在hook源码中hook存在于Dispatcher中，Dispatcher就是一个对象，不同hook 调用的函数不一样，全局变量ReactCurrentDispatcher.current会根据是mount还是update赋值为HooksDispatcherOnMount或HooksDispatcherOnUpdate

ReactCurrentDispatcher.current = 
  current === null || current.memoizedState === null//mount or update
  ? HooksDispatcherOnMount
    : HooksDispatcherOnUpdate;

const HooksDispatcherOnMount: Dispatcher = {//mount时
  useCallback: mountCallback,
  useContext: readContext,
  useEffect: mountEffect,
  useImperativeHandle: mountImperativeHandle,
  useLayoutEffect: mountLayoutEffect,
  useMemo: mountMemo,
  useReducer: mountReducer,
  useRef: mountRef,
  useState: mountState,
  //...
};

const HooksDispatcherOnUpdate: Dispatcher = {//update时
  useCallback: updateCallback,
  useContext: readContext,
  useEffect: updateEffect,
  useImperativeHandle: updateImperativeHandle,
  useLayoutEffect: updateLayoutEffect,
  useMemo: updateMemo,
  useReducer: updateReducer,
  useRef: updateRef,
  useState: updateState,
  //...
};

hook数据结构

在FunctionComponent中，多个hook会形成hook链表，保存在Fiber的memoizedState的上，而需要更新的Update保存在hook.queue.pending中

const hook: Hook = {
  memoizedState: null,//对于不同hook，有不同的值
  baseState: null,//初始state
  baseQueue: null,//初始queue队列
  queue: null,//需要更新的update
  next: null,//下一个hook
};

下面来看下memoizedState对应的值

useState：例如const [state, updateState] = useState(initialState)，memoizedState等于state的值
useReducer：例如const [state, dispatch] = useReducer(reducer, {});，memoizedState等于state的值
useEffect：在mountEffect时会调用pushEffect创建effect链表，memoizedState就等于effect链表，effect链表也会挂载到fiber.updateQueue上，每个effect上存在useEffect的第一个参数回调和第二个参数依赖数组，例如，useEffect(callback, [dep])，effect就是{create:callback, dep:dep,...}
useRef：例如useRef(0)，memoizedState就等于{current: 0}
useMemo：例如useMemo(callback, [dep])，memoizedState等于[callback(), dep]
useCallback：例如useCallback(callback, [dep])，memoizedState等于[callback, dep]。useCallback保存callback函数，useMemo保存callback的执行结果

useState&useReducer

之所以把useState和useReducer放在一起，是因为在源码中useState就是有默认reducer参数的useReducer。

useState&useReducer声明

resolveDispatcher函数会获取当前的Dispatcher

function useState(initialState) {
  var dispatcher = resolveDispatcher();
  return dispatcher.useState(initialState);
}
function useReducer(reducer, initialArg, init) {
  var dispatcher = resolveDispatcher();
  return dispatcher.useReducer(reducer, initialArg, init);
}

mount阶段

mount阶段useState调用mountState，useReducer调用mountReducer，唯一区别就是它们创建的queue中lastRenderedReducer不一样，mount有初始值basicStateReducer，所以说useState就是有默认reducer参数的useReducer。

function mountState<S>(//
  initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
  const hook = mountWorkInProgressHook();//创建当前hook
  if (typeof initialState === 'function') {
    initialState = initialState();
  }
  hook.memoizedState = hook.baseState = initialState;//hook.memoizedState赋值
  const queue = (hook.queue = {//赋值hook.queue
    pending: null,
    dispatch: null,
    lastRenderedReducer: basicStateReducer,//和mountReducer的区别
    lastRenderedState: (initialState: any),
  });
  const dispatch: Dispatch<//创建dispatch函数
    BasicStateAction<S>,
  > = (queue.dispatch = (dispatchAction.bind(
    null,
    currentlyRenderingFiber,
    queue,
  ): any));
  return [hook.memoizedState, dispatch];//返回memoizedState和dispatch
}

function mountReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = mountWorkInProgressHook();//创建当前hook
  let initialState;
  if (init !== undefined) {
    initialState = init(initialArg);
  } else {
    initialState = ((initialArg: any): S);
  }
  hook.memoizedState = hook.baseState = initialState;//hook.memoizedState赋值
  const queue = (hook.queue = {//创建queue
    pending: null,
    dispatch: null,
    lastRenderedReducer: reducer,
    lastRenderedState: (initialState: any),
  });
  const dispatch: Dispatch<A> = (queue.dispatch = (dispatchAction.bind(//创建dispatch函数
    null,
    currentlyRenderingFiber,
    queue,
  ): any));
  return [hook.memoizedState, dispatch];//返回memoizedState和dispatch
}

function basicStateReducer<S>(state: S, action: BasicStateAction<S>): S {
  return typeof action === 'function' ? action(state) : action;
}

update阶段

update时会根据hook中的update计算新的state

function updateReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
): [S, Dispatch<A>] {
  const hook = updateWorkInProgressHook();//获取hook
  const queue = hook.queue;
  queue.lastRenderedReducer = reducer;

