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通过示例源码解读React首次渲染流程

2023-03-27 11:38:30 作者：Aaaaaaaaaaayou

这篇文章主要为大家通过示例源码解读React的首次渲染流程，有需要的朋友可以借鉴参考下，希望能够有所帮助，祝大家多多进步，早日升职加薪

说明

本文结论均基于 React 16.13.1 得出，若有出入请参考对应版本源码。参考了 React 技术揭秘。

题目

在开始进行源码分析前，我们先来看几个题目：

题目一：

渲染下面的组件，打印顺序是什么？

import React from 'react'
const channel = new MessageChannel()
// onmessage 是一个宏任务
channel.port1.onmessage = () => {
  console.log('1 message channel')
}
export default function App() {
  React.useEffect(() => {
    console.log('2 use effect')
  }, [])
  Promise.resolve().then(() => {
    console.log('3 promise')
  })
  React.useLayoutEffect(() => {
    console.log('4 use layout effect')
    channel.port2.postMessage('')
  }, [])
  return <div>App</div>
}

答案：4 3 2 1

题目二：

点击 p 标签后，下面事件发生的顺序

页面显示 xingzhi
console.log('useLayoutEffect ayou')
console.log('useLayoutEffect xingzhi')
console.log('useEffect ayou')
console.log('useEffect xingzhi')

import React from 'react'
import {useState} from 'react'
function Name({name}) {
  React.useEffect(() => {
    console.log(`useEffect ${name}`)
    return () => {
      console.log(`useEffect destroy ${name}`)
    }
  }, [name])
  React.useLayoutEffect(() => {
    console.log(`useLayoutEffect ${name}`)
    return () => {
      console.log(`useLayoutEffect destroy ${name}`)
    }
  }, [name])
  return <span>{name}</span>
}
// 点击后，下面事件发生的顺序
// 1. 页面显示 xingzhi
// 2. console.log('useLayoutEffect ayou')
// 3. console.log('useLayoutEffect xingzhi')
// 4. console.log('useEffect ayou')
// 5. console.log('useEffect xingzhi')
export default function App() {
  const [name, setName] = useState('ayou')
  const onClick = React.useCallback(() => setName('xingzhi'), [])
  return (
    <div>
      <Name name={name} />
      <p onClick={onClick}>I am 18</p>
    </div>
  )
}

答案：1 2 3 4 5

你是不是都答对了呢？

首次渲染流程

我们以下面这个例子来阐述下首次渲染的流程：

function Name({name}) {
  React.useEffect(() => {
    console.log(`useEffect ${name}`)
    return () => {
      console.log('useEffect destroy')
    }
  }, [name])
  React.useLayoutEffect(() => {
    console.log(`useLayoutEffect ${name}`)
    return () => {
      console.log('useLayoutEffect destroy')
    }
  }, [name])
  return <span>{name}</span>
}
function Gender() {
  return <i>Male</i>
}
export default function App() {
  const [name, setName] = useState('ayou')
  return (
    <div>
      <Name name={name} />
      <p onClick={() => setName('xingzhi')}>I am 18</p>
      <Gender />
    </div>
  )
}
...
ReactDOM.render(<App />, document.getElementById('root'))

首先，我们看看 render，它是从 ReactDOMLegacy 中导出的，并最后调用了 legacyRenderSubtreeIntoContainer：

function legacyRenderSubtreeIntoContainer(
  parentComponent: ?React$Component<any, any>,
  children: ReactNodeList,
  container: Container,
  forceHydrate: boolean,
  callback: ?Function
) {
  // TODO: Without `any` type, Flow says "Property cannot be accessed on any
  // member of intersection type." Whyyyyyy.
  let root: RootType = (container._reactRootContainer: any)
  let fiberRoot
  if (!root) {
    // 首次渲染
    root = container._reactRootContainer = legacyCreateRootFromDOMContainer(
      container,
      forceHydrate
    )
    fiberRoot = root._internalRoot
    if (typeof callback === 'function') {
      const originalCallback = callback
      callback = function () {
        const instance = getPublicRootInstance(fiberRoot)
        originalCallback.call(instance)
      }
    }
    // Initial mount should not be batched.
    unbatchedUpdates(() => {
      updateContainer(children, fiberRoot, parentComponent, callback)
    })
  } else {
    // 更新
    fiberRoot = root._internalRoot
    if (typeof callback === 'function') {
      const originalCallback = callback
      callback = function () {
        const instance = getPublicRootInstance(fiberRoot)
        originalCallback.call(instance)
      }
    }
    updateContainer(children, fiberRoot, parentComponent, callback)
  }
  return getPublicRootInstance(fiberRoot)
}

