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hooks 实现 ​

在beginWork阶段，为了获取函数式组件的children，在updateFunctionComponent函数中会调用renderWithHooks方法：

javascript
nextChildren = renderWithHooks(
  current,
  workInProgress,
  Component,
  nextProps,
  context,
  renderLanes,
);
nextChildren = renderWithHooks(
  current,
  workInProgress,
  Component,
  nextProps,
  context,
  renderLanes,
);

renderWithHooks方法在ReactFiberHooks.new.js文件中定义，它的关键代码为：

javascript
ReactCurrentDispatcher.current =
  current === null || current.memoizedState === null
  ? HooksDispatcherOnMount
 : HooksDispatcherOnUpdate;
let children = Component(props, secondArg);
ReactCurrentDispatcher.current = ContextOnlyDispatcher;
ReactCurrentDispatcher.current =
  current === null || current.memoizedState === null
  ? HooksDispatcherOnMount
 : HooksDispatcherOnUpdate;
let children = Component(props, secondArg);
ReactCurrentDispatcher.current = ContextOnlyDispatcher;

首先会根据current.memoizedState来判断当前是mount阶段还是update阶段，从而决定使用HooksDispatcherOnMount还是HooksDispatcherOnUpdate。然后执行Component，在执行的过程中，如果我们使用了hook，那么就会执行相应的hook方法。最后将ReactCurrentDispatcher.current置空。下面看一下hooks的实现。

useState ​

useState在React包中定义，其他的hooks也同样在这里定义：

javascript
export function useState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  const dispatcher = resolveDispatcher();
  return dispatcher.useState(initialState);
}
export function useState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  const dispatcher = resolveDispatcher();
  return dispatcher.useState(initialState);
}

第一步是resolveDispatcher获取当前的dispatcher。在renderWithHooks函数执行前已经通过判断mount/update将当前的dispatcher存放到全局，所以这里可以获取到。

mount 阶段 ​

如果是mount阶段，useState对应的是mountState方法：

javascript
const HooksDispatcherOnMount: Dispatcher = {
  useState: mountState,
}

function mountState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  // 创建一个 hook，挂载到 fiber上，并返回
  const hook = mountWorkInProgressHook();
  if (typeof initialState === 'function') {
    initialState = initialState();
  }
  hook.memoizedState = hook.baseState = initialState;
  // 初始化 queue
  const queue = (hook.queue = {
    pending: null,
    interleaved: null,
    lanes: NoLanes,
    dispatch: null,
    lastRenderedReducer: basicStateReducer,
    lastRenderedState: (initialState: any),
  });
  const dispatch: Dispatch<
    BasicStateAction<S>,
    > = (queue.dispatch = (dispatchAction.bind(
      null,
      currentlyRenderingFiber,
      queue,
    ): any));
  return [hook.memoizedState, dispatch];
}
const HooksDispatcherOnMount: Dispatcher = {
  useState: mountState,
}

function mountState<S>(
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  // 创建一个 hook，挂载到 fiber上，并返回
  const hook = mountWorkInProgressHook();
  if (typeof initialState === 'function') {
    initialState = initialState();
  }
  hook.memoizedState = hook.baseState = initialState;
  // 初始化 queue
  const queue = (hook.queue = {
    pending: null,
    interleaved: null,
    lanes: NoLanes,
    dispatch: null,
    lastRenderedReducer: basicStateReducer,
    lastRenderedState: (initialState: any),
  });
  const dispatch: Dispatch<
    BasicStateAction<S>,
    > = (queue.dispatch = (dispatchAction.bind(
      null,
      currentlyRenderingFiber,
      queue,
    ): any));
  return [hook.memoizedState, dispatch];
}

mountWorkInProgressHook方法 ​

第一步执行的是mountWorkInProgressHook，该函数首先会创建一个hook，用于存放对应hook的状态：

javascript
const hook: Hook = {
  // 记录该 hook 的 state
  memoizedState: null,
  // base state
  baseState: null,
  // base queue
  baseQueue: null,
  // 更新queue
  queue: null,
  // 指向下一个 hook
  next: null,
};
const hook: Hook = {
  // 记录该 hook 的 state
  memoizedState: null,
  // base state
  baseState: null,
  // base queue
  baseQueue: null,
  // 更新queue
  queue: null,
  // 指向下一个 hook
  next: null,
};

