当您调用返回的 setter 时，该函数会发生大部分情况useState is dispatchAction within ReactFiberHooks.js https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberHooks.js（当前从第 1009 行开始）。
检查状态是否已更改（如果未更改，则可能会跳过重新渲染）的代码块当前包含以下条件：
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之后评估为错误setTimer称呼。为了验证这一点，我复制了开发 CDN React 文件并将一些控制台日志添加到dispatchAction功能：
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);
}
}
为了清楚起见，这是控制台输出以及一些附加注释：
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其值为零。可以看到第一条日志fiber.expirationTime after setTimer具有非零值。在第二个日志中setTimer打电话，那个fiber.expirationTime已移至alternate.expirationTime仍然阻止状态比较，因此重新渲染将是无条件的。此后，双方fiber and alternate过期时间为 0（NoWork），然后进行状态比较并避免重新渲染。
React Fiber 架构的描述 https://github.com/acdlite/react-fiber-architecture是尝试理解其目的的一个很好的起点expirationTime.
源代码中与理解它最相关的部分是：
- ReactFiberExpirationTime.js https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberExpirationTime.js
- ReactFiberScheduler.js https://github.com/facebook/react/blob/master/packages/react-reconciler/src/ReactFiberScheduler.js
我相信过期时间主要与默认情况下尚未启用的并发模式相关。过期时间表示 React 将强制尽早提交工作的时间点。在此时间点之前，React 可能会选择批量更新。某些更新（例如来自用户交互的更新）具有非常短（高优先级）的过期时间，而其他更新（例如来自提取完成后的异步代码的更新）具有较长（低优先级）的过期时间。触发的更新setTimer从内部setInterval回调将属于低优先级类别，并且可能会被批量处理（如果启用了并发模式）。由于该工作有可能已被批处理或可能被丢弃，如果先前的更新有expirationTime.
你可以看看我的回答here https://stackoverflow.com/questions/53974865/how-do-react-hooks-determine-the-component-that-they-are-for/53980190#53980190详细了解如何通过 React 代码找到实现此目标的方法dispatchAction功能。
对于其他想要自己挖掘的人，这里有一个包含我修改后的 React 版本的 CodeSandbox：
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