首次渲染时，经过下面这一系列的操作，会初始化一些东西：

ReactDOMLegacy.js
function legacyCreateRootFromDOMContainer(
  container: Container,
  forceHydrate: boolean
): RootType {
  ...
  return createLegacyRoot(
    container,
    shouldHydrate
      ? {
          hydrate: true,
        }
      : undefined
  )
}
ReactDOMRoot.js
function createLegacyRoot(
  container: Container,
  options?: RootOptions,
): RootType {
  return new ReactDOMBlockingRoot(container, LegacyRoot, options);
}
function ReactDOMBlockingRoot(
  container: Container,
  tag: RootTag,
  options: void | RootOptions,
) {
  this._internalRoot = createRootImpl(container, tag, options);
}
function createRootImpl(
  container: Container,
  tag: RootTag,
  options: void | RootOptions,
) {
  ...
  const root = createContainer(container, tag, hydrate, hydrationCallbacks)
  ...
}
ReactFiberReconciler.old.js
function createContainer(
  containerInfo: Container,
  tag: RootTag,
  hydrate: boolean,
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
): OpaqueRoot {
  return createFiberRoot(containerInfo, tag, hydrate, hydrationCallbacks);
}
ReactFiberRoot.old.js
function createFiberRoot(
  containerInfo: any,
  tag: RootTag,
  hydrate: boolean,
  hydrationCallbacks: null | SuspenseHydrationCallbacks,
): FiberRoot {
  ...
  const root: FiberRoot = (new FiberRootNode(containerInfo, tag, hydrate): any)
  const uninitializedFiber = createHostRootFiber(tag)
  root.current = uninitializedFiber
  uninitializedFiber.stateNode = root
  initializeUpdateQueue(uninitializedFiber)
  return root
}

经过这一系列的操作以后，会形成如下的数据结构：

然后，会来到：

unbatchedUpdates(() => {
  // 这里的 children 是 App 对应的这个 ReactElement
  updateContainer(children, fiberRoot, parentComponent, callback)
})

这里 unbatchedUpdates 会设置当前的 executionContext：

export function unbatchedUpdates<A, R>(fn: (a: A) => R, a: A): R {
  const prevExecutionContext = executionContext
  // 去掉 BatchedContext
  executionContext &= ~BatchedContext
  // 加上 LegacyUnbatchedContext
  executionContext |= LegacyUnbatchedContext
  try {
    return fn(a)
  } finally {
    executionContext = prevExecutionContext
    if (executionContext === NoContext) {
      // Flush the immediate callbacks that were scheduled during this batch
      flushSyncCallbackQueue()
    }
  }
}

然后执行 updateContainer：

export function updateContainer(
  element: ReactNodeList,
  container: OpaqueRoot,
  parentComponent: ?React$Component<any, any>,
  callback: ?Function
): ExpirationTime {
  const current = container.current
  const currentTime = requestCurrentTimeForUpdate()
  const suspenseConfig = requestCurrentSuspenseConfig()
  const expirationTime = computeExpirationForFiber(
    currentTime,
    current,
    suspenseConfig
  )
  const context = getContextForSubtree(parentComponent)
  if (container.context === null) {
    container.context = context
  } else {
    container.pendingContext = context
  }
  const update = createUpdate(expirationTime, suspenseConfig)
  // Caution: React DevTools currently depends on this property
  // being called "element".
  update.payload = {element}
  callback = callback === undefined ? null : callback
  if (callback !== null) {
    update.callback = callback
  }
  enqueueUpdate(current, update)
  scheduleUpdateOnFiber(current, expirationTime)
  return expirationTime
}

这里，会创建一个 update，然后入队，我们的数据结构会变成这样：

接下来就到了 scheduleUpdateOnFiber:

export function scheduleUpdateOnFiber(
  fiber: Fiber,
  expirationTime: ExpirationTime
) {
  checkForNestedUpdates()
  warnAboutRenderPhaseUpdatesInDEV(fiber)
  const root = markUpdateTimeFromFiberToRoot(fiber, expirationTime)
  if (root === null) {
    warnAboutUpdateOnUnmountedFiberInDEV(fiber)
    return
  }
  // TODO: computeExpirationForFiber also reads the priority. Pass the
  // priority as an argument to that function and this one.
  const priorityLevel = getCurrentPriorityLevel()
  if (expirationTime === Sync) {
    if (
      // Check if we're inside unbatchedUpdates
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      // Check if we're not already rendering
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // Register pending interactions on the root to avoid losing traced interaction data.
      schedulePendingInteractions(root, expirationTime)
      // This is a legacy edge case. The initial mount of a ReactDOM.render-ed
      // root inside of batchedUpdates should be synchronous, but layout updates
      // should be deferred until the end of the batch.
      performSyncWorkOnRoot(root)
    } else {
      // 暂时不看
    }
  } else {
    // 暂时不看
  }
}

