React16源码: React中的renderRoot的源码实现

renderRoot

1 ）概述

• renderRoot 是一个非常复杂的方法
• 这个方法里处理很多各种各样的逻辑, 它主要的工作内容是什么？
• A. 它调用 workLoop 进行循环单元更新
  • 遍历整个 Fiber Tree，把每一个组件或者 dom 节点对应的
  • Fiber 节点拿出来单一的进行更新，这是一个循环的操作
  • 把整棵 Fiber Tree 都遍历一遍，这就是 workLoop
• B. 捕获错误并进行处理
  • 在进行每一个单元更新的时候，这个遍历逻辑的时候，有可能会出现一些错误
  • 有些是可预期的，比如说是 Suspense 的功能, throw 一个 Promise 对象
  • 这个时候，是我们要特殊的对它进行处理的
  • 有一些是不可预期的，比如说, render 里面报了一个错误
  • 我们也要对它进行一些处理，要告诉我们的react这个地方
  • 我们出现了一个错误，在react16之后，有了 Error Boundary
  • 可以在组件内捕获渲染的错误
• C. 在走完流程之后要进行善后
  • 因为流程走完之后会有各种不同的情况
  • 比如说有错误的情况，比如说有任务被挂起的情况，也就是Suspense的情况
  • 这些任务，都要按照特定的逻辑给它进行一些处理
• 这就是 renderRoot 这个方法，它的主要的核心工作内容

2 ）流程图

• 进入 renderRoot, 它里面有一个很核心的循环 do while workLoop
• 这个while循环就是调用 workLoop 对整棵树，它每个节点进行一个遍历，并且拿出来单独进行更新
• 因为每个 Fiber节点上，如果有更新的话，它会记入 updateQueen
• 我们通过 updateQueen 上是否有内容来判断它是否要进行更新
• 以及可以计算出它的新的 state，得到最新的 children，拿到所有最新的节点
• 在 workLoop 的过程当中，它在做什么呢？
• nextUnitOfWork 就是每一个节点在遍历的过程当中，它自己更新完之后，它会返回它的第一个child
• 它的第一个child 就是作为 nextUnitOfWork，因为我们执行了一个节点的更新之后，我们需要返回
• 返回之后，我们要判断一些逻辑
• 比如，对于异步的操作，每个节点更新完之后都要判断 !shouldYield()
• 判断我们现在的时间片是否还有？如果还有的话，再继续，如果没有的话，就要跳出了
• 接下去就会执行 performUnitOfWork
• 之后，执行 beginWork, completeUnitOfWork 这些，当然中间会判断是否有 next
• next就是我在更新完一个结点之后，它是否还有下一个节点需要更新
• 如果有next的情况，我们就返回，然后去判断这个逻辑是否还有
• 这就是整个对整个 fiber tree 每个节点的遍历更新
• 在这个更新过程当中，如果有任何catch，就是捕获到异常
• 那么首先会进行一系列的判断，然后对它执行 throwException 或者是 onUncaughtError
• 它们对应的逻辑会不一样, 如果这个节点它是可处理的错误
• 我会直接对它进行 completeUnitOfWork() 因为更新到这个节点之后，它抛出错误了
• 说明我们这个节点下面的所有子节点都不需要再更新了
• 执行完成之后，我们就会调用 continue，对于这个 do while 循环，它又继续调用 workLoop
• 也就是说我们把一棵子数的错误处理完之后，它还可以继续对别的子树进行更新
• 整体更新完之后，就会 break，之后会有各种不同的情况，比如说有致命的错误等
• 它们都会调用不同的逻辑进行一个处理，这里 nextRenderDidError 是一个可处理的错误
• 比如说, 是可以被捕获的错误，有组件能捕获它，而 nextLatestAbsoluteTimeoutMs 是抛出promise的错误
• 它是一个被挂起的一个任务，对应要执行一个被被挂起的操作, 最后如果上面的情况都没有出现
• 直接 onComplete，之后就可以在root节点上 set finishedwork，这样, 就可以对它整体进行一个更新
• 就可以执行 completeRoot，就可以把 fiber树 变成我们真正的dom 树去更新整个页面
• 这就是整个 renderRoot 它的一个逻辑，简单总结
  • 先正常的执行每个单元的更新
  • 然后捕获到任何错误进行一定的处理
  • 最终把整个树遍历完之后根据不同的情况再进行一个处理

