React钩子渲染额外的时间

Question

我的代码导致了意外的重新渲染。

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行上的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。当通过按钮单击更改“
isOn”状态时，第二个日志用于重新渲染。第三个日志是setInterval调用setTimer时，因此它再次被重新渲染。这是我实际上得到的：

重新渲染0

重新渲染0

重新渲染1

重新渲染1

我不知道为什么还有第四个日志。这是它的REPL的链接：

https://codesandbox.io/s/kx393n58r7

***请澄清一下，我知道解决方案是使用setTimer（timer => timer + 1），但是我想知道为什么上面的代码会导致第四个渲染。

Answer 1

该函数具有大量的功能，这些功能在您调用由返回的setter时发生的事情useState都dispatchAction在ReactFiberHooks.js中（当前从1009行开始）。

当前，检查状态是否已更改的代码块（如果尚未更改，则可能跳过重新渲染）被以下条件包围：

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

我对此的假设是，在第二次setTimer调用后，此条件评估为false 。为了验证这一点，我复制了开发CDN
React文件，并向该dispatchAction函数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);
  }
}

这是控制台输出，为清晰起见还带有一些其他注释：

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。在第二个setTimer调用的日志中，该日志fiber.expirationTime已移至alternate.expirationTime仍阻止状态比较，因此重新呈现将是无条件的。之后，fiber和alternate到期时间均为0（NoWork），然后进行状态比较并避免重新渲染。

对React Fiber Architecture的描述是尝试了解的目的的一个很好的起点expirationTime。

用于理解它的源代码中最相关的部分是：

• ReactFiberExpirationTime.js
• ReactFiberScheduler.js

我相信到期时间主要与并发模式有关，并发模式默认情况下尚未启用。到期时间表示React将尽早强制提交工作的时间点。在该时间点之前，React可以选择批量更新。某些更新（例如，来自用户交互的更新）的到期时间非常短（高优先级），而其他更新（例如，提取完成后的来自异步代码的更新）的到期时间则更长（低优先级）。由setTimer内触发的更新setInterval回调将属于低优先级类别，并且有可能被分批处理（如果启用了并发模式）。由于有可能该工作已被批处理或可能被丢弃，因此如果先前的更新具有，React将无条件地将重新渲染排队（即使自上次更新以来状态不变）expirationTime。

您可以在这里看到我的答案，以了解更多有关通过React代码找到实现此dispatchAction功能的方法的知识。

对于其他想要自己挖掘的人，这是一个带有我的React修改版的CodeSandbox：

react文件是这些文件的修改后的副本：

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