【JS】React源码 commit阶段详解

• Z时代
• 2024-01-10
• 分类：技术分享

function commitRoot(root) {
const renderPriorityLevel = getCurrentPriorityLevel();
runWithPriority(
ImmediateSchedulerPriority,
commitRootImpl.bind(null, root, renderPriorityLevel),
);
return null;
}

function commitRootImpl(root, renderPriorityLevel) {
// 进入commit阶段，先执行一次之前未执行的useEffect
do {
flushPassiveEffects();
} while (rootWithPendingPassiveEffects !== null);
// 准备阶段-----------------------------------------------
const finishedWork = root.finishedWork;
const lanes = root.finishedLanes;
if (finishedWork === null) {
return null;
}
root.finishedWork = null;
root.finishedLanes = NoLanes;
root.callbackNode = null;
root.callbackId = NoLanes;
// effectList的整理，将root上的effect连到effectList的末尾
let firstEffect;
if (finishedWork.effectTag > PerformedWork) {
if (finishedWork.lastEffect !== null) {
finishedWork.lastEffect.nextEffect = finishedWork;
firstEffect = finishedWork.firstEffect;
} else {
firstEffect = finishedWork;
}
} else {
// There is no effect on the root.
firstEffect = finishedWork.firstEffect;
}
// 准备阶段结束，开始处理effectList
if (firstEffect !== null) {
...
// before mutation阶段--------------------------------
nextEffect = firstEffect;
do {...} while (nextEffect !== null);
...
// mutation阶段---------------------------------------
nextEffect = firstEffect;
do {...} while (nextEffect !== null);
// 将wprkInProgress树切换为current树
root.current = finishedWork;
// layout阶段-----------------------------------------
nextEffect = firstEffect;
do {...} while (nextEffect !== null);
nextEffect = null;
// 通知浏览器去绘制
requestPaint();
} else {
// 没有effectList，直接将wprkInProgress树切换为current树
root.current = finishedWork;
}
const rootDidHavePassiveEffects = rootDoesHavePassiveEffects;
// 获取尚未处理的优先级，比如之前被跳过的任务的优先级
remainingLanes = root.pendingLanes;
// 将被跳过的优先级放到root上的pendingLanes（待处理的优先级）上
markRootFinished(root, remainingLanes);
/*
* 每次commit阶段完成后，再执行一遍ensureRootIsScheduled，确保是否还有任务需要被调度。
* 例如，高优先级插队的更新完成后，commit完成后，还会再执行一遍，保证之前跳过的低优先级任务
* 重新调度
*
* */
ensureRootIsScheduled(root, now());
...
return null;
}

nextEffect = firstEffect;
do {
try {
commitBeforeMutationEffects();
} catch (error) {
...
}
} while (nextEffect !== null);

function commitBeforeMutationEffects() {
while (nextEffect !== null) {
const current = nextEffect.alternate;
...
const flags = nextEffect.flags;
if ((flags & Snapshot) !== NoFlags) {
// 通过commitBeforeMutationEffectOnFiber调用getSnapshotBeforeUpdate
commitBeforeMutationEffectOnFiber(current, nextEffect);
}
if ((flags & Passive) !== NoFlags) {
// 异步调度useEffect
if (!rootDoesHavePassiveEffects) {
rootDoesHavePassiveEffects = true;
scheduleCallback(NormalSchedulerPriority, () => {
flushPassiveEffects();
return null;
});
}
}
nextEffect = nextEffect.nextEffect;
}
}

function commitBeforeMutationLifeCycles(
current: Fiber | null,
finishedWork: Fiber,
): void {
switch (finishedWork.tag) {
...
case ClassComponent: {
if (finishedWork.flags & Snapshot) {
if (current !== null) {
const prevProps = current.memoizedProps;
const prevState = current.memoizedState;
const instance = finishedWork.stateNode;
// 调用getSnapshotBeforeUpdate
const snapshot = instance.getSnapshotBeforeUpdate(
finishedWork.elementType === finishedWork.type
? prevProps
: resolveDefaultProps(finishedWork.type, prevProps),
prevState,
);
// 将返回值存储在内部属性上，方便componentDidUpdate获取
instance.__reactInternalSnapshotBeforeUpdate = snapshot;
}
}
return;
}
...
}
}

