react源码解析10.commit阶段

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

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

相关参考视频讲解：进入学习

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;

​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

function 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

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;
      }
  }

function updateDOMProperties(
    domElement: Element,
    updatePayload: Array<any>,
    wasCustomComponentTag: boolean,
    isCustomComponentTag: boolean,
  ): void {
    // TODO: Handle wasCustomComponentTag
    for (let i = 0; i < updatePayload.length; i += 2) {
      const propKey = updatePayload[i];
      const propValue = 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

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);
    }
  }

• 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

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

ReactDOM.render(<App />, 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

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

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

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;
          }
        }
      }