React hook渲染额外的时间

Question

我的代码导致意外数量的重新渲染.

function App() {    
    const [isOn, setIsOn] = useState(false)
    const [timer, setTimer] = useState(0)
    console.log('re-rendered', timer)

    useEffect(() => {
        let interval

        if (isOn) {
            interval = setInterval(() => setTimer(timer + 1), 1000)
        }

        return () => clearInterval(interval)
    }, [isOn])

    return (
      <div>
        {timer}
        {!isOn && (
          <button type="button" onClick={() => setIsOn(true)}>
            Start
          </button>
        )}

        {isOn && (
          <button type="button" onClick={() => setIsOn(false)}>
            Stop
          </button>
        )}
      </div>
    );
 }

请注意第4行的console.log.我期望以下内容被注销:

重新渲染0

重新渲染0

重新渲染1

第一个日志用于初始渲染.当"isOn"状态通过按钮单击更改时,第二个日志用于重新呈现.第三个日志是setInterval调用setTimer时,它会再次重新渲染.这是我实际得到的:

重新渲染0

重新渲染0

重新渲染1

重新渲染1

我无法弄清楚为什么会有第四个日志.这是一个链接到它的REPL:

https://codesandbox.io/s/kx393n58r7

***只是为了澄清,我知道解决方案是使用setTimer(timer => timer + 1),但我想知道为什么上面的代码会导致第四次渲染.

Answer 1

当你调用返回的setter时发生的大部分函数useState都dispatchAction在ReactFiberHooks.js内(当前从第1009行开始).

检查状态是否已更改的代码块(如果未更改,则可能会跳过重新呈现)当前包含以下条件:

if (
  fiber.expirationTime === NoWork &&
  (alternate === null || alternate.expirationTime === NoWork)
) {

我看到这个的假设是这个条件在第二次setTimer调用后被评估为假.为了验证这一点,我复制了开发CDN React文件并在dispatchAction函数中添加了一些控制台日志:

function dispatchAction(fiber, queue, action) {
  !(numberOfReRenders < RE_RENDER_LIMIT) ? invariant(false, 'Too many re-renders. React limits the number of renders to prevent an infinite loop.') : void 0;

  {
    !(arguments.length <= 3) ? warning$1(false, "State updates from the useState() and useReducer() Hooks don't support the " + 'second callback argument. To execute a side effect after ' + 'rendering, declare it in the component body with useEffect().') : void 0;
  }
  console.log("dispatchAction1");
  var alternate = fiber.alternate;
  if (fiber === currentlyRenderingFiber$1 || alternate !== null && alternate === currentlyRenderingFiber$1) {
    // This is a render phase update. Stash it in a lazily-created map of
    // queue -> linked list of updates. After this render pass, we'll restart
    // and apply the stashed updates on top of the work-in-progress hook.
    didScheduleRenderPhaseUpdate = true;
    var update = {
      expirationTime: renderExpirationTime,
      action: action,
      eagerReducer: null,
      eagerState: null,
      next: null
    };
    if (renderPhaseUpdates === null) {
      renderPhaseUpdates = new Map();
    }
    var firstRenderPhaseUpdate = renderPhaseUpdates.get(queue);
    if (firstRenderPhaseUpdate === undefined) {
      renderPhaseUpdates.set(queue, update);
    } else {
      // Append the update to the end of the list.
      var lastRenderPhaseUpdate = firstRenderPhaseUpdate;
      while (lastRenderPhaseUpdate.next !== null) {
        lastRenderPhaseUpdate = lastRenderPhaseUpdate.next;
      }
      lastRenderPhaseUpdate.next = update;
    }
  } else {
    flushPassiveEffects();

    console.log("dispatchAction2");
    var currentTime = requestCurrentTime();
    var _expirationTime = computeExpirationForFiber(currentTime, fiber);

    var _update2 = {
      expirationTime: _expirationTime,
      action: action,
      eagerReducer: null,
      eagerState: null,
      next: null
    };