最后走到了 performSyncWorkOnRoot：

function performSyncWorkOnRoot(root) {
  invariant(
    (executionContext &amp; (RenderContext | CommitContext)) === NoContext,
    'Should not already be working.'
  )
  flushPassiveEffects()
  const lastExpiredTime = root.lastExpiredTime
  let expirationTime
  if (lastExpiredTime !== NoWork) {
    ...
  } else {
    // There's no expired work. This must be a new, synchronous render.
    expirationTime = Sync
  }
  let exitStatus = renderRootSync(root, expirationTime)
  ...
  const finishedWork: Fiber = (root.current.alternate: any);
  root.finishedWork = finishedWork;
  root.finishedExpirationTime = expirationTime;
  root.nextKnownPendingLevel = getRemainingExpirationTime(finishedWork);
  commitRoot(root);
  return null
}

这里，可以分为两个大的步骤：

render
commit

render

首先看看 renderRootSync：

function renderRootSync(root, expirationTime) {
  const prevExecutionContext = executionContext
  executionContext |= RenderContext
  const prevDispatcher = pushDispatcher(root)
  // If the root or expiration time have changed, throw out the existing stack
  // and prepare a fresh one. Otherwise we'll continue where we left off.
  if (root !== workInProgressRoot || expirationTime !== renderExpirationTime) {
    // 主要是给 workInProgress 赋值
    prepareFreshStack(root, expirationTime)
    startWorkOnPendingInteractions(root, expirationTime)
  }
  const prevInteractions = pushInteractions(root)
  do {
    try {
      workLoopSync()
      break
    } catch (thrownValue) {
      handleError(root, thrownValue)
    }
  } while (true)
  resetContextDependencies()
  if (enableSchedulerTracing) {
    popInteractions(((prevInteractions: any): Set&lt;Interaction&gt;))
  }
  executionContext = prevExecutionContext
  popDispatcher(prevDispatcher)
  if (workInProgress !== null) {
    // This is a sync render, so we should have finished the whole tree.
    invariant(
      false,
      'Cannot commit an incomplete root. This error is likely caused by a ' +
        'bug in React. Please file an issue.'
    )
  }
  // Set this to null to indicate there's no in-progress render.
  workInProgressRoot = null
  return workInProgressRootExitStatus
}

这里首先调用 prepareFreshStack(root, expirationTime)，这一句主要是通过 root.current 来创建 workInProgress。调用后，数据结构成了这样：

跳过中间的一些语句，我们来到 workLoopSync：

function workLoopSync() {
  // Already timed out, so perform work without checking if we need to yield.
  while (workInProgress !== null) {
    performUnitOfWork(workInProgress)
  }
}

function performUnitOfWork(unitOfWork: Fiber): void {
  // The current, flushed, state of this fiber is the alternate. Ideally
  // nothing should rely on this, but relying on it here means that we don't
  // need an additional field on the work in progress.
  const current = unitOfWork.alternate
  setCurrentDebugFiberInDEV(unitOfWork)
  let next
  if (enableProfilerTimer &amp;&amp; (unitOfWork.mode &amp; ProfileMode) !== NoMode) {
    startProfilerTimer(unitOfWork)
    next = beginWork(current, unitOfWork, renderExpirationTime)
    stopProfilerTimerIfRunningAndRecordDelta(unitOfWork, true)
  } else {
    next = beginWork(current, unitOfWork, renderExpirationTime)
  }
  resetCurrentDebugFiberInDEV()
  unitOfWork.memoizedProps = unitOfWork.pendingProps
  if (next === null) {
    // If this doesn't spawn new work, complete the current work.
    completeUnitOfWork(unitOfWork)
  } else {
    workInProgress = next
  }
  ReactCurrentOwner.current = null
}