3 ）源码

定位到 packages/react-reconciler/src/ReactFiberScheduler.js

先看 renderRoot 的代码

function renderRoot(root: FiberRoot,isYieldy: boolean,isExpired: boolean,
): void {invariant(!isWorking,'renderRoot was called recursively. This error is likely caused ' +'by a bug in React. Please file an issue.',);isWorking = true;ReactCurrentOwner.currentDispatcher = Dispatcher;const expirationTime = root.nextExpirationTimeToWorkOn;// Check if we're starting from a fresh stack, or if we're resuming from// previously yielded work.// 刚进来的时候，进行这个判断// nextRoot 和 nextRenderExpirationTime 对应着接下来要渲染的节点和对应的过期时间 这是两个公共变量// 在这种情况下，说明调用这个方法的时候，接收到的参数和之前的不一样，可能就是之前的异步任务被新进来的高优先级的任务给打断了if (expirationTime !== nextRenderExpirationTime ||root !== nextRoot ||nextUnitOfWork === null) {// Reset the stack and start working from the root.resetStack();nextRoot = root;nextRenderExpirationTime = expirationTime;// nextUnitOfWork 来自于 createWorkInProgress// 就是把当前的应用的状态对应的Fiber节点，拷贝了一份叫做 workInProgress 的对象// 因为我们不能直接在当前对象的Fiber节点上操作，它会影响我们目前的dom节点展示的样子// 所以要复制一份拷贝，对拷贝进行操作，workInProgress 和 current 之间会有一个转换的关系// 在renderRoot开始之后，我们真正操作的节点都是 workInProgress，没有直接在 current 上操作nextUnitOfWork = createWorkInProgress(nextRoot.current,null,nextRenderExpirationTime,);// 这种和不同expirationTime会有关系root.pendingCommitExpirationTime = NoWork;if (enableSchedulerTracing) {// Determine which interactions this batch of work currently includes,// So that we can accurately attribute time spent working on it,// And so that cascading work triggered during the render phase will be associated with it.const interactions: Set<Interaction> = new Set();root.pendingInteractionMap.forEach((scheduledInteractions, scheduledExpirationTime) => {if (scheduledExpirationTime <= expirationTime) {scheduledInteractions.forEach(interaction =>interactions.add(interaction),);}},);// Store the current set of interactions on the FiberRoot for a few reasons:// We can re-use it in hot functions like renderRoot() without having to recalculate it.// We will also use it in commitWork() to pass to any Profiler onRender() hooks.// This also provides DevTools with a way to access it when the onCommitRoot() hook is called.root.memoizedInteractions = interactions;if (interactions.size > 0) {const subscriber = __subscriberRef.current;if (subscriber !== null) {const threadID = computeThreadID(expirationTime,root.interactionThreadID,);try {subscriber.onWorkStarted(interactions, threadID);} catch (error) {// Work thrown by an interaction tracing subscriber should be rethrown,// But only once it's safe (to avoid leaveing the scheduler in an invalid state).// Store the error for now and we'll re-throw in finishRendering().if (!hasUnhandledError) {hasUnhandledError = true;unhandledError = error;}}}}}}let prevInteractions: Set<Interaction> = (null: any);if (enableSchedulerTracing) {// We're about to start new traced work.// Restore pending interactions so cascading work triggered during the render phase will be accounted for.prevInteractions = __interactionsRef.current;__interactionsRef.current = root.memoizedInteractions;}let didFatal = false;startWorkLoopTimer(nextUnitOfWork);// 上面初始化工作做完之后，就开始 workLoop// 如果有 catch 就会有一大段处理逻辑, 这里先跳过// do {try {workLoop(isYieldy);} catch (thrownValue) {if (nextUnitOfWork === null) {// This is a fatal error.didFatal = true;onUncaughtError(thrownValue);} else {if (__DEV__) {// Reset global debug state// We assume this is defined in DEV(resetCurrentlyProcessingQueue: any)();}const failedUnitOfWork: Fiber = nextUnitOfWork;if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {replayUnitOfWork(failedUnitOfWork, thrownValue, isYieldy);}// TODO: we already know this isn't true in some cases.// At least this shows a nicer error message until we figure out the cause.// https://github.com/facebook/react/issues/12449#issuecomment-386727431invariant(nextUnitOfWork !== null,'Failed to replay rendering after an error. This ' +'is likely caused by a bug in React. Please file an issue ' +'with a reproducing case to help us find it.',);const sourceFiber: Fiber = nextUnitOfWork;let returnFiber = sourceFiber.return;if (returnFiber === null) {// This is the root. The root could capture its own errors. However,// we don't know if it errors before or after we pushed the host// context. This information is needed to avoid a stack mismatch.