useEffect和useLayoutEffect的区别

使用方式
这两个函数的使用方式其实非常简单，他们都接受一个函数``一个数组，只有在数组里面的值改变的情况下才会再次执行 effect。

差异

useEffect 是异步执行的，而useLayoutEffect是同步执行的。
useEffect 的执行时机是浏览器完成渲染之后，而 useLayoutEffect 的执行时机是浏览器把内容真正渲染到界面之前，和 componentDidMount 等价。

具体表现

我们用一个例子说明

import React, { useEffect, useLayoutEffect, useState } from 'react';

function App() {
  const [state, setState] = useState("hello world")

  useEffect(() => {
    let i = 0;
    while(i <= 100000000) {
      i++;
    };
    setState("world hello");
  }, []);

  // useLayoutEffect(() => {
  //   let i = 0;
  //   while(i <= 100000000) {
  //     i++;
  //   };
  //   setState("world hello");
  // }, []);

  return (
    <>
      <div>{state}</div>
    </>
  );
}

export default App;

在这里插入图片描述

而换成 useLayoutEffect 之后闪烁现象就消失了。

·useEffect· 是渲染完之后异步执行的，所以会导致 ·hello world ·先被渲染到了屏幕上，再变成 ·world hello·，就会出现·闪烁现象·。而 useLayoutEffect是渲染之前同步执行的，所以会等它执行完再渲染上去，就避免了闪烁现象。也就是说我们最好把操作 dom 的相关操作放到 useLayouteEffect 中去，避免导致闪烁。

源码剖析
useEffect
首先找到 useEffect 调用的入口

function updateEffect(create, deps) {
  {
    // $FlowExpectedError - jest isn't a global, and isn't recognized outside of tests
    if ('undefined' !== typeof jest) {
      warnIfNotCurrentlyActingEffectsInDEV(currentlyRenderingFiber$1);
    }
  }

  return updateEffectImpl(Update | Passive, Passive$1, create, deps);
}

调用 updateEffectImpl 时传入的 hookEffectTag 为 Passive$1 , 所以我们找一下：Passive$1。

然后我们找到是在这里传入了 Passive$1 类型来调用 useEffect 。

function commitPassiveHookEffects(finishedWork) {
  if ((finishedWork.effectTag & Passive) !== NoEffect) {
    switch (finishedWork.tag) {
      case FunctionComponent:
      case ForwardRef:
      case SimpleMemoComponent:
      case Block:
        {
          // TODO (#17945) We should call all passive destroy functions (for all fibers)
          // before calling any create functions. The current approach only serializes
          // these for a single fiber.
          commitHookEffectListUnmount(Passive$1 | HasEffect, finishedWork);
          commitHookEffectListMount(Passive$1 | HasEffect, finishedWork);
          break;
        }
    }
  }
}

那我们继续顺藤摸瓜找 commitPassiveHookEffects

function flushPassiveEffectsImpl() {
    //...省略
    while (_effect2 !== null) {
      {
        setCurrentFiber(_effect2);
        invokeGuardedCallback(null, commitPassiveHookEffects, null, _effect2);
      }
   }
    //...省略
}

找flushPassiveEffectsImpl

function flushPassiveEffects() {
  if (pendingPassiveEffectsRenderPriority !== NoPriority) {
    var priorityLevel = pendingPassiveEffectsRenderPriority > NormalPriority ? NormalPriority : pendingPassiveEffectsRenderPriority;
    pendingPassiveEffectsRenderPriority = NoPriority;
    return runWithPriority$1(priorityLevel, flushPassiveEffectsImpl);
  }
}