    nextEffect = firstEffect;
do {
try {
commitMutationEffects(root, renderPriorityLevel);
} catch (error) {
...
nextEffect = nextEffect.nextEffect;
}
} while (nextEffect !== null);

nextEffect = firstEffect;
do {
try {
commitLayoutEffects(root, lanes);
} catch (error) {
...
nextEffect = nextEffect.nextEffect;
}
} while (nextEffect !== null);

function commitLifeCycles(
finishedRoot: FiberRoot,
current: Fiber | null,
finishedWork: Fiber,
committedLanes: Lanes,
): void {
switch (finishedWork.tag) {
case FunctionComponent:
case ForwardRef:
case SimpleMemoComponent:
case Block: {
// 执行useLayoutEffect的创建
commitHookEffectListMount(HookLayout | HookHasEffect, finishedWork);
// 填充useEffect的effect执行数组
schedulePassiveEffects(finishedWork);
return;
}
case ClassComponent: {
const instance = finishedWork.stateNode;
if (finishedWork.flags & Update) {
if (current === null) {
// 如果是初始挂载阶段，调用componentDidMount
instance.componentDidMount();
} else {
// 如果是更新阶段，调用componentDidUpdate
const prevProps =
finishedWork.elementType === finishedWork.type
? current.memoizedProps
: resolveDefaultProps(finishedWork.type, current.memoizedProps);
const prevState = current.memoizedState;
instance.componentDidUpdate(
prevProps,
prevState,
// 将getSnapshotBeforeUpdate的结果传入
instance.__reactInternalSnapshotBeforeUpdate,
);
}
}
// 调用setState的回调
const updateQueue: UpdateQueue<
*,
> | null = (finishedWork.updateQueue: any);
if (updateQueue !== null) {
commitUpdateQueue(finishedWork, updateQueue, instance);
}
return;
}
...
}
}

点击进入React源码调试仓库。

当render阶段完成后，意味着在内存中构建的workInProgress树所有更新工作已经完成，这包括树中fiber节点的更新、diff、effectTag的标记、effectList的收集。此时workInProgress树的完整形态如下：

和current树相比，它们的结构上固然存在区别，变化的fiber节点也存在于workInProgress树，但要将这些节点应用到DOM上却不会循环整棵树，而是通过循环effectList这个链表来实现，这样保证了只针对有变化的节点做工作。

所以循环effectList链表去将有更新的fiber节点应用到页面上是commit阶段的主要工作。

commit阶段的入口是commitRoot函数，它会告知scheduler以立即执行的优先级去调度commit阶段的工作。

function commitRoot(root) {
const renderPriorityLevel = getCurrentPriorityLevel();
runWithPriority(
ImmediateSchedulerPriority,
commitRootImpl.bind(null, root, renderPriorityLevel),
);
return null;
}

scheduler去调度的是commitRootImpl，它是commit阶段的核心实现，整个commit阶段被划分成三个部分。

commit流程概览

commit阶段主要是针对root上收集的effectList进行处理。在真正的工作开始之前，有一个准备阶段，主要是变量的赋值，以及将root的effect加入到effectList中。随后开始针对effectList分三个阶段进行工作：