    // Append the update to the end of the list.
    var _last = queue.last;
    if (_last === null) {
      // This is the first update. Create a circular list.
      _update2.next = _update2;
    } else {
      var first = _last.next;
      if (first !== null) {
        // Still circular.
        _update2.next = first;
      }
      _last.next = _update2;
    }
    queue.last = _update2;

    console.log("expiration: " + fiber.expirationTime);
    if (alternate) {
      console.log("alternate expiration: " + alternate.expirationTime);
    }
    if (fiber.expirationTime === NoWork && (alternate === null || alternate.expirationTime === NoWork)) {
      console.log("dispatchAction3");

      // The queue is currently empty, which means we can eagerly compute the
      // next state before entering the render phase. If the new state is the
      // same as the current state, we may be able to bail out entirely.
      var _eagerReducer = queue.eagerReducer;
      if (_eagerReducer !== null) {
        var prevDispatcher = void 0;
        {
          prevDispatcher = ReactCurrentDispatcher$1.current;
          ReactCurrentDispatcher$1.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
        }
        try {
          var currentState = queue.eagerState;
          var _eagerState = _eagerReducer(currentState, action);
          // Stash the eagerly computed state, and the reducer used to compute
          // it, on the update object. If the reducer hasn't changed by the
          // time we enter the render phase, then the eager state can be used
          // without calling the reducer again.
          _update2.eagerReducer = _eagerReducer;
          _update2.eagerState = _eagerState;
          if (is(_eagerState, currentState)) {
            // Fast path. We can bail out without scheduling React to re-render.
            // It's still possible that we'll need to rebase this update later,
            // if the component re-renders for a different reason and by that
            // time the reducer has changed.
            return;
          }
        } catch (error) {
          // Suppress the error. It will throw again in the render phase.
        } finally {
          {
            ReactCurrentDispatcher$1.current = prevDispatcher;
          }
        }
      }
    }
    {
      if (shouldWarnForUnbatchedSetState === true) {
        warnIfNotCurrentlyBatchingInDev(fiber);
      }
    }
    scheduleWork(fiber, _expirationTime);
  }
}

为了清晰起见,这里是控制台输出和一些额外的注释:

re-rendered 0 // initial render

dispatchAction1 // setIsOn
dispatchAction2
expiration: 0
dispatchAction3
re-rendered 0

dispatchAction1 // first call to setTimer
dispatchAction2
expiration: 1073741823
alternate expiration: 0
re-rendered 1

dispatchAction1 // second call to setTimer
dispatchAction2
expiration: 0
alternate expiration: 1073741823
re-rendered 1

dispatchAction1 // third and subsequent calls to setTimer all look like this
dispatchAction2
expiration: 0
alternate expiration: 0
dispatchAction3

NoWork值为零.您可以看到fiber.expirationTimeafter 的第一个日志setTimer具有非零值.在第二次setTimer调用的日志中,fiber.expirationTime已经移动到alternate.expirationTime仍然阻止状态比较,因此重新呈现将是无条件的.之后,fiber和alternate失效时间均为0(NoWork),然后进行状态比较并避免重新渲染.

React Fiber Architecture的这种描述是尝试理解其目的的良好起点expirationTime.

理解它的源代码中最相关的部分是:

• ReactFiberExpirationTime.js
• ReactFiberScheduler.js

我认为到期时间主要与默认情况下尚未启用的并发模式相关.到期时间表示React将尽早强制提交作品的时间点.在此之前,React可能会选择批量更新.某些更新(例如来自用户交互)具有非常短(高优先级)的到期,并且其他更新(例如,在获取完成后来自异步代码)具有更长(低优先级)到期.setTimer在setInterval回调中触发的更新将属于低优先级类别,并且可能被批处理(如果启用了并发模式).由于可能已经批量处理或可能丢弃该工作,因此如果先前的更新具有,则React无条件地排队重新呈现(即使自上次更新后状态未改变)expirationTime.

你可以在这里看到我的答案,以了解更多关于通过React代码找到你的方法来获得这个dispatchAction功能.

对于那些想要挖掘自己的人来说,这里有一个CodeSandbox和我修改过的React版本:

react文件是这些文件的修改副本:

https://unpkg.com/react@16/umd/react.development.js
https://unpkg.com/react-dom@16/umd/react-dom.development.js