  //...更新state和第12章的state计算逻辑基本一致

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

执行阶段

useState执行setState后会调用dispatchAction，dispatchAction做的事情就是讲Update加入queue.pending中，然后开始调度

function dispatchAction(fiber, queue, action) {

  var update = {//创建update
    eventTime: eventTime,
    lane: lane,
    suspenseConfig: suspenseConfig,
    action: action,
    eagerReducer: null,
    eagerState: null,
    next: null
  }; 

  //queue.pending中加入update
  
  var alternate = fiber.alternate;

  if (fiber === currentlyRenderingFiber$1 || alternate !== null && alternate === currentlyRenderingFiber$1) {
    //如果是render阶段执行的更新didScheduleRenderPhaseUpdate=true
}
    didScheduleRenderPhaseUpdateDuringThisPass = didScheduleRenderPhaseUpdate = true;
  } else {
    if (fiber.lanes === NoLanes && (alternate === null || alternate.lanes === NoLanes)) {
      //如果fiber不存在优先级并且当前alternate不存在或者没有优先级，那就不需要更新了
      //优化的步骤
    }

    scheduleUpdateOnFiber(fiber, lane, eventTime);
  }
}

useEffect

声明

获取并返回useEffect函数

export function useEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  const dispatcher = resolveDispatcher();
  return dispatcher.useEffect(create, deps);
}

mount阶段

调用mountEffect，mountEffect调用mountEffectImpl，hook.memoizedState赋值为effect链表

function mountEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = mountWorkInProgressHook();//获取hook
  const nextDeps = deps === undefined ? null : deps;//依赖
  currentlyRenderingFiber.flags |= fiberFlags;//增加flag
  hook.memoizedState = pushEffect(//memoizedState=effects环状链表
    HookHasEffect | hookFlags,
    create,
    undefined,
    nextDeps,
  );
}

update阶段

浅比较依赖，如果依赖性变了pushEffect第一个参数传HookHasEffect | hookFlags，HookHasEffect表示useEffect依赖项改变了，需要在commit阶段重新执行

function updateEffectImpl(fiberFlags, hookFlags, create, deps): void {
  const hook = updateWorkInProgressHook();
  const nextDeps = deps === undefined ? null : deps;
  let destroy = undefined;

  if (currentHook !== null) {
    const prevEffect = currentHook.memoizedState;
    destroy = prevEffect.destroy;//
    if (nextDeps !== null) {
      const prevDeps = prevEffect.deps;
      if (areHookInputsEqual(nextDeps, prevDeps)) {//比较deps
        //即使依赖相等也要将effect加入链表，以保证顺序一致
        pushEffect(hookFlags, create, destroy, nextDeps);
        return;
      }
    }
  }

  currentlyRenderingFiber.flags |= fiberFlags;

  hook.memoizedState = pushEffect(
    //参数传HookHasEffect | hookFlags，包含hookFlags的useEffect会在commit阶段执行这个effect
    HookHasEffect | hookFlags,
    create,
    destroy,
    nextDeps,
  );
}

执行阶段

在第9章commit阶段的commitLayoutEffects函数中会调用schedulePassiveEffects，将useEffect的销毁和回调函数push到pendingPassiveHookEffectsUnmount和pendingPassiveHookEffectsMount中，然后在mutation之后调用flushPassiveEffects依次执行上次render的销毁函数回调和本次render 的回调函数

const unmountEffects = pendingPassiveHookEffectsUnmount;
pendingPassiveHookEffectsUnmount = [];
for (let i = 0; i < unmountEffects.length; i += 2) {
  const effect = ((unmountEffects[i]: any): HookEffect);
  const fiber = ((unmountEffects[i + 1]: any): Fiber);
  const destroy = effect.destroy;
  effect.destroy = undefined;

  if (typeof destroy === 'function') {
    try {
      destroy();//销毁函数执行
    } catch (error) {
      captureCommitPhaseError(fiber, error);
    }
  }
}

const mountEffects = pendingPassiveHookEffectsMount;
pendingPassiveHookEffectsMount = [];
for (let i = 0; i < mountEffects.length; i += 2) {
  const effect = ((mountEffects[i]: any): HookEffect);
  const fiber = ((mountEffects[i + 1]: any): Fiber);
  
  try {
    const create = effect.create;//本次render的创建函数
   effect.destroy = create();
  } catch (error) {
    captureCommitPhaseError(fiber, error);
  }
}

useRef

sring类型的ref已经不在推荐使用(源码中string会生成refs，发生在coerceRef函数中)，ForwardRef只是把ref通过传参传下去，createRef也是{current: any这种结构，所以我们只讨论function或者{current: any}的useRef

//createRef返回{current: any}
export function createRef(): RefObject {
  const refObject = {
    current: null,
  };
  return refObject;
}