• before mutation：读取组件变更前的状态，针对类组件，调用getSnapshotBeforeUpdate，让我们可以在DOM变更前获取组件实例的信息；针对函数组件，异步调度useEffect。
• mutation：针对HostComponent，进行相应的DOM操作；针对类组件，调用componentWillUnmount；针对函数组件，执行useLayoutEffect的销毁函数。
• layout：在DOM操作完成后，读取组件的状态，针对类组件，调用生命周期componentDidMount和componentDidUpdate，调用setState的回调；针对函数组件填充useEffect 的 effect执行数组，并调度useEffect

before mutation和layout针对函数组件的useEffect调度是互斥的，只能发起一次调度

workInProgress 树切换到current树的时机是在mutation结束后，layout开始前。这样做的原因是在mutation阶段调用类组件的componentWillUnmount的时候，
还可以获取到卸载前的组件信息；在layout阶段调用componentDidMount/Update时，获取的组件信息更新后的。

function commitRootImpl(root, renderPriorityLevel) {
// 进入commit阶段，先执行一次之前未执行的useEffect
do {
flushPassiveEffects();
} while (rootWithPendingPassiveEffects !== null);
// 准备阶段-----------------------------------------------
const finishedWork = root.finishedWork;
const lanes = root.finishedLanes;
if (finishedWork === null) {
return null;
}
root.finishedWork = null;
root.finishedLanes = NoLanes;
root.callbackNode = null;
root.callbackId = NoLanes;
// effectList的整理，将root上的effect连到effectList的末尾
let firstEffect;
if (finishedWork.effectTag > PerformedWork) {
if (finishedWork.lastEffect !== null) {
finishedWork.lastEffect.nextEffect = finishedWork;
firstEffect = finishedWork.firstEffect;
} else {
firstEffect = finishedWork;
}
} else {
// There is no effect on the root.
firstEffect = finishedWork.firstEffect;
}
// 准备阶段结束，开始处理effectList
if (firstEffect !== null) {
...
// before mutation阶段--------------------------------
nextEffect = firstEffect;
do {...} while (nextEffect !== null);
...
// mutation阶段---------------------------------------
nextEffect = firstEffect;
do {...} while (nextEffect !== null);
// 将wprkInProgress树切换为current树
root.current = finishedWork;
// layout阶段-----------------------------------------
nextEffect = firstEffect;
do {...} while (nextEffect !== null);
nextEffect = null;
// 通知浏览器去绘制
requestPaint();
} else {
// 没有effectList，直接将wprkInProgress树切换为current树
root.current = finishedWork;
}
const rootDidHavePassiveEffects = rootDoesHavePassiveEffects;
// 获取尚未处理的优先级，比如之前被跳过的任务的优先级
remainingLanes = root.pendingLanes;
// 将被跳过的优先级放到root上的pendingLanes（待处理的优先级）上
markRootFinished(root, remainingLanes);
/*
* 每次commit阶段完成后，再执行一遍ensureRootIsScheduled，确保是否还有任务需要被调度。
* 例如，高优先级插队的更新完成后，commit完成后，还会再执行一遍，保证之前跳过的低优先级任务
* 重新调度
*
* */
ensureRootIsScheduled(root, now());
...
return null;
}

下面的部分，是对这三个阶段分别进行的详细讲解。

before Mutation

beforeMutation阶段的入口函数是commitBeforeMutationEffects

nextEffect = firstEffect;
do {
try {
commitBeforeMutationEffects();
} catch (error) {
...
}
} while (nextEffect !== null);

它的作用主要是调用类组件的getSnapshotBeforeUpdate，针对函数组件，异步调度useEffect。

function commitBeforeMutationEffects() {
while (nextEffect !== null) {
const current = nextEffect.alternate;
...
const flags = nextEffect.flags;
if ((flags & Snapshot) !== NoFlags) {
// 通过commitBeforeMutationEffectOnFiber调用getSnapshotBeforeUpdate
commitBeforeMutationEffectOnFiber(current, nextEffect);
}
if ((flags & Passive) !== NoFlags) {
// 异步调度useEffect
if (!rootDoesHavePassiveEffects) {
rootDoesHavePassiveEffects = true;
scheduleCallback(NormalSchedulerPriority, () => {
flushPassiveEffects();
return null;
});
}
}
nextEffect = nextEffect.nextEffect;
}
}