这里又分为两个步骤：

beginWork，传入当前 Fiber 节点，创建子 Fiber 节点。
completeUnitOfWork，通过 Fiber 节点创建真实 DOM 节点。

这两个步骤会交替的执行，其目标是：

构建出新的 Fiber 树
与旧 Fiber 比较得到 effect 链表（插入、更新、删除、useEffect 等都会产生 effect）

beginWork

function beginWork(
  current: Fiber | null,
  workInProgress: Fiber,
  renderExpirationTime: ExpirationTime
): Fiber | null {
  const updateExpirationTime = workInProgress.expirationTime
  if (current !== null) {
    const oldProps = current.memoizedProps
    const newProps = workInProgress.pendingProps
    if (
      oldProps !== newProps ||
      hasLegacyContextChanged() ||
      // Force a re-render if the implementation changed due to hot reload:
      (__DEV__ ? workInProgress.type !== current.type : false)
    ) {
      // 略
    } else if (updateExpirationTime &lt; renderExpirationTime) {
      // 略
    } else {
      // An update was scheduled on this fiber, but there are no new props
      // nor legacy context. Set this to false. If an update queue or context
      // consumer produces a changed value, it will set this to true. Otherwise,
      // the component will assume the children have not changed and bail out.
      didReceiveUpdate = false
    }
  } else {
    didReceiveUpdate = false
  }
  // Before entering the begin phase, clear pending update priority.
  // TODO: This assumes that we're about to evaluate the component and process
  // the update queue. However, there's an exception: SimpleMemoComponent
  // sometimes bails out later in the begin phase. This indicates that we should
  // move this assignment out of the common path and into each branch.
  workInProgress.expirationTime = NoWork
  switch (workInProgress.tag) {
    case IndeterminateComponent:
    // ...省略
    case LazyComponent:
    // ...省略
    case FunctionComponent:
    // ...省略
    case ClassComponent:
    // ...省略
    case HostRoot:
      return updateHostRoot(current, workInProgress, renderExpirationTime)
    case HostComponent:
    // ...省略
    case HostText:
    // ...省略
    // ...省略其他类型
  }
}

这里因为是 rootFiber，所以会走到 updateHostRoot：

function updateHostRoot(current, workInProgress, renderExpirationTime) {
  // 暂时不看
  pushHostRootContext(workInProgress)
  const updateQueue = workInProgress.updateQueue
  const nextProps = workInProgress.pendingProps
  const prevState = workInProgress.memoizedState
  const prevChildren = prevState !== null ? prevState.element : null
  cloneUpdateQueue(current, workInProgress)
  processUpdateQueue(workInProgress, nextProps, null, renderExpirationTime)
  const nextState = workInProgress.memoizedState
  // Caution: React DevTools currently depends on this property
  // being called "element".
  const nextChildren = nextState.element
  if (nextChildren === prevChildren) {
    // 省略
  }
  const root: FiberRoot = workInProgress.stateNode
  if (root.hydrate &amp;&amp; enterHydrationState(workInProgress)) {
    // 省略
  } else {
    // 给 rootFiber 生成子 fiber
    reconcileChildren(
      current,
      workInProgress,
      nextChildren,
      renderExpirationTime
    )
    resetHydrationState()
  }
  return workInProgress.child
}

经过 updateHostRoot 后，会返回 workInProgress.child 作为下一个 workInProgress，最后的数据结构如下（这里先忽略 reconcileChildren 这个比较复杂的函数）：

接着会继续进行 beginWork，这次会来到 mountIndeterminateComponent （暂时忽略）。总之，经过不断的 beginWork 后，我们会得到如下的一个结构：

此时 next 为空，我们会走到：

if (next === null) {
  // If this doesn't spawn new work, complete the current work.
  completeUnitOfWork(unitOfWork)
} else {
  ...
}

