react源码解析10.commit阶段 视频讲解（高效学习）：进入学习 往期文章： 1.开篇介绍和面试题
2.react的设计理念
3.react源码架构
4.源码目录结构和调试
5.jsx&核心api
6.legacy和concurrent模式入口函数
7.Fiber架构
8.render阶段
9.diff算法
10.commit阶段
11.生命周期
12.状态更新流程
13.hooks源码
14.手写hooks
15.scheduler&Lane
16.concurrent模式
17.context
18事件系统
19.手写迷你版react
20.总结&第一章的面试题解答
在render阶段的末尾会调用commitRoot(root); 进入commit阶段，这里的root指的就是fiberRoot，然后会遍历render阶段生成的effectList，effectList上的Fiber节点保存着对应的props变化。之后会遍历effectList进行对应的dom操作和生命周期、hooks回调或销毁函数，各个函数做的事情如下

文章图片
在commitRoot函数中其实是调度了commitRootImpl函数

//ReactFiberWorkLoop.old.js function commitRoot(root) { var renderPriorityLevel = getCurrentPriorityLevel(); runWithPriority$1(ImmediatePriority$1, commitRootImpl.bind(null, root, renderPriorityLevel)); return null; }

在commitRootImpl的函数中主要分三个部分：

• commit阶段前置工作
  
  1. 调用flushPassiveEffects执行完所有effect的任务
  2. 初始化相关变量
  3. 赋值firstEffect给后面遍历effectList用
     

     //ReactFiberWorkLoop.old.js do { // 调用flushPassiveEffects执行完所有effect的任务 flushPassiveEffects(); } while (rootWithPendingPassiveEffects !== null); //...// 重置变量 finishedWork指rooFiber root.finishedWork = null; //重置优先级 root.finishedLanes = NoLanes; // Scheduler回调函数重置 root.callbackNode = null; root.callbackId = NoLanes; // 重置全局变量 if (root === workInProgressRoot) { workInProgressRoot = null; workInProgress = null; workInProgressRootRenderLanes = NoLanes; } else { }//rootFiber可能会有新的副作用 将它也加入到effectLis let firstEffect; if (finishedWork.effectTag > PerformedWork) { if (finishedWork.lastEffect !== null) { finishedWork.lastEffect.nextEffect = finishedWork; firstEffect = finishedWork.firstEffect; } else { firstEffect = finishedWork; } } else { firstEffect = finishedWork.firstEffect; }

     
     

• mutation阶段
  ? 遍历effectList分别执行三个方法commitBeforeMutationEffects、commitMutationEffects、commitLayoutEffects执行对应的dom操作和生命周期
  ? 在介绍双缓存Fiber树的时候，我们在构建完workInProgress Fiber树之后会将fiberRoot的current指向workInProgress Fiber，让workInProgress Fiber成为current，这个步骤发生在commitMutationEffects函数执行之后，commitLayoutEffects之前，因为componentWillUnmount发生在commitMutationEffects函数中，这时还可以获取之前的Update，而componentDidMount和componentDidUpdate会在commitLayoutEffects中执行，这时已经可以获取更新后的真实dom了
  

  function commitRootImpl(root, renderPriorityLevel) { //... do { //... commitBeforeMutationEffects(); } while (nextEffect !== null); do { //... commitMutationEffects(root, renderPriorityLevel); //commitMutationEffects } while (nextEffect !== null); root.current = finishedWork; //切换current Fiber树do { //... commitLayoutEffects(root, lanes); //commitLayoutEffects } while (nextEffect !== null); //... }

  
  

• mutation 后
  
  1. 根据rootDoesHavePassiveEffects赋值相关变量
  2. 执行flushSyncCallbackQueue处理componentDidMount等生命周期或者useLayoutEffect等同步任务
     

     const rootDidHavePassiveEffects = rootDoesHavePassiveEffects; // 根据rootDoesHavePassiveEffects赋值相关变量 if (rootDoesHavePassiveEffects) { rootDoesHavePassiveEffects = false; rootWithPendingPassiveEffects = root; pendingPassiveEffectsLanes = lanes; pendingPassiveEffectsRenderPriority = renderPriorityLevel; } else {} //...// 确保被调度 ensureRootIsScheduled(root, now()); // ...// 执行flushSyncCallbackQueue处理componentDidMount等生命周期或者useLayoutEffect等同步任务 flushSyncCallbackQueue(); return null;