commitBeforeMutationEffectOnFiber代码如下

function commitBeforeMutationLifeCycles(
current: Fiber | null,
finishedWork: Fiber,
): void {
switch (finishedWork.tag) {
...
case ClassComponent: {
if (finishedWork.flags & Snapshot) {
if (current !== null) {
const prevProps = current.memoizedProps;
const prevState = current.memoizedState;
const instance = finishedWork.stateNode;
// 调用getSnapshotBeforeUpdate
const snapshot = instance.getSnapshotBeforeUpdate(
finishedWork.elementType === finishedWork.type
? prevProps
: resolveDefaultProps(finishedWork.type, prevProps),
prevState,
);
// 将返回值存储在内部属性上，方便componentDidUpdate获取
instance.__reactInternalSnapshotBeforeUpdate = snapshot;
}
}
return;
}
...
}
}

mutation

mutation阶段会真正操作DOM节点，涉及到的操作有增、删、改。入口函数是commitMutationEffects

    nextEffect = firstEffect;
do {
try {
commitMutationEffects(root, renderPriorityLevel);
} catch (error) {
...
nextEffect = nextEffect.nextEffect;
}
} while (nextEffect !== null);

由于过程较为复杂，所以我写了三篇文章来说明这三种DOM操作，如果想要了解细节，可以看一下。文章写于17还未正式发布的时候，所以里面的源码版本取自17.0.0-alpha0。

React和DOM的那些事-节点新增算法

React和DOM的那些事-节点删除算法

React和DOM的那些事-节点更新

layout阶段

layout阶段的入口函数是commitLayoutEffects。

nextEffect = firstEffect;
do {
try {
commitLayoutEffects(root, lanes);
} catch (error) {
...
nextEffect = nextEffect.nextEffect;
}
} while (nextEffect !== null);

我们只关注classComponent和functionComponent。针对前者，调用生命周期componentDidMount和componentDidUpdate，调用setState的回调；针对后者，填充useEffect 的 effect执行数组，并调度useEffect（具体的原理在我的这篇文章：梳理useEffect和useLayoutEffect的原理与区别中有讲解）。

function commitLifeCycles(
finishedRoot: FiberRoot,
current: Fiber | null,
finishedWork: Fiber,
committedLanes: Lanes,
): void {
switch (finishedWork.tag) {
case FunctionComponent:
case ForwardRef:
case SimpleMemoComponent:
case Block: {
// 执行useLayoutEffect的创建
commitHookEffectListMount(HookLayout | HookHasEffect, finishedWork);
// 填充useEffect的effect执行数组
schedulePassiveEffects(finishedWork);
return;
}
case ClassComponent: {
const instance = finishedWork.stateNode;
if (finishedWork.flags & Update) {
if (current === null) {
// 如果是初始挂载阶段，调用componentDidMount
instance.componentDidMount();
} else {
// 如果是更新阶段，调用componentDidUpdate
const prevProps =
finishedWork.elementType === finishedWork.type
? current.memoizedProps
: resolveDefaultProps(finishedWork.type, current.memoizedProps);
const prevState = current.memoizedState;
instance.componentDidUpdate(
prevProps,
prevState,
// 将getSnapshotBeforeUpdate的结果传入
instance.__reactInternalSnapshotBeforeUpdate,
);
}
}
// 调用setState的回调
const updateQueue: UpdateQueue<
*,
> | null = (finishedWork.updateQueue: any);
if (updateQueue !== null) {
commitUpdateQueue(finishedWork, updateQueue, instance);
}
return;
}
...
}
}

总结

commit阶段将effectList的处理分成三个阶段保证了不同生命周期函数的适时调用。相对于同步执行的useEffectLayout，useEffect的异步调度提供了一种不阻塞页面渲染的副作用操作入口。另外，标记root上还未处理的优先级和调用ensureRootIsScheduled使得被跳过的低优先级任务得以再次被调度。commit阶段的完成，也就意味着本次更新已经结束。

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