beginWork

1 ）概述

• 在 renderRoot 之后，要对我们的 Fiber 树每一个节点进行对应的更新
• 更新节点的一个入口方法，就是 beginWork
• 这个入口方法会有帮助我们去优化整棵树的更新过程
  • react 它的节点其实是非常多的，如果每一次子节点的一个更新
  • 就需要每一个节点都执行一遍更新的话，它整体的性能肯定会不好，而且是没有必要的
  • 我们一个子节点的更新，可能不会影响到它的兄弟节点的更新
  • 所以这部分肯定是要优化的
• 具体它是如何进行优化的，如下
  • 首先，要判断组件的更新是否可以优化
  • 然后要根据节点的类型分发进行处理，每一个节点它的更新方式会不一样
  • 最后是要根据 expirationTime 等信息判断这个节点的更新是否可以跳过

2 ）源码

• 在 renderRoot 中，它里面调用一个方法叫做 workLoop
• 在 workLoop 中调用的一个方法叫做 performUnitOfWork
• 而 beginWork 方法就在 performUnitOfWork 中调用
• beginWork 就是执行对整棵树的每一个节点进行更新的一个操作
• beginWork 来自于

  import {beginWork} from './ReactFiberBeginWork';
  

定位到 packages/react-reconciler/src/ReactFiberBeginWork.js
整个文件 export 出去的只有 beginWork 方法