     
     

现在让我们来看看mutation阶段的三个函数分别做了什么事情

• commitBeforeMutationEffects
  该函数主要做了如下两件事
  
  1. 执行getSnapshotBeforeUpdate
     

     在源码中commitBeforeMutationEffectOnFiber对应的函数是commitBeforeMutationLifeCycles在该函数中会调用getSnapshotBeforeUpdate，现在我们知道了getSnapshotBeforeUpdate是在mutation阶段中的commitBeforeMutationEffect函数中执行的，而commit阶段是同步的，所以getSnapshotBeforeUpdate也同步执行

     
     

     function commitBeforeMutationLifeCycles( current: Fiber | null, finishedWork: Fiber, ): void { switch (finishedWork.tag) { //... case ClassComponent: { if const instance = finishedWork.stateNode; const snapshot = instance.getSnapshotBeforeUpdate(//getSnapshotBeforeUpdate finishedWork.elementType === finishedWork.type ? prevProps : resolveDefaultProps(finishedWork.type, prevProps), prevState, ); } }

     
     

2. 调度useEffect
   ? 在flushPassiveEffects函数中调用flushPassiveEffectsImpl遍历pendingPassiveHookEffectsUnmount和pendingPassiveHookEffectsMount，执行对应的effect回调和销毁函数，而这两个数组是在commitLayoutEffects函数中赋值的（待会就会讲到），mutation后effectList赋值给rootWithPendingPassiveEffects，然后scheduleCallback调度执行flushPassiveEffects
   

   function flushPassiveEffectsImpl() { if (rootWithPendingPassiveEffects === null) {//在mutation后变成了root return false; } const unmountEffects = pendingPassiveHookEffectsUnmount; pendingPassiveHookEffectsUnmount = []; //useEffect的回调函数 for (let i = 0; i < unmountEffects.length; i += 2) { const effect = ((unmountEffects[i]: any): HookEffect); //... const destroy = effect.destroy; destroy(); }const mountEffects = pendingPassiveHookEffectsMount; //useEffect的销毁函数 pendingPassiveHookEffectsMount = []; for (let i = 0; i < mountEffects.length; i += 2) { const effect = ((unmountEffects[i]: any): HookEffect); //... const create = effect.create; effect.destroy = create(); } }

   
   

?componentDidUpdate或componentDidMount会在commit阶段同步执行(这个后面会讲到)，而useEffect会在commit阶段异步调度，所以适用于数据请求等副作用的处理 > 注意，和在render阶段的fiber node会打上Placement等标签一样，useEffect或useLayoutEffect也有对应的effect Tag，在源码中对应export const Passive = /**/ 0b0000000001000000000; ```js function commitBeforeMutationEffects() { while (nextEffect !== null) { const current = nextEffect.alternate; const effectTag = nextEffect.effectTag; // 在commitBeforeMutationEffectOnFiber函数中会执行getSnapshotBeforeUpdate if ((effectTag & Snapshot) !== NoEffect) { commitBeforeMutationEffectOnFiber(current, nextEffect); } // scheduleCallback调度useEffect if ((effectTag & Passive) !== NoEffect) { if (!rootDoesHavePassiveEffects) { rootDoesHavePassiveEffects = true; scheduleCallback(NormalSchedulerPriority, () => { flushPassiveEffects(); return null; }); } } nextEffect = nextEffect.nextEffect; //遍历effectList } } ```