completeUnitOfWork

function completeUnitOfWork(unitOfWork: Fiber): void {
  // Attempt to complete the current unit of work, then move to the next
  // sibling. If there are no more siblings, return to the parent fiber.
  let completedWork = unitOfWork
  do {
    // The current, flushed, state of this fiber is the alternate. Ideally
    // nothing should rely on this, but relying on it here means that we don't
    // need an additional field on the work in progress.
    const current = completedWork.alternate
    const returnFiber = completedWork.return
    // Check if the work completed or if something threw.
    if ((completedWork.effectTag & Incomplete) === NoEffect) {
      setCurrentDebugFiberInDEV(completedWork)
      let next
      if (
        !enableProfilerTimer ||
        (completedWork.mode & ProfileMode) === NoMode
      ) {
        next = completeWork(current, completedWork, renderExpirationTime)
      } else {
        startProfilerTimer(completedWork)
        next = completeWork(current, completedWork, renderExpirationTime)
        // Update render duration assuming we didn't error.
        stopProfilerTimerIfRunningAndRecordDelta(completedWork, false)
      }
      resetCurrentDebugFiberInDEV()
      resetChildExpirationTime(completedWork)
      if (next !== null) {
        // Completing this fiber spawned new work. Work on that next.
        workInProgress = next
        return
      }
      if (
        returnFiber !== null &&
        // Do not append effects to parents if a sibling failed to complete
        (returnFiber.effectTag & Incomplete) === NoEffect
      ) {
        // Append all the effects of the subtree and this fiber onto the effect
        // list of the parent. The completion order of the children affects the
        // side-effect order.
        if (returnFiber.firstEffect === null) {
          returnFiber.firstEffect = completedWork.firstEffect
        }
        if (completedWork.lastEffect !== null) {
          if (returnFiber.lastEffect !== null) {
            returnFiber.lastEffect.nextEffect = completedWork.firstEffect
          }
          returnFiber.lastEffect = completedWork.lastEffect
        }
        // If this fiber had side-effects, we append it AFTER the children's
        // side-effects. We can perform certain side-effects earlier if needed,
        // by doing multiple passes over the effect list. We don't want to
        // schedule our own side-effect on our own list because if end up
        // reusing children we'll schedule this effect onto itself since we're
        // at the end.
        const effectTag = completedWork.effectTag
        // Skip both NoWork and PerformedWork tags when creating the effect
        // list. PerformedWork effect is read by React DevTools but shouldn't be
        // committed.
        if (effectTag > PerformedWork) {
          if (returnFiber.lastEffect !== null) {
            returnFiber.lastEffect.nextEffect = completedWork
          } else {
            returnFiber.firstEffect = completedWork
          }
          returnFiber.lastEffect = completedWork
        }
      }
    } else {
      // This fiber did not complete because something threw. Pop values off
      // the stack without entering the complete phase. If this is a boundary,
      // capture values if possible.
      const next = unwindWork(completedWork, renderExpirationTime)
      // Because this fiber did not complete, don't reset its expiration time.
      if (
        enableProfilerTimer &&
        (completedWork.mode & ProfileMode) !== NoMode
      ) {
        // Record the render duration for the fiber that errored.
        stopProfilerTimerIfRunningAndRecordDelta(completedWork, false)
        // Include the time spent working on failed children before continuing.
        let actualDuration = completedWork.actualDuration
        let child = completedWork.child
        while (child !== null) {
          actualDuration += child.actualDuration
          child = child.sibling
        }
        completedWork.actualDuration = actualDuration
      }
      if (next !== null) {
        // If completing this work spawned new work, do that next. We'll come
        // back here again.
        // Since we're restarting, remove anything that is not a host effect
        // from the effect tag.
        next.effectTag &= HostEffectMask
        workInProgress = next
        return
      }
      if (returnFiber !== null) {
        // Mark the parent fiber as incomplete and clear its effect list.
        returnFiber.firstEffect = returnFiber.lastEffect = null
        returnFiber.effectTag |= Incomplete
      }
    }
    const siblingFiber = completedWork.sibling
    if (siblingFiber !== null) {
      // If there is more work to do in this returnFiber, do that next.
      workInProgress = siblingFiber
      return
    }
    // Otherwise, return to the parent
    completedWork = returnFiber
    // Update the next thing we're working on in case something throws.
    workInProgress = completedWork
  } while (completedWork !== null)
  // We've reached the root.
  if (workInProgressRootExitStatus === RootIncomplete) {
    workInProgressRootExitStatus = RootCompleted
  }
}

此时这里的 unitOfWork 是 span 对应的 fiber。从函数头部的注释我们可以大致知道该函数的功能：

// Attempt to complete the current unit of work, then move to the next
// sibling. If there are no more siblings, return to the parent fiber.
// 尝试去完成当前的工作单元，然后处理下一个 sibling。如果没有 sibling 了，就返回去完成父 fiber

这里一路走下去最后会来到 completeWork 这里：

case HostComponent:
  ...
  // 会调用 ReactDOMComponent.js 中的 createELement 方法创建 span 标签
  const instance = createInstance(
    type,
    newProps,
    rootContainerInstance,
    currentHostContext,
    workInProgress
  )
  // 将子元素 append 到 instance 中
  appendAllChildren(instance, workInProgress, false, false)
  workInProgress.stateNode = instance;