function beginWork(current: Fiber | null,workInProgress: Fiber,renderExpirationTime: ExpirationTime,
): Fiber | null {// 读取了 workInProgress.expirationTime 这个值，这个 expirationTime 是节点上的 expirationTime// 函数第三个参数 renderExpirationTime 是 nextExpirationTimeToWorkOn 和 下面这个 expirationTime 有区别const updateExpirationTime = workInProgress.expirationTime;// 在 ReactDOM.render 第一次渲染时，第一个节点 current 是有值的, 接下去的节点都是没有的// 因为我们操作都是在 workInProgress 上操作的// 所以，workInProgress 创建的 child 节点也是一个 workInProgress 而不是 current// performUnitOfWork 接收的就是 workInProgressif (current !== null) {const oldProps = current.memoizedProps;const newProps = workInProgress.pendingProps;// 下面 renderExpirationTime 标志优先级最大都到哪个点，如果节点上的 expirationTime 比 renderExpirationTime 小// 说明节点上有优先级更高的任务，在这次渲染里是会执行的，反之，则不需要进行渲染，这个节点可以跳过// (updateExpirationTime === NoWork || updateExpirationTime > renderExpirationTime) 代表没有更新 或 有更新，但是优先级不高// hasLegacyContextChanged 属 context 相关api, childContextType 先跳过// oldProps === newProps 表示 props 一样if (oldProps === newProps &&!hasLegacyContextChanged() &&(updateExpirationTime === NoWork ||updateExpirationTime > renderExpirationTime)) {// This fiber does not have any pending work. Bailout without entering// the begin phase. There's still some bookkeeping we that needs to be done// in this optimized path, mostly pushing stuff onto the stack.// switch 先跳过switch (workInProgress.tag) {case HostRoot:pushHostRootContext(workInProgress);resetHydrationState();break;case HostComponent:pushHostContext(workInProgress);break;case ClassComponent: {const Component = workInProgress.type;if (isLegacyContextProvider(Component)) {pushLegacyContextProvider(workInProgress);}break;}case HostPortal:pushHostContainer(workInProgress,workInProgress.stateNode.containerInfo,);break;case ContextProvider: {const newValue = workInProgress.memoizedProps.value;pushProvider(workInProgress, newValue);break;}case Profiler:if (enableProfilerTimer) {workInProgress.effectTag |= Update;}break;case SuspenseComponent: {const state: SuspenseState | null = workInProgress.memoizedState;const didTimeout = state !== null && state.didTimeout;if (didTimeout) {// If this boundary is currently timed out, we need to decide// whether to retry the primary children, or to skip over it and// go straight to the fallback. Check the priority of the primary// child fragment.const primaryChildFragment: Fiber = (workInProgress.child: any);const primaryChildExpirationTime =primaryChildFragment.childExpirationTime;if (primaryChildExpirationTime !== NoWork &&primaryChildExpirationTime <= renderExpirationTime) {// The primary children have pending work. Use the normal path// to attempt to render the primary children again.return updateSuspenseComponent(current,workInProgress,renderExpirationTime,);} else {// The primary children do not have pending work with sufficient// priority. Bailout.const child = bailoutOnAlreadyFinishedWork(current,workInProgress,renderExpirationTime,);if (child !== null) {// The fallback children have pending work. Skip over the// primary children and work on the fallback.return child.sibling;} else {return null;}}}break;}}// 这个方法跳过该节点与其所有子节点的更新return bailoutOnAlreadyFinishedWork(current,workInProgress,renderExpirationTime,);}}// Before entering the begin phase, clear the expiration time.workInProgress.expirationTime = NoWork;switch (workInProgress.tag) {case IndeterminateComponent: {const elementType = workInProgress.elementType;return mountIndeterminateComponent(current,workInProgress,elementType,renderExpirationTime,);}case LazyComponent: {const elementType = workInProgress.elementType;return mountLazyComponent(current,workInProgress,elementType,updateExpirationTime,renderExpirationTime,);}case FunctionComponent: {const Component = workInProgress.type;const unresolvedProps = workInProgress.pendingProps;const resolvedProps =workInProgress.elementType === Component? unresolvedProps: resolveDefaultProps(Component, unresolvedProps);return updateFunctionComponent(current,workInProgress,Component,resolvedProps,renderExpirationTime,);}case ClassComponent: {const Component = workInProgress.type;const unresolvedProps = workInProgress.pendingProps;const resolvedProps =workInProgress.elementType === Component? unresolvedProps: resolveDefaultProps(Component, unresolvedProps);return updateClassComponent(current,workInProgress,Component,resolvedProps,renderExpirationTime,);}case HostRoot:return updateHostRoot(current, workInProgress, renderExpirationTime);case HostComponent:return updateHostComponent(current, workInProgress, renderExpirationTime);case HostText:return updateHostText(current, workInProgress);case SuspenseComponent:return updateSuspenseComponent(current,workInProgress,renderExpirationTime,);case HostPortal:return updatePortalComponent(current,workInProgress,renderExpirationTime,);case ForwardRef: {const type = workInProgress.type;const unresolvedProps = workInProgress.pendingProps;const resolvedProps =workInProgress.elementType === type? unresolvedProps: resolveDefaultProps(type, unresolvedProps);return updateForwardRef(current,workInProgress,type,resolvedProps,renderExpirationTime,);}case Fragment:return updateFragment(current, workInProgress, renderExpirationTime);case Mode:return updateMode(current, workInProgress, renderExpirationTime);case Profiler:return updateProfiler(current, workInProgress, renderExpirationTime);case ContextProvider:return updateContextProvider(current,workInProgress,renderExpirationTime,);case ContextConsumer:return updateContextConsumer(current,workInProgress,renderExpirationTime,);case MemoComponent: {const type = workInProgress.type;const unresolvedProps = workInProgress.pendingProps;const resolvedProps = resolveDefaultProps(type.type, unresolvedProps);return updateMemoComponent(current,workInProgress,type,resolvedProps,updateExpirationTime,renderExpirationTime,);}case SimpleMemoComponent: {return updateSimpleMemoComponent(current,workInProgress,workInProgress.type,workInProgress.pendingProps,updateExpirationTime,renderExpirationTime,);}case IncompleteClassComponent: {const Component = workInProgress.type;const unresolvedProps = workInProgress.pendingProps;const resolvedProps =workInProgress.elementType === Component? unresolvedProps: resolveDefaultProps(Component, unresolvedProps);return mountIncompleteClassComponent(current,workInProgress,Component,resolvedProps,renderExpirationTime,);}default:invariant(false,'Unknown unit of work tag. This error is likely caused by a bug in ' +'React. Please file an issue.',);}
}

• 注意，最顶层 workInProgress.expirationTime; 这个 expirationTime Fiber 节点上的 expirationTime
  • 对比 ReactFiberScheduler.js 中的 renderRoot 中读取的 root.nextExpirationTimeToWorkOn
  • 这个 root 是 FiberRoot, 而 FiberRoot 对应的 Fiber对象是 RootFiber
  • RootFiber 是 FiberRoot 的 current 属性，它没有对应React组件树的任何节点, 它只对应 FiberRoot 节点
    • FiberRoot 节点也就是挂载整个应用到的 Dom 节点
    • FiberRoot 上面存储整个应用的 expirationTime 和
  • RootFiber 的 stateNode 属性是 FiberRoot
  • FiberRoot 上面会存储整个应用上面的 expirationTime 和 nextExpirationTimeToWorkOn
  • 上面的两个值意义是不一样的, 在 renderRoot 函数中的一段代码如下