随后将hook形成链表结构，并将该链表存放到fiber.memoizedState当中：

javascript
if (workInProgressHook === null) {
  // currentlyRenderingFiber 表示的是当前正在执行的的 函数 fiber。
  currentlyRenderingFiber.memoizedState = workInProgressHook = hook;
} else {
  // 遇到的 hooks 形成链表形式。
  workInProgressHook = workInProgressHook.next = hook;
}
if (workInProgressHook === null) {
  // currentlyRenderingFiber 表示的是当前正在执行的的 函数 fiber。
  currentlyRenderingFiber.memoizedState = workInProgressHook = hook;
} else {
  // 遇到的 hooks 形成链表形式。
  workInProgressHook = workInProgressHook.next = hook;
}

这样后续就可以通过链表来访问每一个使用到的hook的状态了。

dispatchAction ​

在拿到hook后会初始化memoizedState，并将相应的memoizedState和dispatch函数返回。当调用dispatch函数时，实际触发的是dispatchAction函数，首先会创建一个update:

javascript
const eventTime = requestEventTime();
const lane = requestUpdateLane(fiber);

const update: Update<S, A> = {
  lane,
  action,
  // update.eagerReducer = lastRenderedReducer;
  eagerReducer: null,
  // update.eagerState = lastRenderedReducer(currentState, action);
  eagerState: null,
  next: (null: any),
};
const eventTime = requestEventTime();
const lane = requestUpdateLane(fiber);

const update: Update<S, A> = {
  lane,
  action,
  // update.eagerReducer = lastRenderedReducer;
  eagerReducer: null,
  // update.eagerState = lastRenderedReducer(currentState, action);
  eagerState: null,
  next: (null: any),
};

如果当前正在处理这个fiber，触发了dispatch，那么会执行如下分支：

javascript
if (
  fiber === currentlyRenderingFiber ||
  (alternate !== null && alternate === currentlyRenderingFiber)
) {
  // 注意这个参数用于防止循环渲染
  didScheduleRenderPhaseUpdateDuringThisPass = didScheduleRenderPhaseUpdate = true;
  const pending = queue.pending;
  // 将 update 形成链表形式
  if (pending === null) {
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }
  queue.pending = update;
}
if (
  fiber === currentlyRenderingFiber ||
  (alternate !== null && alternate === currentlyRenderingFiber)
) {
  // 注意这个参数用于防止循环渲染
  didScheduleRenderPhaseUpdateDuringThisPass = didScheduleRenderPhaseUpdate = true;
  const pending = queue.pending;
  // 将 update 形成链表形式
  if (pending === null) {
    update.next = update;
  } else {
    update.next = pending.next;
    pending.next = update;
  }
  queue.pending = update;
}

此时会将update形成循环链表，存放到queue.pending当中，并将didScheduleRenderPhaseUpdateDuringThisPass标记为true，这个标记在renderWithHooks中有用到：

javascript
if (didScheduleRenderPhaseUpdateDuringThisPass) {
  let numberOfReRenders: number = 0;
  do {
    didScheduleRenderPhaseUpdateDuringThisPass = false;
    invariant(
      numberOfReRenders < RE_RENDER_LIMIT,
      'Too many re-renders. React limits the number of renders to prevent ' +
      'an infinite loop.',
    );

    numberOfReRenders += 1;

    currentHook = null;
    workInProgressHook = null;
    workInProgress.updateQueue = null;