• commitMutationEffects
  commitMutationEffects主要做了如下几件事
  
  1. 调用commitDetachRef解绑ref（第11章hook会讲解）
  2. 根据effectTag执行对应的dom操作
  3. useLayoutEffect销毁函数在UpdateTag时执行
     

     function commitMutationEffects(root: FiberRoot, renderPriorityLevel) { //遍历effectList while (nextEffect !== null) {const effectTag = nextEffect.effectTag; // 调用commitDetachRef解绑ref if (effectTag & Ref) { const current = nextEffect.alternate; if (current !== null) { commitDetachRef(current); } }// 根据effectTag执行对应的dom操作 const primaryEffectTag = effectTag & (Placement | Update | Deletion | Hydrating); switch (primaryEffectTag) { // 插入dom case Placement: { commitPlacement(nextEffect); nextEffect.effectTag &= ~Placement; break; } // 插入更新dom case PlacementAndUpdate: { // 插入 commitPlacement(nextEffect); nextEffect.effectTag &= ~Placement; // 更新 const current = nextEffect.alternate; commitWork(current, nextEffect); break; } //... // 更新dom case Update: { const current = nextEffect.alternate; commitWork(current, nextEffect); break; } // 删除dom case Deletion: { commitDeletion(root, nextEffect, renderPriorityLevel); break; } }nextEffect = nextEffect.nextEffect; } }

     
     

现在让我们来看看操作dom的这几个函数 **commitPlacement插入节点：** ?简化后的代码很清晰，找到该节点最近的parent节点和兄弟节点，然后根据isContainer来判断是插入到兄弟节点前还是append到parent节点后 ```js unction commitPlacement(finishedWork: Fiber): void { //... const parentFiber = getHostParentFiber(finishedWork); //找到最近的parent let parent; let isContainer; const parentStateNode = parentFiber.stateNode; switch (parentFiber.tag) { case HostComponent: parent = parentStateNode; isContainer = false; break; //... } const before = getHostSibling(finishedWork); //找兄弟节点 if (isContainer) { insertOrAppendPlacementNodeIntoContainer(finishedWork, before, parent); } else { insertOrAppendPlacementNode(finishedWork, before, parent); } } ``` **commitWork更新节点：** ?在简化后的源码中可以看到 ?如果fiber的tag是SimpleMemoComponent会调用commitHookEffectListUnmount执行对应的hook的销毁函数，可以看到传入的参数是HookLayout | HookHasEffect，也就是说执行useLayoutEffect的销毁函数。 ?如果是HostComponent，那么调用commitUpdate，commitUpdate最后会调用updateDOMProperties处理对应Update的dom操作 ```js function commitWork(current: Fiber | null, finishedWork: Fiber): void { if (!supportsMutation) { switch (finishedWork.tag) { //... case SimpleMemoComponent: { commitHookEffectListUnmount(HookLayout | HookHasEffect, finishedWork); } //... } } switch (finishedWork.tag) { //... case HostComponent: { //... commitUpdate( instance, updatePayload, type, oldProps, newProps, finishedWork, ); } return; } } ``` ```js function updateDOMProperties( domElement: Element, updatePayload: Array, wasCustomComponentTag: boolean, isCustomComponentTag: boolean, ): void { // TODO: Handle wasCustomComponentTag for (let i = 0; i < updatePayload.length; i += 2) { const propKey = updatePayload[i]; const propValue = https://www.it610.com/article/updatePayload[i + 1]; if (propKey === STYLE) { setValueForStyles(domElement, propValue); } else if (propKey === DANGEROUSLY_SET_INNER_HTML) { setInnerHTML(domElement, propValue); } else if (propKey === CHILDREN) { setTextContent(domElement, propValue); } else { setValueForProperty(domElement, propKey, propValue, isCustomComponentTag); } } } ``` **commitDeletion删除节点:** 如果是ClassComponent会执行componentWillUnmount，删除fiber，如果是FunctionComponent 会删除ref、并执行useEffect的销毁函数，具体可在源码中查看unmountHostComponents、commitNestedUnmounts、detachFiberMutation这几个函数 ```js function commitDeletion( finishedRoot: FiberRoot, current: Fiber, renderPriorityLevel: ReactPriorityLevel, ): void { if (supportsMutation) { // Recursively delete all host nodes from the parent. // Detach refs and call componentWillUnmount() on the whole subtree. unmountHostComponents(finishedRoot, current, renderPriorityLevel); } else { // Detach refs and call componentWillUnmount() on the whole subtree. commitNestedUnmounts(finishedRoot, current, renderPriorityLevel); } const alternate = current.alternate; detachFiberMutation(current); if (alternate !== null) { detachFiberMutation(alternate); } } ```

