commitBeforeMutationLifecycles

这里在ReactFiberScheduler.js里面声明了这个方法,但还从ReactFiberCommitWork引入了一个同名方法,嗯~~~

这其实就是调用getSnapshotBeforeUpdate,在后面调用componentDidUpdate声明周期方法的时候,会传入这里计算出来的值。

function commitBeforeMutationLifecycles() {
  while (nextEffect !== null) {
    const effectTag = nextEffect.effectTag
    if (effectTag & Snapshot) {
      recordEffect()
      const current = nextEffect.alternate
      commitBeforeMutationLifeCycles(current, nextEffect)
    }

    // Don't cleanup effects yet;
    // This will be done by commitAllLifeCycles()
    nextEffect = nextEffect.nextEffect
  }
}

function commitBeforeMutationLifeCycles(
  current: Fiber | null,
  finishedWork: Fiber,
): void {
  switch (finishedWork.tag) {
    case ClassComponent: {
      if (finishedWork.effectTag & Snapshot) {
        if (current !== null) {
          const prevProps = current.memoizedProps
          const prevState = current.memoizedState
          startPhaseTimer(finishedWork, 'getSnapshotBeforeUpdate')
          const instance = finishedWork.stateNode
          instance.props = finishedWork.memoizedProps
          instance.state = finishedWork.memoizedState
          const snapshot = instance.getSnapshotBeforeUpdate(
            prevProps,
            prevState,
          )
          instance.__reactInternalSnapshotBeforeUpdate = snapshot
          stopPhaseTimer()
        }
      }
      return
    }
    case HostRoot:
    case HostComponent:
    case HostText:
    case HostPortal:
    case IncompleteClassComponent:
      // Nothing to do for these component types
      return
    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.',
      )
    }
  }
}

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