    ReactCurrentDispatcher.current = __DEV__
      ? HooksDispatcherOnRerenderInDEV
      : HooksDispatcherOnRerender;

    children = Component(props, secondArg);
  } while (didScheduleRenderPhaseUpdateDuringThisPass);
}
if (didScheduleRenderPhaseUpdateDuringThisPass) {
  let numberOfReRenders: number = 0;
  do {
    didScheduleRenderPhaseUpdateDuringThisPass = false;
    invariant(
      numberOfReRenders < RE_RENDER_LIMIT,
      'Too many re-renders. React limits the number of renders to prevent ' +
      'an infinite loop.',
    );

    numberOfReRenders += 1;

    currentHook = null;
    workInProgressHook = null;
    workInProgress.updateQueue = null;


    ReactCurrentDispatcher.current = __DEV__
      ? HooksDispatcherOnRerenderInDEV
      : HooksDispatcherOnRerender;

    children = Component(props, secondArg);
  } while (didScheduleRenderPhaseUpdateDuringThisPass);
}

如果为true的话，在函数组件执行完成后，会将dispatcher改为HooksDispatcherOnRerender并再次执行Component函数。并且这个重新渲染的次数不能大于最大次数RE_RENDER_LIMIT。

如果触发时的fiber与当前fiber不相等时，同样会建立循环链表的update存放到queue.pending当中，不同的是，此时会进行schedule：

javascript
const root = scheduleUpdateOnFiber(fiber, lane, eventTime);
const root = scheduleUpdateOnFiber(fiber, lane, eventTime);

rerender 阶段 ​

在mount阶段我们提到了didScheduleRenderPhaseUpdateDuringThisPass字段为true时，如果当前函数执行完毕，此时会立即触发rerender，使用的是HooksDispatcherOnRerender，因此useState对应于rerenderState也是rerenderReducer函数。

updateWorkInProgressHook ​

updateWorkInProgressHook与mountWorkInProgressHook对应，它是用于取存放在fiber.memoizedState上的hook。

javascript
let nextCurrentHook: null | Hook;
if (currentHook === null) {
  const current = currentlyRenderingFiber.alternate;
  if (current !== null) {
    nextCurrentHook = current.memoizedState;
  } else {
    nextCurrentHook = null;
  }
} else {
  nextCurrentHook = currentHook.next;
}

// 这个对应的 fiber 是 workInProgress
let nextWorkInProgressHook: null | Hook;
if (workInProgressHook === null) {
  nextWorkInProgressHook = currentlyRenderingFiber.memoizedState;
} else {
  nextWorkInProgressHook = workInProgressHook.next;
}
let nextCurrentHook: null | Hook;
if (currentHook === null) {
  const current = currentlyRenderingFiber.alternate;
  if (current !== null) {
    nextCurrentHook = current.memoizedState;
  } else {
    nextCurrentHook = null;
  }
} else {
  nextCurrentHook = currentHook.next;
}

// 这个对应的 fiber 是 workInProgress
let nextWorkInProgressHook: null | Hook;
if (workInProgressHook === null) {
  nextWorkInProgressHook = currentlyRenderingFiber.memoizedState;
} else {
  nextWorkInProgressHook = workInProgressHook.next;
}

nextCurrentHook和currentHook对应于老fiber上的hooks，而nextWorkInProgressHook和workInProgressHook对应于新fiber上的hooks。当新fiber上的hook存在时，直接使用即可：

javascript
if (nextWorkInProgressHook !== null) {
  // 如果 新fiber上存在该 hook， 可以直接使用
  workInProgressHook = nextWorkInProgressHook;
  nextWorkInProgressHook = workInProgressHook.next;

  currentHook = nextCurrentHook;
}
if (nextWorkInProgressHook !== null) {
  // 如果 新fiber上存在该 hook， 可以直接使用
  workInProgressHook = nextWorkInProgressHook;
  nextWorkInProgressHook = workInProgressHook.next;

  currentHook = nextCurrentHook;
}

否则会复用老fiber上的hook：

javascript
currentHook = nextCurrentHook;