    // Reset the stack and start working from the root.
    resetStack();
    nextRoot = root;
    nextRenderExpirationTime = expirationTime;
    // nextUnitOfWork 是一个Fiber对象, 对应 RootFiber, 而非 FiberRoot
    // 更新过程使用的都是 Fiber 对象，不会是 FiberRoot
    nextUnitOfWork = createWorkInProgress(nextRoot.current, // 注意这里null,nextRenderExpirationTime,
    );
    

    • nextUnitOfWork 才是我们每一个节点去更新时要操作的节点对象
  • 所以，workInProgress.expirationTime 是对应Fiber节点产生更新的过期时间
    • 这里页面点击按钮让某个组件更新，最多到App上面创建更新
    • 不可能到 RootFiber 上面创建更新
  • 而 RootFiber 产生更新时在 ReactDOM.render 的时候才会有
  • 注意，在 beginWork 第一个参数是 current 是 Fiber对象，它对应的tag是 HostRoot
    • 在创建 FibeRoot 时, 在 ReactFiberReconciler.js 中的 createContainer 中
    • 有一个方法叫做 createFiberRoot, 定位到 ReactFiberRoot.js 中
    • 定位到 createHostRootFiber 方法，而这个方法又来自于 ReactFiber.js
    • 找到 createFiber, 是最终创建Fiber对象的时候调用的方法

      return createFiber(HostRoot, null, null, mode);
      

    • 参数是 HostRoot, null, null, mode
    • 第一个参数 HostRoot 就是 Fiber的tag
  • 在 ReactFiberScheduler.js 中 performUnitOfWork 接收的就是一个 workInProgress
    • 它传给 beginWork 的第一个参数 current 是 workInProgress.alternate
    • 在第一次渲染，也就是 ReactDOM.render 的时候, workInProgress 是刚刚创建的
    • 是不会在创建一个 current, 要等渲染结束后，把 current 和 workInProgress 的指针进行一个调换
    • 它才会变成有 current, 没有 workInProgress，再下次有更新产生，进行渲染时，才会创建一个新的 workInProgress
    • 这时候，current 和 workInProgress 都有了
    • 在最顶层 判断 current 是否 为 null, 也就是 判断是否是第一次渲染
• 关于 bailoutOnAlreadyFinishedWork 这个方法帮助跳过该节点与其所有子节点的更新

  function bailoutOnAlreadyFinishedWork(current: Fiber | null,workInProgress: Fiber,renderExpirationTime: ExpirationTime,
  ): Fiber | null {cancelWorkTimer(workInProgress);if (current !== null) {// Reuse previous context listworkInProgress.firstContextDependency = current.firstContextDependency;}if (enableProfilerTimer) {// Don't update "base" render times for bailouts.stopProfilerTimerIfRunning(workInProgress);}// Check if the children have any pending work.const childExpirationTime = workInProgress.childExpirationTime; // 这个值是 React 16.5 加上的，更早跳过更新// 子树上无更新，或高优先级任务在这次渲染中完成的 则跳过，是一个非常大的优化if (childExpirationTime === NoWork ||childExpirationTime > renderExpirationTime) {// The children don't have any work either. We can skip them.// TODO: Once we add back resuming, we should check if the children are// a work-in-progress set. If so, we need to transfer their effects.return null;} else {// This fiber doesn't have work, but its subtree does. Clone the child// fibers and continue.// 如果子树上有更新要执行，拷贝老的childcloneChildFibers(current, workInProgress);return workInProgress.child;}
  }
  

  • 回过头看 performUnitOfWork, return 了 child 之后，赋值给next

    let next;
    if (enableProfilerTimer) {// ... 跳过很多代码next = beginWork(current, workInProgress, nextRenderExpirationTime);// ... 跳过很多代码
    } else {next = beginWork(current, workInProgress, nextRenderExpirationTime);workInProgress.memoizedProps = workInProgress.pendingProps;
    }// ... 跳过很多代码if (next === null) {// If this doesn't spawn new work, complete the current work.next = completeUnitOfWork(workInProgress);
    }ReactCurrentOwner.current = null;
    // next 不为 null, 直接 return
    return next;
    

  • return next; 之后 回调 workLoop, 之后赋值给 nextUnitOfWork

    while (nextUnitOfWork !== null) {nextUnitOfWork = performUnitOfWork(nextUnitOfWork);}
    

  • 通过 while 循环继续执行 child 的更新