人人都能读懂的react源码解析(大厂高薪必备)

7.commit阶段(听说renderer帮我们打好标记了,映射真实节点吧)

视频课程&调试demos

​ 视频课程的目的是为了快速掌握react源码运行的过程和react中的scheduler、reconciler、renderer、fiber等，并且详细debug源码和分析，过程更清晰。

​ 视频课程：进入课程

​ demos：demo

课程结构:

1. 开篇(听说你还在艰难的啃react源码)
2. react心智模型(来来来,让大脑有react思维吧)
3. Fiber(我是在内存中的dom)
4. 从legacy或concurrent开始(从入口开始,然后让我们奔向未来)
5. state更新流程(setState里到底发生了什么)
6. render阶段(厉害了,我有创建Fiber的技能)
7. commit阶段(听说renderer帮我们打好标记了,映射真实节点吧)
8. diff算法(妈妈再也不担心我的diff面试了)
9. hooks源码(想知道Function Component是怎样保存状态的嘛)
10. scheduler&lane模型(来看看任务是暂停、继续和插队的)
11. concurrent mode(并发模式是什么样的)
12. 手写迷你react(短小精悍就是我)

​ 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

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：

• mutation前

  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

     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

     onst 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;

          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:

​ commitAttachRef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;
    }
  }
}

各阶段生命周期执行情况

mount和update发生的生命周期的调用如下