// Because we're not sure, treat this as a fatal error. We could track// which phase it fails in, but doesn't seem worth it. At least// for now.didFatal = true;onUncaughtError(thrownValue);} else {throwException(root,returnFiber,sourceFiber,thrownValue,nextRenderExpirationTime,);nextUnitOfWork = completeUnitOfWork(sourceFiber);continue;}}}break;} while (true);if (enableSchedulerTracing) {// Traced work is done for now; restore the previous interactions.__interactionsRef.current = prevInteractions;}// We're done performing work. Time to clean up.isWorking = false;ReactCurrentOwner.currentDispatcher = null;resetContextDependences();// 在处理完 workLoop 后这里会有各种不同的判断// Yield back to main thread.// 这里代表有致命的错误if (didFatal) {const didCompleteRoot = false;stopWorkLoopTimer(interruptedBy, didCompleteRoot);interruptedBy = null;// There was a fatal error.if (__DEV__) {resetStackAfterFatalErrorInDev();}// `nextRoot` points to the in-progress root. A non-null value indicates// that we're in the middle of an async render. Set it to null to indicate// there's no more work to be done in the current batch.nextRoot = null;onFatal(root);return;}// 正常流程走完，这个if一定会匹配// 因为已经跳出 workLoop 了，说明一定有 react没有意识到的错误，所以调用 onYieldif (nextUnitOfWork !== null) {// There's still remaining async work in this tree, but we ran out of time// in the current frame. Yield back to the renderer. Unless we're// interrupted by a higher priority update, we'll continue later from where// we left off.const didCompleteRoot = false;stopWorkLoopTimer(interruptedBy, didCompleteRoot);interruptedBy = null;onYield(root);return;}// We completed the whole tree.const didCompleteRoot = true;stopWorkLoopTimer(interruptedBy, didCompleteRoot);const rootWorkInProgress = root.current.alternate;invariant(rootWorkInProgress !== null,'Finished root should have a work-in-progress. This error is likely ' +'caused by a bug in React. Please file an issue.',);// `nextRoot` points to the in-progress root. A non-null value indicates// that we're in the middle of an async render. Set it to null to indicate// there's no more work to be done in the current batch.nextRoot = null;interruptedBy = null;// 这里也是if (nextRenderDidError) {// There was an errorif (hasLowerPriorityWork(root, expirationTime)) {// There's lower priority work. If so, it may have the effect of fixing// the exception that was just thrown. Exit without committing. This is// similar to a suspend, but without a timeout because we're not waiting// for a promise to resolve. React will restart at the lower// priority level.markSuspendedPriorityLevel(root, expirationTime);const suspendedExpirationTime = expirationTime;const rootExpirationTime = root.expirationTime;onSuspend(root,rootWorkInProgress,suspendedExpirationTime,rootExpirationTime,-1, // Indicates no timeout);return;} else if (// There's no lower priority work, but we're rendering asynchronously.// Synchronsouly attempt to render the same level one more time. This is// similar to a suspend, but without a timeout because we're not waiting// for a promise to resolve.!root.didError &&!isExpired) {root.didError = true;const suspendedExpirationTime = (root.nextExpirationTimeToWorkOn = expirationTime);const rootExpirationTime = (root.expirationTime = Sync);onSuspend(root,rootWorkInProgress,suspendedExpirationTime,rootExpirationTime,-1, // Indicates no timeout);return;}}// 注意这里的错误if (!isExpired && nextLatestAbsoluteTimeoutMs !== -1) {// The tree was suspended.const suspendedExpirationTime = expirationTime;markSuspendedPriorityLevel(root, suspendedExpirationTime);// Find the earliest uncommitted expiration time in the tree, including// work that is suspended. The timeout threshold cannot be longer than// the overall expiration.const earliestExpirationTime = findEarliestOutstandingPriorityLevel(root,expirationTime,);const earliestExpirationTimeMs = expirationTimeToMs(earliestExpirationTime);if (earliestExpirationTimeMs < nextLatestAbsoluteTimeoutMs) {nextLatestAbsoluteTimeoutMs = earliestExpirationTimeMs;}// Subtract the current time from the absolute timeout to get the number// of milliseconds until the timeout. In other words, convert an absolute// timestamp to a relative time. This is the value that is passed// to `setTimeout`.const currentTimeMs = expirationTimeToMs(requestCurrentTime());let msUntilTimeout = nextLatestAbsoluteTimeoutMs - currentTimeMs;msUntilTimeout = msUntilTimeout < 0 ? 0 : msUntilTimeout;// TODO: Account for the Just Noticeable Differenceconst rootExpirationTime = root.expirationTime;onSuspend(root,rootWorkInProgress,suspendedExpirationTime,rootExpirationTime,msUntilTimeout,);return;}// Ready to commit.onComplete(root, rootWorkInProgress, expirationTime);
}