// 如果新的 hook 为 null 时，需要将 current hook 作为 base,
// 进行创建新的 hook
const newHook: Hook = {
  memoizedState: currentHook.memoizedState,

  baseState: currentHook.baseState,
  baseQueue: currentHook.baseQueue,
  queue: currentHook.queue,

  next: null,
};

if (workInProgressHook === null) {
  currentlyRenderingFiber.memoizedState = workInProgressHook = newHook;
} else {
  workInProgressHook = workInProgressHook.next = newHook;
}
currentHook = nextCurrentHook;

// 如果新的 hook 为 null 时，需要将 current hook 作为 base,
// 进行创建新的 hook
const newHook: Hook = {
  memoizedState: currentHook.memoizedState,

  baseState: currentHook.baseState,
  baseQueue: currentHook.baseQueue,
  queue: currentHook.queue,

  next: null,
};

if (workInProgressHook === null) {
  currentlyRenderingFiber.memoizedState = workInProgressHook = newHook;
} else {
  workInProgressHook = workInProgressHook.next = newHook;
}

更新state ​

在拿到对应的hook后，就会依次执行所有的update，更新当前的state:

javascript
// 循环执行
do {
  const action = update.action;
  newState = reducer(newState, action);
  update = update.next;
} while (update !== firstRenderPhaseUpdate);

// 更新 state
hook.memoizedState = newState;
if (hook.baseQueue === null) {
  hook.baseState = newState;
}
queue.lastRenderedState = newState;
// 循环执行
do {
  const action = update.action;
  newState = reducer(newState, action);
  update = update.next;
} while (update !== firstRenderPhaseUpdate);

// 更新 state
hook.memoizedState = newState;
if (hook.baseQueue === null) {
  hook.baseState = newState;
}
queue.lastRenderedState = newState;

update 阶段 ​

update阶段调用的是updateReducer，与rerender不同的是，循环执行update队列时，会判断updateLane是否在renderLanes中:

javascript
if (!isSubsetOfLanes(renderLanes, updateLane)) {}
if (!isSubsetOfLanes(renderLanes, updateLane)) {}

这个阶段的更新与UpdateQueue章节中class组件的processUpdate方法更新类似，这里不再赘述。

useReducer ​

useReducer的流程就与useState一致，不同的是传入的reducer是用户定义的reducer，并且dispatch的时候会携带action。在useState中，reducer是state => state，dispatch的时候无需action。

useEffect ​

mount阶段 ​

mount阶段useEffect调用的是mountEffect：

javascript
return mountEffectImpl(
  PassiveEffect | PassiveStaticEffect,
  HookPassive,
  create,
  deps,
)
function mountEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  currentlyRenderingFiber.flags |= fiberFlags;
  hook.memoizedState = pushEffect(
    HookHasEffect | hookFlags,
    create,
    undefined,
    nextDeps,
  );
}
return mountEffectImpl(
  PassiveEffect | PassiveStaticEffect,
  HookPassive,
  create,
  deps,
)
function mountEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  currentlyRenderingFiber.flags |= fiberFlags;
  hook.memoizedState = pushEffect(
    HookHasEffect | hookFlags,
    create,
    undefined,
    nextDeps,
  );
}

mountWorkInProgressHook在useState中已经讲过，其主要作用是创建一个hook，并将该hook与其他hook形成链表存放到fiber.memoizedState属性上。

currentlyRenderingFiber.flags |= fiberFlags这里表示如果使用了useEffect这个hook，那么fiber.flags上会增加Passive和PassiveStatic这两个标识符。