执行完后，我们的结构如下所示（我们用绿色的圆来表示真实 dom）：

此时 next 将会是 null，我们需要往上找到下一个 completedWork，即 Name，因为 Name 是一个 FunctionComponent，所以在 completeWork 中直接返回了 null。又因为它有 sibling，所以会将它的 sibling 赋值给 workInProgress，并返回对其进行 beginWork。

const siblingFiber = completedWork.sibling
if (siblingFiber !== null) {
  // If there is more work to do in this returnFiber, do that next.
  // workInProgress 更新为 sibling
  workInProgress = siblingFiber
  // 直接返回，回到了 performUnitOfWork
  return
}

function performUnitOfWork(unitOfWork: Fiber): void {
  ...
  if (next === null) {
    // If this doesn't spawn new work, complete the current work.
    // 上面的代码回到了这里
    completeUnitOfWork(unitOfWork)
  } else {
    workInProgress = next
  }
  ReactCurrentOwner.current = null
}

这样 beginWork 和 completeWork 不断交替的执行，当我们执行到 div 的时候，我们的结构如下所示：

之所以要额外的分析 div 的 complete 过程，是因为这个例子方便我们分析 appendAllChildren：

appendAllChildren = function (
  parent: Instance,
  workInProgress: Fiber,
  needsVisibilityToggle: boolean,
  isHidden: boolean
) {
  // We only have the top Fiber that was created but we need recurse down its
  // children to find all the terminal nodes.
  let node = workInProgress.child
  while (node !== null) {
    if (node.tag === HostComponent || node.tag === HostText) {
      appendInitialChild(parent, node.stateNode)
    } else if (enableFundamentalAPI && node.tag === FundamentalComponent) {
      appendInitialChild(parent, node.stateNode.instance)
    } else if (node.tag === HostPortal) {
      // If we have a portal child, then we don't want to traverse
      // down its children. Instead, we'll get insertions from each child in
      // the portal directly.
    } else if (node.child !== null) {
      node.child.return = node
      node = node.child
      continue
    }
    if (node === workInProgress) {
      return
    }
    while (node.sibling === null) {
      if (node.return === null || node.return === workInProgress) {
        return
      }
      node = node.return
    }
    node.sibling.return = node.return
    node = node.sibling
  }
}

由于 workInProgress 指向 div 这个 fiber，他的 child 是 Name，会进入 else if (node.child !== null) 这个条件分支。然后继续下一个循环，此时 node 为 span 这个 fiber，会进入第一个分支，将 span 对应的 dom 元素插入到 parent 之中。

这样不停的循环，最后会执行到 if (node === workInProgress) 退出，此时所有的子元素都 append 到了 parent 之中：

然后继续 beginWork 和 completeWork，最后会来到 rootFiber。不同的是，该节点的 alternate 并不为空，且该节点 tag 为 HootRoot，所以 completeWork 时会来到这里：

case HostRoot: {
  ...
  updateHostContainer(workInProgress);
  return null;
}

updateHostContainer = function (workInProgress: Fiber) {
  // Noop
}

看来几乎没有做什么事情，到这我们的 render 阶段就结束了，最后的结构如下所示：

其中蓝色表示是有 effect 的 Fiber 节点，他们组成了一个链表，方便 commit 过程进行遍历。

可以查看 render 过程动画。

commit

commit 大致可分为以下过程：

准备阶段
before mutation 阶段（执行 DOM 操作前）
mutation 阶段（执行 DOM 操作）
切换 Fiber Tree
layout 阶段（执行 DOM 操作后）
收尾阶段