• renderRoot 代码会相对比较长，要把代码的区块进行一个区分
• 一些原版英文注释，和我添加的中文注释如上

现在来看下 workLoop 的源码

function workLoop(isYieldy) {if (!isYieldy) {// Flush work without yieldingwhile (nextUnitOfWork !== null) {nextUnitOfWork = performUnitOfWork(nextUnitOfWork);}} else {// Flush asynchronous work until the deadline runs out of time.while (nextUnitOfWork !== null && !shouldYield()) {nextUnitOfWork = performUnitOfWork(nextUnitOfWork);}}
}

• 它接收一个 isYieldy 作为参数
• 这个参数意味着是否可以被中断
• Sync的任务和已超时的异步任务都是不可中断的
• 如果是不可中断的，只要有 nextUnitOfWork
• 就会继续调用 performUnitOfWork
• 如果是可以中断的，就通过判断 !shouldYield()
• 来看当前时间片中是否还有足够的时间继续渲染下一个节点

再来看下 performUnitOfWork

function performUnitOfWork(workInProgress: Fiber): Fiber | null {// 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 = workInProgress.alternate;// See if beginning this work spawns more work.startWorkTimer(workInProgress);if (__DEV__) {ReactCurrentFiber.setCurrentFiber(workInProgress);}if (__DEV__ && replayFailedUnitOfWorkWithInvokeGuardedCallback) {stashedWorkInProgressProperties = assignFiberPropertiesInDEV(stashedWorkInProgressProperties,workInProgress,);}let next;if (enableProfilerTimer) {if (workInProgress.mode & ProfileMode) {startProfilerTimer(workInProgress);}next = beginWork(current, workInProgress, nextRenderExpirationTime);workInProgress.memoizedProps = workInProgress.pendingProps;if (workInProgress.mode & ProfileMode) {// Record the render duration assuming we didn't bailout (or error).stopProfilerTimerIfRunningAndRecordDelta(workInProgress, true);}} else {next = beginWork(current, workInProgress, nextRenderExpirationTime);workInProgress.memoizedProps = workInProgress.pendingProps;}if (__DEV__) {ReactCurrentFiber.resetCurrentFiber();if (isReplayingFailedUnitOfWork) {// Currently replaying a failed unit of work. This should be unreachable,// because the render phase is meant to be idempotent, and it should// have thrown again. Since it didn't, rethrow the original error, so// React's internal stack is not misaligned.rethrowOriginalError();}}if (__DEV__ && ReactFiberInstrumentation.debugTool) {ReactFiberInstrumentation.debugTool.onBeginWork(workInProgress);}if (next === null) {// If this doesn't spawn new work, complete the current work.next = completeUnitOfWork(workInProgress);}ReactCurrentOwner.current = null;return next;
}

• 它声明了一个 next 变量，next = beginWork(…), 这里涉及到对每个节点的更新
  • 更新完一个节点之后，它会返回它的下一个节点
  • 会更新 workInProgress.memoizedProps, 节点已经更新完了
  • 最新的 props 已经变成目前正在用的 props
  • 先跳过
• 跳过 DEV 的代码
• 如果 next === null 说明这个节点已经更新到子树的叶子节点了
  • 这棵子树就可以结束了
  • 结束就调用 completeUnitOfWork
  • 它也会返回它的下一个节点
• 最后，return next
  • 在 workLoop 函数中可看到，它会赋值给 nextUnitOfWork

    // 参考其中一个 while
    while (nextUnitOfWork !== null) {nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
    

  • 所以，真正到 nextUnitOfWork 为 null 的情况是它到了根节点，即 FiberRoot 节点
  • 它的 return 是 null，这时就跳出了 while 循环了