reconciler 运作流程

// 唯一接收输入信号的函数
export function scheduleUpdateOnFiber(
  fiber: Fiber,
  lane: Lane,
  eventTime: number,
) {
  // ... 省略部分无关代码
  const root = markUpdateLaneFromFiberToRoot(fiber, lane);
  if (lane === SyncLane) {
    if (
      (executionContext & LegacyUnbatchedContext) !== NoContext &&
      (executionContext & (RenderContext | CommitContext)) === NoContext
    ) {
      // 直接进行`fiber构造`
      performSyncWorkOnRoot(root);
    } else {
      // 注册调度任务, 经过`Scheduler`包的调度, 间接进行`fiber构造`
      ensureRootIsScheduled(root, eventTime);
    }
  } else {
    // 注册调度任务, 经过`Scheduler`包的调度, 间接进行`fiber构造`
    ensureRootIsScheduled(root, eventTime);
  }
}

// ... 省略部分无关代码
function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
  // 前半部分: 判断是否需要注册新的调度
  const existingCallbackNode = root.callbackNode;
  const nextLanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  const newCallbackPriority = returnNextLanesPriority();
  if (nextLanes === NoLanes) {
    return;
  }
  if (existingCallbackNode !== null) {
    const existingCallbackPriority = root.callbackPriority;
    if (existingCallbackPriority === newCallbackPriority) {
      return;
    }
    cancelCallback(existingCallbackNode);
  }

  // 后半部分: 注册调度任务
  let newCallbackNode;
  if (newCallbackPriority === SyncLanePriority) {
    newCallbackNode = scheduleSyncCallback(
      performSyncWorkOnRoot.bind(null, root),
    );
  } else if (newCallbackPriority === SyncBatchedLanePriority) {
    newCallbackNode = scheduleCallback(
      ImmediateSchedulerPriority,
      performSyncWorkOnRoot.bind(null, root),
    );
  } else {
    const schedulerPriorityLevel =
      lanePriorityToSchedulerPriority(newCallbackPriority);
    newCallbackNode = scheduleCallback(
      schedulerPriorityLevel,
      performConcurrentWorkOnRoot.bind(null, root),
    );
  }
  root.callbackPriority = newCallbackPriority;
  root.callbackNode = newCallbackNode;
}

// ... 省略部分无关代码
function performSyncWorkOnRoot(root) {
  let lanes;
  let exitStatus;

  lanes = getNextLanes(root, NoLanes);
  // 1. fiber树构造
  exitStatus = renderRootSync(root, lanes);

  // 2. 异常处理: 有可能fiber构造过程中出现异常
  if (root.tag !== LegacyRoot && exitStatus === RootErrored) {
    // ...
  }

  // 3. 输出: 渲染fiber树
  const finishedWork: Fiber = (root.current.alternate: any);
  root.finishedWork = finishedWork;
  root.finishedLanes = lanes;
  commitRoot(root);

  // 退出前再次检测, 是否还有其他更新, 是否需要发起新调度
  ensureRootIsScheduled(root, now());
  return null;
}

// ... 省略部分无关代码
function performConcurrentWorkOnRoot(root) {

  const originalCallbackNode = root.callbackNode;

  // 1. 刷新pending状态的effects, 有可能某些effect会取消本次任务
  const didFlushPassiveEffects = flushPassiveEffects();
  if (didFlushPassiveEffects) {
    if (root.callbackNode !== originalCallbackNode) {
      // 任务被取消, 退出调用
      return null;
    } else {
      // Current task was not canceled. Continue.
    }
  }
  // 2. 获取本次渲染的优先级
  let lanes = getNextLanes(
    root,
    root === workInProgressRoot ? workInProgressRootRenderLanes : NoLanes,
  );
  // 3. 构造fiber树
  let exitStatus = renderRootConcurrent(root, lanes);

  if (
    includesSomeLane(
      workInProgressRootIncludedLanes,
      workInProgressRootUpdatedLanes,
    )
  ) {
    // 如果在render过程中产生了新的update, 且新update的优先级与最初render的优先级有交集
    // 那么最初render无效, 丢弃最初render的结果, 等待下一次调度
    prepareFreshStack(root, NoLanes);
  } else if (exitStatus !== RootIncomplete) {
    // 4. 异常处理: 有可能fiber构造过程中出现异常
    if (exitStatus === RootErrored) {
      // ...
    }.
    const finishedWork: Fiber = (root.current.alternate: any);
    root.finishedWork = finishedWork;
    root.finishedLanes = lanes;
    // 5. 输出: 渲染fiber树
    finishConcurrentRender(root, exitStatus, lanes);
  }

  // 退出前再次检测, 是否还有其他更新, 是否需要发起新调度
  ensureRootIsScheduled(root, now());
  if (root.callbackNode === originalCallbackNode) {
    // 渲染被阻断, 返回一个新的performConcurrentWorkOnRoot函数, 等待下一次调用
    return performConcurrentWorkOnRoot.bind(null, root);
  }
  return null;
}

// ... 省略部分无关代码
function commitRootImpl(root, renderPriorityLevel) {
  // 设置局部变量
  const finishedWork = root.finishedWork;
  const lanes = root.finishedLanes;

  // 清空FiberRoot对象上的属性
  root.finishedWork = null;
  root.finishedLanes = NoLanes;
  root.callbackNode = null;

  // 提交阶段
  let firstEffect = finishedWork.firstEffect;
  if (firstEffect !== null) {
    const prevExecutionContext = executionContext;
    executionContext |= CommitContext;
    // 阶段1: dom突变之前
    nextEffect = firstEffect;
    do {
      commitBeforeMutationEffects();
    } while (nextEffect !== null);

    // 阶段2: dom突变, 界面发生改变
    nextEffect = firstEffect;
    do {
      commitMutationEffects(root, renderPriorityLevel);
    } while (nextEffect !== null);
    root.current = finishedWork;

    // 阶段3: layout阶段, 调用生命周期componentDidUpdate和回调函数等
    nextEffect = firstEffect;
    do {
      commitLayoutEffects(root, lanes);
    } while (nextEffect !== null);
    nextEffect = null;
    executionContext = prevExecutionContext;
  }
  ensureRootIsScheduled(root, now());
  return null;
}