function commitBeforeMutationEffects() {
  while (nextEffect !== null) {
    const current = nextEffect.alternate;

    if (!shouldFireAfterActiveInstanceBlur && focusedInstanceHandle !== null) {
      /// 处理 focus/blur 状态
    }
		// 根据之前生成的 effectList 进行处理
    const flags = nextEffect.flags;
    if ((flags & Snapshot) !== NoFlags)
	    // 内部调用`getSnapshotBeforeUpdate`生命周期钩子
      commitBeforeMutationEffectOnFiber(current, nextEffect);
    }
    // 如果存在 useEffect 依赖发生更改的 对应的tag，调度 useffect 回调
    if ((flags & Passive) !== NoFlags) {
      if (!rootDoesHavePassiveEffects) {
        rootDoesHavePassiveEffects = true;
        // 调度 useEffect 的 hook 执行
        scheduleCallback(NormalSchedulerPriority, () => {
	        // 在commit阶段之后 触发 useEffect
		      flushPassiveEffects();
          return null;
        });
      }
    }
    // commit 阶段是同步执行的，useEffect 以 Normal 优先级进行调度
    nextEffect = nextEffect.nextEffect;
  }
}

function commitBeforeMutationLifeCycles(current, finishedWork) {
	// 引入别名commitBeforeMutationEffectOnFiber
	switch (finishedWork.tag) {
	 // 类组件
		case ClassComponent: {
			if (finishedWork.flags & Snapshot) {
				if (current !== null) {
          const prevProps = current.memoizedProps;
          const prevState = current.memoizedState;
					// ReactComponent实例
          const instance = finishedWork.stateNode;
          // 调用实例的 getSnapshotBeforeUpdate
          // 此时页面还没有更新
          const snapshot = instance.getSnapshotBeforeUpdate(
            finishedWork.elementType === finishedWork.type
              ? prevProps
              : resolveDefaultProps(finishedWork.type, prevProps),
            prevState,
          );        
        }	
			}
    }
	}
}