与散装的，当你调用返回的制定者会发生什么功能useState是dispatchAction内ReactFiberHooks.js（目前开始行1009）。
检查状态是否已更改（如果未更改则可能跳过重新渲染）的代码块当前被以下条件包围：
if (
fiber.expirationTime === NoWork &&
(alternate === null || alternate.expirationTime === NoWork)
) {
我看到这一点的假设是，在第二次setTimer调用后，这种情况评估为假。为了验证这一点，我复制了开发 CDN React 文件并向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);
}
}
为清楚起见，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，它fiber.expirationTime已被移至alternate.expirationTime仍然阻止状态比较，因此重新渲染将是无条件的。之后，过期时间fiber和alternate过期时间都为 0（NoWork），然后它会进行状态比较并避免重新渲染。
这篇 React Fiber 架构的描述是尝试理解expirationTime.
理解它的源代码中最相关的部分是：
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 将尽早强制提交工作的时间点。在那个时间点之前，React 可能会选择批量更新。某些更新（例如来自用户交互）的过期时间非常短（高优先级），而其他更新（例如获取完成后的异步代码）的过期时间更长（低优先级）。setTimer从内部触发的更新setInterval回调将属于低优先级类别，并且可能会被批处理（如果启用了并发模式）。由于该工作有可能被批处理或可能被丢弃，如果上一次更新有一个expirationTime.
您可以在此处查看我的答案，以了解更多有关如何通过 React 代码找到该dispatchAction功能的方法。
对于其他想要自己挖掘的人，这里有一个带有我修改过的 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