接下来是pushEffect方法，首先会创建一个effect：

javascript
const effect: Effect = {
  // 表示 hook 的 tag
  tag,
  // 创建时的回调函数
  create,
  // 销毁时的回调函数
  destroy,
  // hook 的依赖
  deps,
  // 指向下一个 effect
  next: (null: any),
};
const effect: Effect = {
  // 表示 hook 的 tag
  tag,
  // 创建时的回调函数
  create,
  // 销毁时的回调函数
  destroy,
  // hook 的依赖
  deps,
  // 指向下一个 effect
  next: (null: any),
};

该effect的tag为HasEffect | Passive，随后将这个effect与其他的effect形成链表结构存放到fiber.updateQueue中：

javascript
let componentUpdateQueue: null | FunctionComponentUpdateQueue = (currentlyRenderingFiber.updateQueue: any);
// 如果不存在 updateQueue，进行初始化
if (componentUpdateQueue === null) {
  // 建立循环链表
  componentUpdateQueue = createFunctionComponentUpdateQueue();
  currentlyRenderingFiber.updateQueue = (componentUpdateQueue: any);
  componentUpdateQueue.lastEffect = effect.next = effect;
} else {
  // 如果存在updateQueue，将effect添加到链表中
  const lastEffect = componentUpdateQueue.lastEffect;
  if (lastEffect === null) {
    componentUpdateQueue.lastEffect = effect.next = effect;
  } else {
    // 添加到链表的前面
    const firstEffect = lastEffect.next;
    lastEffect.next = effect;
    effect.next = firstEffect;
    componentUpdateQueue.lastEffect = effect;
  }
}
let componentUpdateQueue: null | FunctionComponentUpdateQueue = (currentlyRenderingFiber.updateQueue: any);
// 如果不存在 updateQueue，进行初始化
if (componentUpdateQueue === null) {
  // 建立循环链表
  componentUpdateQueue = createFunctionComponentUpdateQueue();
  currentlyRenderingFiber.updateQueue = (componentUpdateQueue: any);
  componentUpdateQueue.lastEffect = effect.next = effect;
} else {
  // 如果存在updateQueue，将effect添加到链表中
  const lastEffect = componentUpdateQueue.lastEffect;
  if (lastEffect === null) {
    componentUpdateQueue.lastEffect = effect.next = effect;
  } else {
    // 添加到链表的前面
    const firstEffect = lastEffect.next;
    lastEffect.next = effect;
    effect.next = firstEffect;
    componentUpdateQueue.lastEffect = effect;
  }
}

这里fiber.updateQueue.lastEffect指向的是新添加的effect。

rerender和update阶段 ​

rerender和update阶段都是调用的updateEffect方法，也是调用的updateEffectImpl方法。首先会获取相应的hook:

javascript
const hook = updateWorkInProgressHook()
const hook = updateWorkInProgressHook()

随后会判断依赖deps有没有发生变化。如果没有发生变化，那么pushEffect的时候，effect.tags不包含HasEffect。反之，则会包含。

javascript
hook.memoizedState = pushEffect(hookFlags, create, destroy, nextDeps);
hook.memoizedState = pushEffect(hookFlags, create, destroy, nextDeps);

useLayoutEffect ​

mount 阶段 ​

useLayoutEffect的mount阶段与useEffect几乎一致，主要的区别有两点：

  • fiber的flags为Update | LayoutStatic。
  • effect的tag为HasEffect | Layout。

rerender和update阶段 ​

与useEffect一致，主要的区别在于：

  • fiber的flags为UpdateEffect。
  • effect的tag为HasEffect | Layout。

useMemo ​

mount 阶段 ​

javascript
function mountMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null,
): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const nextValue = nextCreate();
  hook.memoizedState = [nextValue, nextDeps];
  return nextValue;
}
function mountMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null,
): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  const nextValue = nextCreate();
  hook.memoizedState = [nextValue, nextDeps];
  return nextValue;
}

