React16源码: React中commit阶段的commitBeforeMutationLifecycles的源码实现

commitBeforeMutationLifecycles

1 ）概述

在 react commit阶段的 commitRoot 第一个while循环中
调用了 commitBeforeMutationLifeCycles
现在来看下，里面发生了什么

2 ）源码

回到 commit 阶段的第一个循环中，在 commitRoot 函数里
定位到 packages/react-reconciler/src/ReactFiberScheduler.js#L647

看这里

// Invoke instances of getSnapshotBeforeUpdate before mutation.
nextEffect = firstEffect;
startCommitSnapshotEffectsTimer();// 接下去就进入了三个循环：
// 第一个循环主要是 commitBeforeMutationLifecycles，这个方法它其实很简单，它是这三个循环当中最简单的一个方法
// 它唯一的作用就是调用 ClassComponent 上面可能会存在的 getSnapshotBeforeUpdate 这么一个生命周期方法
while (nextEffect !== null) {let didError = false;let error;if (__DEV__) {invokeGuardedCallback(null, commitBeforeMutationLifecycles, null);if (hasCaughtError()) {didError = true;error = clearCaughtError();}} else {try {commitBeforeMutationLifecycles();} catch (e) {didError = true;error = e;}}if (didError) {invariant(nextEffect !== null,'Should have next effect. This error is likely caused by a bug ' +'in React. Please file an issue.',);captureCommitPhaseError(nextEffect, error);// Clean-upif (nextEffect !== null) {nextEffect = nextEffect.nextEffect;}}
}

在开始循环之前都会用到一个全局变量叫做 nextEffect，当然它一开始是 firstEffect
- 是在 RootFiber 上面，它的 firstEffect 到 lastEffect 这个链表上面的第一个effect
- 也就是第一个需要我们commit的fiber对象

需要注意一点，这边有一个很奇怪的代码，就是我们这个 commitBeforeMutationLifecycles 方法

可以看到 import { commitBeforeMutationLifeCycles } from './ReactFiberCommitWork'

但是又在本文件中，定义了一个同名的方法

// 这个是核心
function commitBeforeMutationLifecycles() {while (nextEffect !== null) {if (__DEV__) {ReactCurrentFiber.setCurrentFiber(nextEffect);}const effectTag = nextEffect.effectTag;if (effectTag & Snapshot) {recordEffect();const current = nextEffect.alternate;commitBeforeMutationLifeCycles(current, nextEffect); // 这个函数，不是上层的自己，而是 import 进来的}nextEffect = nextEffect.nextEffect;}if (__DEV__) {ReactCurrentFiber.resetCurrentFiber();}
}

这个操作会有点奇怪，因为正常来讲，import 进来的内容，它是一个常量，它不应该是能够直接被修改
注意，在 commitRoot 里调用的 commitBeforeMutationLifeCycles 是文件内的这个方法
而文件内的这个方法，调用的 commitBeforeMutationLifeCycles 是 import 进来的
- 它来自于 packages/react-reconciler/src/ReactFiberCommitWork.js#L215
它通过一个while循环，循环的是我们的 effect链
- 在这个循环的后面，可以看到 nextEffect = nextEffect.nextEffect;
这就是说一个一个往下这么去循环，对有 Snapshot 这个 SideEffect 的节点执行了下面的方法

再次定位到 packages/react-reconciler/src/ReactFiberCommitWork.js#L215

进入 commitBeforeMutationLifeCycles 这个就是上面定义的同名函数里面调用的方法

function commitBeforeMutationLifeCycles(current: Fiber | null,finishedWork: Fiber,
): void {switch (finishedWork.tag) {case FunctionComponent:case ForwardRef:case SimpleMemoComponent: {commitHookEffectList(UnmountSnapshot, NoHookEffect, finishedWork);return;}case ClassComponent: {if (finishedWork.effectTag & Snapshot) {if (current !== null) {const prevProps = current.memoizedProps;const prevState = current.memoizedState;startPhaseTimer(finishedWork, 'getSnapshotBeforeUpdate');const instance = finishedWork.stateNode;// We could update instance props and state here,// but instead we rely on them being set during last render.// TODO: revisit this when we implement resuming.if (__DEV__) {if (finishedWork.type === finishedWork.elementType) {warning(instance.props === finishedWork.memoizedProps,'Expected instance props to match memoized props before ' +'getSnapshotBeforeUpdate. This is likely due to a bug in React. ' +'Please file an issue.',);warning(instance.state === finishedWork.memoizedState,'Expected instance state to match memoized state before ' +'getSnapshotBeforeUpdate. This is likely due to a bug in React. ' +'Please file an issue.',);}}const snapshot = instance.getSnapshotBeforeUpdate(finishedWork.elementType === finishedWork.type? prevProps: resolveDefaultProps(finishedWork.type, prevProps),prevState,);if (__DEV__) {const didWarnSet = ((didWarnAboutUndefinedSnapshotBeforeUpdate: any): Set<mixed,>);if (snapshot === undefined && !didWarnSet.has(finishedWork.type)) {didWarnSet.add(finishedWork.type);warningWithoutStack(false,'%s.getSnapshotBeforeUpdate(): A snapshot value (or null) ' +'must be returned. You have returned undefined.',getComponentName(finishedWork.type),);}}instance.__reactInternalSnapshotBeforeUpdate = snapshot;stopPhaseTimer();}}return;}case HostRoot:case HostComponent:case HostText:case HostPortal:case IncompleteClassComponent:// Nothing to do for these component typesreturn;default: {invariant(false,'This unit of work tag should not have side-effects. This error is ' +'likely caused by a bug in React. Please file an issue.',);}}
}