准备阶段

do {
  // 触发useEffect回调与其他同步任务。由于这些任务可能触发新的渲染，所以这里要一直遍历执行直到没有任务
  flushPassiveEffects()
  // 暂时没有复现出 rootWithPendingPassiveEffects !== null 的情景
  // 首次渲染 rootWithPendingPassiveEffects 为 null
} while (rootWithPendingPassiveEffects !== null)
// finishedWork 就是正在工作的 rootFiber
const finishedWork = root.
// 优先级相关暂时不看
const expirationTime = root.finishedExpirationTime
if (finishedWork === null) {
  return null
}
root.finishedWork = null
root.finishedExpirationTime = NoWork
root.callbackNode = null
root.callbackExpirationTime = NoWork
root.callbackPriority_old = NoPriority
const remainingExpirationTimeBeforeCommit = getRemainingExpirationTime(
  finishedWork
)
markRootFinishedAtTime(
  root,
  expirationTime,
  remainingExpirationTimeBeforeCommit
)
if (rootsWithPendingDiscreteUpdates !== null) {
  const lastDiscreteTime = rootsWithPendingDiscreteUpdates.get(root)
  if (
    lastDiscreteTime !== undefined &&
    remainingExpirationTimeBeforeCommit < lastDiscreteTime
  ) {
    rootsWithPendingDiscreteUpdates.delete(root)
  }
}
if (root === workInProgressRoot) {
  workInProgressRoot = null
  workInProgress = null
  renderExpirationTime = NoWork
} else {
}
// 将effectList赋值给firstEffect
// 由于每个fiber的effectList只包含他的子孙节点
// 所以根节点如果有effectTag则不会被包含进来
// 所以这里将有effectTag的根节点插入到effectList尾部
// 这样才能保证有effect的fiber都在effectList中
let firstEffect
if (finishedWork.effectTag > PerformedWork) {
  if (finishedWork.lastEffect !== null) {
    finishedWork.lastEffect.nextEffect = finishedWork
    firstEffect = finishedWork.firstEffect
  } else {
    firstEffect = finishedWork
  }
} else {
  firstEffect = finishedWork.firstEffect
}

准备阶段主要是确定 firstEffect，我们的例子中就是 Name 这个 fiber。

before mutation 阶段

const prevExecutionContext = executionContext
executionContext |= CommitContext
const prevInteractions = pushInteractions(root)
// Reset this to null before calling lifecycles
ReactCurrentOwner.current = null
// The commit phase is broken into several sub-phases. We do a separate pass
// of the effect list for each phase: all mutation effects come before all
// layout effects, and so on.
// The first phase a "before mutation" phase. We use this phase to read the
// state of the host tree right before we mutate it. This is where
// getSnapshotBeforeUpdate is called.
focusedInstanceHandle = prepareForCommit(root.containerInfo)
shouldFireAfterActiveInstanceBlur = false
nextEffect = firstEffect
do {
  if (__DEV__) {
    ...
  } else {
    try {
      commitBeforeMutationEffects()
    } catch (error) {
      invariant(nextEffect !== null, 'Should be working on an effect.')
      captureCommitPhaseError(nextEffect, error)
      nextEffect = nextEffect.nextEffect
    }
  }
} while (nextEffect !== null)
// We no longer need to track the active instance fiber
focusedInstanceHandle = null
if (enableProfilerTimer) {
  // Mark the current commit time to be shared by all Profilers in this
  // batch. This enables them to be grouped later.
  recordCommitTime()
}

before mutation 阶段主要是调用了 commitBeforeMutationEffects 方法：

function commitBeforeMutationEffects() {
  while (nextEffect !== null) {
    if (
      !shouldFireAfterActiveInstanceBlur &&
      focusedInstanceHandle !== null &&
      isFiberHiddenOrDeletedAndContains(nextEffect, focusedInstanceHandle)
    ) {
      shouldFireAfterActiveInstanceBlur = true
      beforeActiveInstanceBlur()
    }
    const effectTag = nextEffect.effectTag
    if ((effectTag & Snapshot) !== NoEffect) {
      setCurrentDebugFiberInDEV(nextEffect)
      const current = nextEffect.alternate
      // 调用getSnapshotBeforeUpdate
      commitBeforeMutationEffectOnFiber(current, nextEffect)
      resetCurrentDebugFiberInDEV()
    }
    if ((effectTag & Passive) !== NoEffect) {
      // If there are passive effects, schedule a callback to flush at
      // the earliest opportunity.
      if (!rootDoesHavePassiveEffects) {
        rootDoesHavePassiveEffects = true
        scheduleCallback(NormalPriority, () => {
          flushPassiveEffects()
          return null
        })
      }
    }
    nextEffect = nextEffect.nextEffect
  }
}

因为 Name 中 effectTag 包括了 Passive，所以这里会执行：

scheduleCallback(NormalPriority, () => {
  flushPassiveEffects()
  return null
})