首先获取hook，然后执行nextCreate并将结果返回。

rerender和update阶段 ​

updateMemo方法主要用于判断前后的deps是否一致。如果一致，则跳过函数执行，将先前的值返回，从而达到缓存的目的。

javascript
if (areHookInputsEqual(nextDeps, prevDeps)) {
  return prevState[0];
}
if (areHookInputsEqual(nextDeps, prevDeps)) {
  return prevState[0];
}

useCallback ​

mount 阶段 ​

javascript
function mountCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  hook.memoizedState = [callback, nextDeps];
  return callback;
}
function mountCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = mountWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  hook.memoizedState = [callback, nextDeps];
  return callback;
}

memoizedState存储的是callback和deps。

rerender和update阶段 ​

对比deps看是否发生变化，如果没变，则将原callback返回。

javascript
if (areHookInputsEqual(nextDeps, prevDeps)) {
  return prevState[0];
}
if (areHookInputsEqual(nextDeps, prevDeps)) {
  return prevState[0];
}

useContext ​

mount、rerender和update阶段 ​

useContext三个阶段都是执行的readContext方法，首先会获取到context的value值，也就是用户定义的context的值。

javascript
const value = isPrimaryRenderer
  ? context._currentValue
  : context._currentValue2;
const value = isPrimaryRenderer
  ? context._currentValue
  : context._currentValue2;

随后判断lastFullyObservedContext === context，如果为true，说明该context已经被处理了，无需重复处理，跳过即可。

javascript
if (lastFullyObservedContext === context) {
  // Nothing to do. We already observe everything in this context.
}
if (lastFullyObservedContext === context) {
  // Nothing to do. We already observe everything in this context.
}

否则的话，会创建一个contextItem，形成链表并存放到fiber.dependencies.firstContext上：

javascript
const contextItem = {
  context: ((context: any): ReactContext<mixed>),
  memoizedValue: value,
  next: null,
};

if (lastContextDependency === null) {
  lastContextDependency = contextItem;
  currentlyRenderingFiber.dependencies = {
    lanes: NoLanes,
    firstContext: contextItem,
  };
} else {
  lastContextDependency = lastContextDependency.next = contextItem;
}
const contextItem = {
  context: ((context: any): ReactContext<mixed>),
  memoizedValue: value,
  next: null,
};

if (lastContextDependency === null) {
  lastContextDependency = contextItem;
  currentlyRenderingFiber.dependencies = {
    lanes: NoLanes,
    firstContext: contextItem,
  };
} else {
  lastContextDependency = lastContextDependency.next = contextItem;
}

这里的处理与class组件的context处理类似。

useRef ​

mount 阶段 ​

javascript
function mountRef<T>(initialValue: T): {| current: T |} {
  const hook = mountWorkInProgressHook();
  const ref = { current: initialValue };
  hook.memoizedState = ref;
  return ref;
}
function mountRef<T>(initialValue: T): {| current: T |} {
  const hook = mountWorkInProgressHook();
  const ref = { current: initialValue };
  hook.memoizedState = ref;
  return ref;
}

mountRef方法创建了一个{ current: xxx }对象并将其返回。

rerender和update阶段 ​

javascript
function updateRef<T>(initialValue: T): {| current: T |} {
  const hook = updateWorkInProgressHook();
  return hook.memoizedState;
}
function updateRef<T>(initialValue: T): {| current: T |} {
  const hook = updateWorkInProgressHook();
  return hook.memoizedState;
}

updateRef也非常的简单，将hook.memoizedState中存储的{ current: xxx }对象返回。

useTransition ​

mount 阶段 ​

useTransition的实现是基于useState的：

javascript
function mountTransition(): [boolean, (() => void) => void] {
  const [isPending, setPending] = mountState(false);
  const start = startTransition.bind(null, setPending);
  const hook = mountWorkInProgressHook();
  hook.memoizedState = start;
  return [isPending, start];
}
function mountTransition(): [boolean, (() => void) => void] {
  const [isPending, setPending] = mountState(false);
  const start = startTransition.bind(null, setPending);
  const hook = mountWorkInProgressHook();
  hook.memoizedState = start;
  return [isPending, start];
}