• 【react源码解析10.commit阶段】commitLayoutEffects
  在commitMutationEffects之后所有的dom操作都已经完成，可以访问dom了，commitLayoutEffects主要做了
  
  1. 调用commitLayoutEffectOnFiber执行相关生命周期函数或者hook相关callback
  2. 执行commitAttachRef为ref赋值

```js function commitLayoutEffects(root: FiberRoot, committedLanes: Lanes) { while (nextEffect !== null) { const effectTag = nextEffect.effectTag; // 调用commitLayoutEffectOnFiber执行生命周期和hook if (effectTag & (Update | Callback)) { const current = nextEffect.alternate; commitLayoutEffectOnFiber(root, current, nextEffect, committedLanes); } // ref赋值 if (effectTag & Ref) { commitAttachRef(nextEffect); } nextEffect = nextEffect.nextEffect; } } ``` **commitLayoutEffectOnFiber:** ?在源码中commitLayoutEffectOnFiber函数的别名是commitLifeCycles，在简化后的代码中可以看到，commitLifeCycles会判断fiber的类型，SimpleMemoComponent会执行useLayoutEffect的回调，然后调度useEffect，ClassComponent会执行componentDidMount或者componentDidUpdate，this.setState第二个参数也会执行，HostRoot会执行ReactDOM.render函数的第三个参数，例如 ```js ReactDOM.render(, document.querySelector("#root"), function() { console.log("root mount"); }); ``` 现在可以知道useLayoutEffect是在commit阶段同步执行，useEffect会在commit阶段异步调度 ```js function commitLifeCycles( finishedRoot: FiberRoot, current: Fiber | null, finishedWork: Fiber, committedLanes: Lanes, ): void { switch (finishedWork.tag) { case SimpleMemoComponent: { // 此函数会调用useLayoutEffect的回调 commitHookEffectListMount(HookLayout | HookHasEffect, finishedWork); // 向pendingPassiveHookEffectsUnmount和pendingPassiveHookEffectsMount中push effect// 并且调度它们 schedulePassiveEffects(finishedWork); } case ClassComponent: { //条件判断... instance.componentDidMount(); //条件判断... instance.componentDidUpdate(//update 在layout期间同步执行 prevProps, prevState, instance.__reactInternalSnapshotBeforeUpdate, ); } case HostRoot: { commitUpdateQueue(finishedWork, updateQueue, instance); //render第三个参数 }} } ``` ?在schedulePassiveEffects中会将useEffect的销毁和回调函数push到pendingPassiveHookEffectsUnmount和pendingPassiveHookEffectsMount中 ```js function schedulePassiveEffects(finishedWork: Fiber) { const updateQueue: FunctionComponentUpdateQueue | null = (finishedWork.updateQueue: any); const lastEffect = updateQueue !== null ? updateQueue.lastEffect : null; if (lastEffect !== null) { const firstEffect = lastEffect.next; let effect = firstEffect; do { const {next, tag} = effect; if ( (tag & HookPassive) !== NoHookEffect && (tag & HookHasEffect) !== NoHookEffect ) { //push useEffect的销毁函数并且加入调度 enqueuePendingPassiveHookEffectUnmount(finishedWork, effect); //push useEffect的回调函数并且加入调度 enqueuePendingPassiveHookEffectMount(finishedWork, effect); } effect = next; } while (effect !== firstEffect); } } ``` **commitAttachRef:** ?commitAttacRef中会判断ref的类型，执行ref或者给ref.current赋值 ```js function commitAttachRef(finishedWork: Fiber) { const ref = finishedWork.ref; if (ref !== null) { const instance = finishedWork.stateNode; let instanceToUse; switch (finishedWork.tag) { case HostComponent: instanceToUse = getPublicInstance(instance); break; default: instanceToUse = instance; } if (typeof ref === "function") { // 执行ref回调 ref(instanceToUse); } else { // 如果是值的类型则赋值给ref.current ref.current = instanceToUse; } } } ```

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