声明阶段

和其他hook一样

export function useRef<T>(initialValue: T): {|current: T|} {
  const dispatcher = resolveDispatcher();
  return dispatcher.useRef(initialValue);
}

mount阶段

mount时会调用mountRef，创建hook和ref对象。

function mountRef<T>(initialValue: T): {|current: T|} {
  const hook = mountWorkInProgressHook();//获取useRef
  const ref = {current: initialValue};//ref初始化
  hook.memoizedState = ref;
  return ref;
}

render阶段：将带有ref属性的Fiber标记上Ref Tag，这一步发生在beginWork和completeWork函数中的markRef

export const Ref = /*                          */ 0b0000000010000000;

//beginWork中
function markRef(current: Fiber | null, workInProgress: Fiber) {
  const ref = workInProgress.ref;
  if (
    (current === null && ref !== null) ||
    (current !== null && current.ref !== ref)
  ) {
    workInProgress.effectTag |= Ref;
  }
}
//completeWork中
function markRef(workInProgress: Fiber) {
  workInProgress.effectTag |= Ref;
}

commit阶段：

    会在commitMutationEffects函数中判断ref是否改变，如果改变了会先执行commitDetachRef先删除之前的ref，然后在commitLayoutEffect中会执行commitAttachRef赋值ref。

function commitMutationEffects(root: FiberRoot, renderPriorityLevel) {
  while (nextEffect !== null) {
    const effectTag = nextEffect.effectTag;
    // ...
    
    if (effectTag & Ref) {
      const current = nextEffect.alternate;
      if (current !== null) {
        commitDetachRef(current);//移除ref
      }
    }
  }

function commitDetachRef(current: Fiber) {
  const currentRef = current.ref;
  if (currentRef !== null) {
    if (typeof currentRef === 'function') {
      currentRef(null);//类型是function，则调用
    } else {
      currentRef.current = null;//否则赋值{current: null}
    }
  }
}

function commitAttachRef(finishedWork: Fiber) {
  const ref = finishedWork.ref;
  if (ref !== null) {
    const instance = finishedWork.stateNode;//获取ref的实例
    let instanceToUse;
    switch (finishedWork.tag) {
      case HostComponent:
        instanceToUse = getPublicInstance(instance);
        break;
      default:
        instanceToUse = instance;
    }

    if (typeof ref === 'function') {//ref赋值
      ref(instanceToUse);
    } else {
      ref.current = instanceToUse;
    }
  }
}

update阶段

update时调用updateRef获取获取当前useRef，然后返回hook链表

function updateRef<T>(initialValue: T): {|current: T|} {
  const hook = updateWorkInProgressHook();//获取当前useRef
  return hook.memoizedState;//返回hook链表
}

useMemo&useCallback

声明阶段

和其他hook 一样
mount阶段

mount阶段useMemo和useCallback唯一区别是在memoizedState中存贮callback还是callback计算出来的函数

function mountMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null,
): T {
  const hook = mountWorkInProgressHook();//创建hook
  const nextDeps = deps === undefined ? null : deps;
  const nextValue = nextCreate();//计算value
  hook.memoizedState = [nextValue, nextDeps];//把value和依赖保存在memoizedState中
  return nextValue;
}

function mountCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = mountWorkInProgressHook();//创建hook
  const nextDeps = deps === undefined ? null : deps;
  hook.memoizedState = [callback, nextDeps];//把callback和依赖保存在memoizedState中
  return callback;
}

update阶段

update时也一样，唯一区别就是直接用回调函数还是执行回调后返回的value作为[?, nextDeps]赋值给memoizedState

function updateMemo<T>(
  nextCreate: () => T,
  deps: Array<mixed> | void | null,
): T {
  const hook = updateWorkInProgressHook();//获取hook
  const nextDeps = deps === undefined ? null : deps;
  const prevState = hook.memoizedState;

  if (prevState !== null) {
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1];
      if (areHookInputsEqual(nextDeps, prevDeps)) {//浅比较依赖
        return prevState[0];//没变 返回之前的状态
      }
    }
  }
  const nextValue = nextCreate();//有变化重新调用callback
  hook.memoizedState = [nextValue, nextDeps];
  return nextValue;
}

function updateCallback<T>(callback: T, deps: Array<mixed> | void | null): T {
  const hook = updateWorkInProgressHook();//获取hook
  const nextDeps = deps === undefined ? null : deps;
  const prevState = hook.memoizedState;

  if (prevState !== null) {
    if (nextDeps !== null) {
      const prevDeps: Array<mixed> | null = prevState[1];
      if (areHookInputsEqual(nextDeps, prevDeps)) {//浅比较依赖
        return prevState[0];//没变 返回之前的状态
      }
    }
  }

  hook.memoizedState = [callback, nextDeps];//变了重新将[callback, nextDeps]赋值给memoizedState
  return callback;
}

useLayoutEffect

useLayoutEffect和useEffect一样，只是调用的时机不同，它是在commit阶段的commitLayout函数中同步执行

forwardRef

forwardRef也非常简单，就是传递ref属性

export function forwardRef<Props, ElementType: React$ElementType>(
  render: (props: Props, ref: React$Ref<ElementType>) => React$Node,
) {
  
  const elementType = {
    $$typeof: REACT_FORWARD_REF_TYPE,
    render,
  };
  
  return elementType;
}
//ForwardRef第二个参数是ref对象
let children = Component(props, secondArg);

react源码解析13.hooks源码