首先会通过useState定义isPending和setPending方法，然后定义一个start方法并将其返回：

javascript
function startTransition(setPending, callback) {
  const previousPriority = getCurrentUpdatePriority();
  setCurrentUpdatePriority(
    higherEventPriority(previousPriority, ContinuousEventPriority),
  );

  // 添加一个 update，优先级为当前 update 优先级
  setPending(true);
  // 此时进行 schedule 的时候，requestUpdateLane 为原始的 lane
  // 比如在 click 的时候就会是同步 lane
  const prevTransition = ReactCurrentBatchConfig.transition;
  ReactCurrentBatchConfig.transition = 1;
  try {
    // 添加一个 update，requestUpdateLane的结果为 transition lane
    setPending(false);
  // 执行 startTransition 的回调
    callback();
  } finally {
    setCurrentUpdatePriority(previousPriority);
    ReactCurrentBatchConfig.transition = prevTransition;
  }
}
function startTransition(setPending, callback) {
  const previousPriority = getCurrentUpdatePriority();
  setCurrentUpdatePriority(
    higherEventPriority(previousPriority, ContinuousEventPriority),
  );

  // 添加一个 update，优先级为当前 update 优先级
  setPending(true);
  // 此时进行 schedule 的时候，requestUpdateLane 为原始的 lane
  // 比如在 click 的时候就会是同步 lane
  const prevTransition = ReactCurrentBatchConfig.transition;
  ReactCurrentBatchConfig.transition = 1;
  try {
    // 添加一个 update，requestUpdateLane的结果为 transition lane
    setPending(false);
  // 执行 startTransition 的回调
    callback();
  } finally {
    setCurrentUpdatePriority(previousPriority);
    ReactCurrentBatchConfig.transition = prevTransition;
  }
}

首先会设置更新时的优先级，这样在setPending的时候，会进行requestUpdateLane，拿到对应的lane。随后将ReactCurrentBatchConfig.transition置为1，在requestUpdateLane会返回transition lane。因此try内部setPending(false)是一个transition lane的更新。同样callback内部定义的更新也将会是一个transition lane的更新。这样在transition更新的时候，setPending(false)会执行。

rerender和update阶段 ​

javascript
function updateTransition(): [boolean, (() => void) => void] {
  const [isPending] = updateState(false);
  const hook = updateWorkInProgressHook();
  const start = hook.memoizedState;
  return [isPending, start];
}
function updateTransition(): [boolean, (() => void) => void] {
  const [isPending] = updateState(false);
  const hook = updateWorkInProgressHook();
  const start = hook.memoizedState;
  return [isPending, start];
}

update阶段比较简单，拿到isPending和start方法返回即可。

useDeferredValue ​

mount、rerender和update阶段 ​

useDeferredValue的实现是基于useState和useEffect的：

javascript
function mountDeferredValue<T>(value: T): T {
  const [prevValue, setValue] = mountState(value);
  mountEffect(() => {
    const prevTransition = ReactCurrentBatchConfig.transition;
    ReactCurrentBatchConfig.transition = 1;
    try {
      setValue(value);
    } finally {
      ReactCurrentBatchConfig.transition = prevTransition;
    }
  }, [value]);
  return prevValue;
}
function mountDeferredValue<T>(value: T): T {
  const [prevValue, setValue] = mountState(value);
  mountEffect(() => {
    const prevTransition = ReactCurrentBatchConfig.transition;
    ReactCurrentBatchConfig.transition = 1;
    try {
      setValue(value);
    } finally {
      ReactCurrentBatchConfig.transition = prevTransition;
    }
  }, [value]);
  return prevValue;
}

在useEffect内部设置value时，会将ReactCurrentBatchConfig.transition置为1，因此更新对应的lane是transition lane。

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