再往上一层是commitBeforeMutationEffects，这里面调用flushPassiveEffects的方法是scheduleCallback，这是一个调度操作，是异步执行的。

function commitBeforeMutationEffects{
    //...省略
    if ((effectTag & Passive) !== NoEffect) {
      // If there are passive effects, schedule a callback to flush at
      // the earliest opportunity.
      if (!rootDoesHavePassiveEffects) {
        rootDoesHavePassiveEffects = true;
        scheduleCallback(NormalPriority, function () {
          flushPassiveEffects();
          return null;
        });
      }
    }
    //...省略
}

继续顺着 commitBeforeMutationEffects方法往上找的话，我们可以找到最终调用 useEffect 的地方是 commitRootImpl ，这是我们 commit 阶段会调用的一个函数，所以就是在这里面对 useEffect 进行了调度，在完成渲染工作以后去异步执行了 useEffect 。

useLayoutEffect

function updateLayoutEffect(create, deps) {
  return updateEffectImpl(Update, Layout, create, deps);
}

这里传进去的 hookEffectTag 是Layout，那么我们找一下Layout。

function commitLifeCycles(finishedRoot, current, finishedWork, committedExpirationTime) {
  switch (finishedWork.tag) {
    case FunctionComponent:
    case ForwardRef:
    case SimpleMemoComponent:
    case Block:
      {
        // At this point layout effects have already been destroyed (during mutation phase).
        // This is done to prevent sibling component effects from interfering with each other,
        // e.g. a destroy function in one component should never override a ref set
        // by a create function in another component during the same commit.
        commitHookEffectListMount(Layout | HasEffect, finishedWork);

        return;
      }

    case ClassComponent:
      {
        var instance = finishedWork.stateNode;

        if (finishedWork.effectTag & Update) {
          if (current === null) {
            startPhaseTimer(finishedWork, 'componentDidMount'); // We could update instance props and state here,
            // but instead we rely on them being set during last render.
            // TODO: revisit this when we implement resuming.

            {
              if (finishedWork.type === finishedWork.elementType && !didWarnAboutReassigningProps) {
                if (instance.props !== finishedWork.memoizedProps) {
                  error('Expected %s props to match memoized props before ' + 'componentDidMount. ' + 'This might either be because of a bug in React, or because ' + 'a component reassigns its own `this.props`. ' + 'Please file an issue.', getComponentName(finishedWork.type) || 'instance');
                }

                if (instance.state !== finishedWork.memoizedState) {
                  error('Expected %s state to match memoized state before ' + 'componentDidMount. ' + 'This might either be because of a bug in React, or because ' + 'a component reassigns its own `this.props`. ' + 'Please file an issue.', getComponentName(finishedWork.type) || 'instance');
                }
              }
            }

            instance.componentDidMount();
            stopPhaseTimer();
          } else {
            var prevProps = finishedWork.elementType === finishedWork.type ? current.memoizedProps : resolveDefaultProps(finishedWork.type, current.memoizedProps);
            var prevState = current.memoizedState;
            startPhaseTimer(finishedWork, 'componentDidUpdate'); // We could update instance props and state here,
            // but instead we rely on them being set during last render.
            // TODO: revisit this when we implement resuming.

            {
              if (finishedWork.type === finishedWork.elementType && !didWarnAboutReassigningProps) {
                if (instance.props !== finishedWork.memoizedProps) {
                  error('Expected %s props to match memoized props before ' + 'componentDidUpdate. ' + 'This might either be because of a bug in React, or because ' + 'a component reassigns its own `this.props`. ' + 'Please file an issue.', getComponentName(finishedWork.type) || 'instance');
                }

                if (instance.state !== finishedWork.memoizedState) {
                  error('Expected %s state to match memoized state before ' + 'componentDidUpdate. ' + 'This might either be because of a bug in React, or because ' + 'a component reassigns its own `this.props`. ' + 'Please file an issue.', getComponentName(finishedWork.type) || 'instance');
                }
              }
            }

            instance.componentDidUpdate(prevProps, prevState, instance.__reactInternalSnapshotBeforeUpdate);
            stopPhaseTimer();
          }
        }
      ...省略
}