    // different from the current state.
    if (!is(newState, hook.memoizedState)) {
      markWorkInProgressReceivedUpdate();
    }

    hook.memoizedState = newState;
    // Don't persist the state accumulated from the render phase updates to
    // the base state unless the queue is empty.
    // TODO: Not sure if this is the desired semantics, but it's what we
    // do for gDSFP. I can't remember why.
    if (hook.baseQueue === null) {
      hook.baseState = newState;
    }

    queue.lastRenderedState = newState;
  }
  return [newState, dispatch];
}