react源码解析13.hooks源码

react源码解析13.hooks源码

视频讲解(高效学习):进入学习

hook调用入口

​ 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);
    }
    
function basicStateReducer(state: S, action: BasicStateAction): S {
  return typeof action === 'function' ? action(state) : action;
}
  • 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 | 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(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(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(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(
  nextCreate: () => T,
  deps: Array | 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(callback: T, deps: Array | 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(
  nextCreate: () => T,
  deps: Array | 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 | 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(callback: T, deps: Array | 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 | 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(
  render: (props: Props, ref: React$Ref) => React$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);