这里主要是对 useEffect 中的任务进行异步调用，最终会在下个事件循环中执行 commitPassiveHookEffects：

export function commitPassiveHookEffects(finishedWork: Fiber): void {
  if ((finishedWork.effectTag & Passive) !== NoEffect) {
    switch (finishedWork.tag) {
      case FunctionComponent:
      case ForwardRef:
      case SimpleMemoComponent:
      case Block: {
        if (
          enableProfilerTimer &&
          enableProfilerCommitHooks &&
          finishedWork.mode & ProfileMode
        ) {
          try {
            startPassiveEffectTimer();
            commitHookEffectListUnmount(
              HookPassive | HookHasEffect,
              finishedWork,
            );
            commitHookEffectListMount(
              HookPassive | HookHasEffect,
              finishedWork,
            );
          } finally {
            recordPassiveEffectDuration(finishedWork);
          }
        } else {
          commitHookEffectListUnmount(
            HookPassive | HookHasEffect,
            finishedWork,
          );
          commitHookEffectListMount(HookPassive | HookHasEffect, finishedWork);
        }
        break;
      }
      default:
        break;
    }
  }
}
function commitHookEffectListUnmount(tag: number, 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 {
      if ((effect.tag & tag) === tag) {
        // Unmount
        const destroy = effect.destroy;
        effect.destroy = undefined;
        if (destroy !== undefined) {
          destroy();
        }
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}
function commitHookEffectListMount(tag: number, 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 {
      if ((effect.tag & tag) === tag) {
        // Mount
        const create = effect.create;
        effect.destroy = create();
        ...
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}

其中，commitHookEffectListUnmount 会执行 useEffect 上次渲染返回的 destroy 方法，commitHookEffectListMount 会执行 useEffect 本次渲染的 create 方法。具体到我们的例子：

因为是首次渲染，所以 destroy 都是 undefined，所以只会打印 useEffect ayou。

mutation 阶段

mutation 阶段主要是执行了 commitMutationEffects 这个方法：

function commitMutationEffects(root: FiberRoot, renderPriorityLevel) {
  // TODO: Should probably move the bulk of this function to commitWork.
  while (nextEffect !== null) {
    setCurrentDebugFiberInDEV(nextEffect)
    const effectTag = nextEffect.effectTag
    ...
    // The following switch statement is only concerned about placement,
    // updates, and deletions. To avoid needing to add a case for every possible
    // bitmap value, we remove the secondary effects from the effect tag and
    // switch on that value.
    const primaryEffectTag =
      effectTag & (Placement | Update | Deletion | Hydrating)
    switch (primaryEffectTag) {
     case Placement: {
        commitPlacement(nextEffect);
        // Clear the "placement" from effect tag so that we know that this is
        // inserted, before any life-cycles like componentDidMount gets called.
        // TODO: findDOMNode doesn't rely on this any more but isMounted does
        // and isMounted is deprecated anyway so we should be able to kill this.
        nextEffect.effectTag &= ~Placement;
        break;
      }
      case PlacementAndUpdate: {
        // Placement
        commitPlacement(nextEffect);
        // Clear the "placement" from effect tag so that we know that this is
        // inserted, before any life-cycles like componentDidMount gets called.
        nextEffect.effectTag &= ~Placement;
        // Update
        const current = nextEffect.alternate;
        commitWork(current, nextEffect);
        break;
      }
      case Hydrating: {
        nextEffect.effectTag &= ~Hydrating;
        break;
      }
      case HydratingAndUpdate: {
        nextEffect.effectTag &= ~Hydrating;
        // Update
        const current = nextEffect.alternate;
        commitWork(current, nextEffect);
        break;
      }
      case Update: {
        const current = nextEffect.alternate;
        commitWork(current, nextEffect);
        break;
      }
      case Deletion: {
        commitDeletion(root, nextEffect, renderPriorityLevel);
        break;
      }
    }
  }
}

其中，Name 会走 Update 这个分支，执行 commitWork，最终会执行到 commitHookEffectListUnmount：

function commitHookEffectListUnmount(tag: number, 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 {
      if ((effect.tag & tag) === tag) {
        // Unmount
        const destroy = effect.destroy;
        effect.destroy = undefined;
        if (destroy !== undefined) {
          destroy();
        }
      }
      effect = effect.next;
    } while (effect !== firstEffect);
  }
}

这里会同步执行 useLayoutEffect 上次渲染返回的 destroy 方法，我们的例子里是 undefined。

而 App 会走到 Placement 这个分支，执行 commitPlacement，这里的主要工作是把整棵 dom 树插入到了 <div id='root'></div> 之中。

切换 Fiber Tree

mutation 阶段完成后，会执行：

root.current = finishedWork

完成后， fiberRoot 会指向 current Fiber 树。

layout 阶段

对应到我们的例子，layout 阶段主要是同步执行 useLayoutEffect 中的 create 函数，所以这里会打印 useLayoutEffect ayou。

题目解析

现在，我们来分析下文章开始的两个题目：