react 调度

[dailynodejs]

每次处理完一部分之后，会从非常深的调用栈看看有没有其他优先要做的事情，如果有就做其他事情，结束之后在做之前的事情

源码版本：https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.7.0-alpha.2/umd/react-dom.development.js

function dispatchAction(fiber, queue, action) {
  // ...
  scheduleWork(fiber, _expirationTime)
}

scheduleWork

scheduleWorkToRoot

function scheduleWorkToRoot(fiber, expirationTime) {
}

markPendingPriorityLevel

更新root的最高优先级和最低优先级

这里面有几个时间：

• earliestPendingTime
• latestPendingTime

// TODO: Offscreen updates should never suspend. However, a promise that
// suspended inside an offscreen subtree should be able to ping at the priority
// of the outer render.

function markPendingPriorityLevel(root, expirationTime) {
  // If there's a gap between completing a failed root and retrying it,
  // additional updates may be scheduled. Clear `didError`, in case the update
  // is sufficient to fix the error.
  root.didError = false;

  // Update the latest and earliest pending times
  var earliestPendingTime = root.earliestPendingTime;
  if (earliestPendingTime === NoWork) {
    // No other pending updates.
    root.earliestPendingTime = root.latestPendingTime = expirationTime;
  } else {
    if (earliestPendingTime < expirationTime) {
      // This is the earliest pending update.
      root.earliestPendingTime = expirationTime;
    } else {
      var latestPendingTime = root.latestPendingTime;
      if (latestPendingTime > expirationTime) {
        // This is the latest pending update
        root.latestPendingTime = expirationTime;
      }
    }
  }
  findNextExpirationTimeToWorkOn(expirationTime, root);
}

performWork

function performWork(minExpirationTime, isYieldy) {
  // Keep working on roots until there's no more work, or until there's a higher
  // priority event.
  findHighestPriorityRoot();

  if (isYieldy) {
    recomputeCurrentRendererTime();
    currentSchedulerTime = currentRendererTime;

    if (enableUserTimingAPI) {
      var didExpire = nextFlushedExpirationTime > currentRendererTime;
      var timeout = expirationTimeToMs(nextFlushedExpirationTime);
      stopRequestCallbackTimer(didExpire, timeout);
    }

    while (nextFlushedRoot !== null && nextFlushedExpirationTime !== NoWork && minExpirationTime <= nextFlushedExpirationTime && !(didYield && currentRendererTime > nextFlushedExpirationTime)) {
      performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime, currentRendererTime > nextFlushedExpirationTime);
      findHighestPriorityRoot();
      recomputeCurrentRendererTime();
      currentSchedulerTime = currentRendererTime;
    }
  } else {
    while (nextFlushedRoot !== null && nextFlushedExpirationTime !== NoWork && minExpirationTime <= nextFlushedExpirationTime) {
      performWorkOnRoot(nextFlushedRoot, nextFlushedExpirationTime, false);
      findHighestPriorityRoot();
    }
  }

  // We're done flushing work. Either we ran out of time in this callback,
  // or there's no more work left with sufficient priority.

  // If we're inside a callback, set this to false since we just completed it.
  if (isYieldy) {
    callbackExpirationTime = NoWork;
    callbackID = null;
  }
  // If there's work left over, schedule a new callback.
  if (nextFlushedExpirationTime !== NoWork) {
    scheduleCallbackWithExpirationTime(nextFlushedRoot, nextFlushedExpirationTime);
  }

  // Clean-up.
  finishRendering();
}

finishRendering

• 重置 nestedUpdateCount 为 0
• 重置 lastCommittedRootDuringThisBatch 为 null

function finishRendering() {
  nestedUpdateCount = 0;
  lastCommittedRootDuringThisBatch = null;

  if (completedBatches !== null) {
    var batches = completedBatches;
    completedBatches = null;
    for (var i = 0; i < batches.length; i++) {
      var batch = batches[i];
      try {
        batch._onComplete();
      } catch (error) {
        if (!hasUnhandledError) {
          hasUnhandledError = true;
          unhandledError = error;
        }
      }
    }
  }

  if (hasUnhandledError) {
    var error = unhandledError;
    unhandledError = null;
    hasUnhandledError = false;
    throw error;
  }
}

findHighestPriorityRoot

function findHighestPriorityRoot() {
  var highestPriorityWork = NoWork;
  var highestPriorityRoot = null;
  if (lastScheduledRoot !== null) {
    var previousScheduledRoot = lastScheduledRoot;
    var root = firstScheduledRoot;
    while (root !== null) {
      var remainingExpirationTime = root.expirationTime;
      if (remainingExpirationTime === NoWork) {
        // This root no longer has work. Remove it from the scheduler.

        // TODO: This check is redudant, but Flow is confused by the branch
        // below where we set lastScheduledRoot to null, even though we break
        // from the loop right after.
        !(previousScheduledRoot !== null && lastScheduledRoot !== null) ? invariant(false, 'Should have a previous and last root. This error is likely caused by a bug in React. Please file an issue.') : void 0;
        if (root === root.nextScheduledRoot) {
          // This is the only root in the list.
          root.nextScheduledRoot = null;
          firstScheduledRoot = lastScheduledRoot = null;
          break;
        } else if (root === firstScheduledRoot) {
          // This is the first root in the list.
          var next = root.nextScheduledRoot;
          firstScheduledRoot = next;
          lastScheduledRoot.nextScheduledRoot = next;
          root.nextScheduledRoot = null;
        } else if (root === lastScheduledRoot) {
          // This is the last root in the list.
          lastScheduledRoot = previousScheduledRoot;
          lastScheduledRoot.nextScheduledRoot = firstScheduledRoot;
          root.nextScheduledRoot = null;
          break;
        } else {
          previousScheduledRoot.nextScheduledRoot = root.nextScheduledRoot;
          root.nextScheduledRoot = null;
        }
        root = previousScheduledRoot.nextScheduledRoot;
      } else {
        if (remainingExpirationTime > highestPriorityWork) {
          // Update the priority, if it's higher
          highestPriorityWork = remainingExpirationTime;
          highestPriorityRoot = root;
        }
        if (root === lastScheduledRoot) {
          break;
        }
        if (highestPriorityWork === Sync) {
          // Sync is highest priority by definition so
          // we can stop searching.
          break;
        }
        previousScheduledRoot = root;
        root = root.nextScheduledRoot;
      }
    }
  }

  nextFlushedRoot = highestPriorityRoot;
  nextFlushedExpirationTime = highestPriorityWork;
}

requestWork

function requestWork(root, expirationTime) {

}

addRootToSchedule

[dailynodejs]

interruptedWork

被打断的

function resetStack() {
  if (nextUnitOfWork !== null) {
    var interruptedWork = nextUnitOfWork.return;
    while (interruptedWork !== null) {
      unwindInterruptedWork(interruptedWork);
      interruptedWork = interruptedWork.return;
    }
  }

  {
    ReactStrictModeWarnings.discardPendingWarnings();
    checkThatStackIsEmpty();
  }

  nextRoot = null;
  nextRenderExpirationTime = NoWork;
  nextLatestAbsoluteTimeoutMs = -1;
  nextRenderDidError = false;
  nextUnitOfWork = null;
}

[dailynodejs]

renderRoot

function renderRoot(root, isYieldy) {
  // ...
  workLoop(isYieldy)
}

workLoop

function workLoop(isYieldy) {
  if (!isYieldy) {
    // Flush work without yielding
    while (nextUnitOfWork !== null) {
      nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
  } else {
    // Flush asynchronous work until there's a higher priority event
    while (nextUnitOfWork !== null && !shouldYieldToRenderer()) {
      nextUnitOfWork = performUnitOfWork(nextUnitOfWork);
    }
  }
}

performUnitOfWork

startWorkTimer

[dailynodejs]

beginWork

completeWork

[dailynodejs]

workInProgress

• workInProgress.pendingProps
• workInProgress.tag

createWorkInProgress

// This is used to create an alternate fiber to do work on.
function createWorkInProgress(current, pendingProps, expirationTime) {
  var workInProgress = current.alternate;
  if (workInProgress === null) {
    // We use a double buffering pooling technique because we know that we'll
    // only ever need at most two versions of a tree. We pool the "other" unused
    // node that we're free to reuse. This is lazily created to avoid allocating
    // extra objects for things that are never updated. It also allow us to
    // reclaim the extra memory if needed.
    workInProgress = createFiber(current.tag, pendingProps, current.key, current.mode);
    workInProgress.elementType = current.elementType;
    workInProgress.type = current.type;
    workInProgress.stateNode = current.stateNode;

    {
      // DEV-only fields
      workInProgress._debugID = current._debugID;
      workInProgress._debugSource = current._debugSource;
      workInProgress._debugOwner = current._debugOwner;
    }

    workInProgress.alternate = current;
    current.alternate = workInProgress;
  } else {
    workInProgress.pendingProps = pendingProps;

    // We already have an alternate.
    // Reset the effect tag.
    workInProgress.effectTag = NoEffect;

    // The effect list is no longer valid.
    workInProgress.nextEffect = null;
    workInProgress.firstEffect = null;
    workInProgress.lastEffect = null;

    if (enableProfilerTimer) {
      // We intentionally reset, rather than copy, actualDuration & actualStartTime.
      // This prevents time from endlessly accumulating in new commits.
      // This has the downside of resetting values for different priority renders,
      // But works for yielding (the common case) and should support resuming.
      workInProgress.actualDuration = 0;
      workInProgress.actualStartTime = -1;
    }
  }

  workInProgress.childExpirationTime = current.childExpirationTime;
  workInProgress.expirationTime = current.expirationTime;

  workInProgress.child = current.child;
  workInProgress.memoizedProps = current.memoizedProps;
  workInProgress.memoizedState = current.memoizedState;
  workInProgress.updateQueue = current.updateQueue;
  workInProgress.firstContextDependency = current.firstContextDependency;

  // These will be overridden during the parent's reconciliation
  workInProgress.sibling = current.sibling;
  workInProgress.index = current.index;
  workInProgress.ref = current.ref;

  if (enableProfilerTimer) {
    workInProgress.selfBaseDuration = current.selfBaseDuration;
    workInProgress.treeBaseDuration = current.treeBaseDuration;
  }

  return workInProgress;
}

[dailynodejs]

流程

scheduleWork --> requestWork --> performWork --> findHighestPriorityRoot --> performWorkOnRoot --> completeRoot --> renderRoot --> commitRoot