React Scheduler (19.0.0)

export const frameYieldMs = 5;
export const userBlockingPriorityTimeout = 250;
export const normalPriorityTimeout = 5000;
export const lowPriorityTimeout = 10000;

export const NoEventPriority: EventPriority = NoLane;
export const DiscreteEventPriority: EventPriority = SyncLane;
export const ContinuousEventPriority: EventPriority = InputContinuousLane;
export const DefaultEventPriority: EventPriority = DefaultLane;
export const IdleEventPriority: EventPriority = IdleLane;

function scheduleTaskForRootDuringMicrotask(
  root: FiberRoot,
  currentTime: number,
): Lane {
  ...
  // Scheduler does have an "ImmediatePriority", but now that we use
  // microtasks for sync work we no longer use that. Any sync work that
  // reaches this path is meant to be time sliced.
  switch (lanesToEventPriority(nextLanes)) {
    case DiscreteEventPriority:
    case ContinuousEventPriority:
      schedulerPriorityLevel = UserBlockingSchedulerPriority;
      break;
    case DefaultEventPriority:
      schedulerPriorityLevel = NormalSchedulerPriority;
      break;
    case IdleEventPriority:
      schedulerPriorityLevel = IdleSchedulerPriority;
      break;
    default:
      schedulerPriorityLevel = NormalSchedulerPriority;
      break;
  }
  ...
}

export type PriorityLevel = 0 | 1 | 2 | 3 | 4 | 5;
export const NoPriority = 0;
export const ImmediatePriority = 1;
export const UserBlockingPriority = 2;
export const NormalPriority = 3;
export const LowPriority = 4;
export const IdlePriority = 5;

import type { PriorityLevel } from '../SchedulerPriorities';
import {
  frameYieldMs,
  userBlockingPriorityTimeout,
  lowPriorityTimeout,
  normalPriorityTimeout,
} from '../SchedulerFeatureFlags';
import { push, pop, peek } from '../SchedulerMinHeap';
import {
  ImmediatePriority,
  UserBlockingPriority,
  NormalPriority,
  LowPriority,
  IdlePriority,
} from '../SchedulerPriorities';
 
export type Callback = boolean => ?Callback;
 
export opaque type Task = {
  id: number,
  callback: Callback | null,
  priorityLevel: PriorityLevel,
  startTime: number,
  expirationTime: number,
  sortIndex: number,
  isQueued?: boolean,
};
 
let getCurrentTime: () => number | DOMHighResTimeStamp;
const hasPerformanceNow = typeof performance === 'object' && typeof performance.now === 'function';
 
if (hasPerformanceNow) {
  const localPerformance = performance;
  getCurrentTime = () => localPerformance.now();
} else {
  const localDate = Date;
  const initialTime = localDate.now();
  getCurrentTime = () => localDate.now() - initialTime;
}
 
const maxSigned31BitInt = 1073741823;
 
let taskQueue: Array<Task> = [];
let timerQueue: Array<Task> = [];
let taskIdCounter = 1;
let isSchedulerPaused = false;
 
let currentTask = null;
let currentPriorityLevel = NormalPriority;
let isPerformingWork = false;
let isHostCallbackScheduled = false;
let isHostTimeoutScheduled = false;
 
const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;
const localClearTimeout = typeof clearTimeout === 'function' ? clearTimeout : null;
const localSetImmediate = typeof setImmediate !== 'undefined' ? setImmediate : null;
 
function advanceTimers(currentTime: number) {
  let timer = peek(timerQueue);
  while (timer !== null) {
    if (timer.callback === null) {
      pop(timerQueue);
    } else if (timer.startTime <= currentTime) {
      pop(timerQueue);
      timer.sortIndex = timer.expirationTime;
      push(taskQueue, timer);
    } else {
      return;
    }
    timer = peek(timerQueue);
  }
}
 
function handleTimeout(currentTime: number) {
  isHostTimeoutScheduled = false;
  advanceTimers(currentTime);
 
  if (!isHostCallbackScheduled) {
    if (peek(taskQueue) !== null) {
      isHostCallbackScheduled = true;
      requestHostCallback();
    } else {
      const firstTimer = peek(timerQueue);
      if (firstTimer !== null) {
        requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
      }
    }
  }
}
 
function flushWork(initialTime: number) {
  isHostCallbackScheduled = false;
  if (isHostTimeoutScheduled) {
    isHostTimeoutScheduled = false;
    cancelHostTimeout();
  }
 
  isPerformingWork = true;
  const previousPriorityLevel = currentPriorityLevel;
  try {
    return workLoop(initialTime);
  } finally {
    currentTask = null;
    currentPriorityLevel = previousPriorityLevel;
    isPerformingWork = false;
  }
}
 
function workLoop(initialTime: number) {
  let currentTime = initialTime;
  advanceTimers(currentTime);
  currentTask = peek(taskQueue);
  while (currentTask !== null && !(enableSchedulerDebugging && isSchedulerPaused)) {
    if (currentTask.expirationTime > currentTime && shouldYieldToHost()) {
      break;
    }
    const callback = currentTask.callback;
    if (typeof callback === 'function') {
      currentTask.callback = null;
      currentPriorityLevel = currentTask.priorityLevel;
      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
      const continuationCallback = callback(didUserCallbackTimeout);
      currentTime = getCurrentTime();
      if (typeof continuationCallback === 'function') {
        currentTask.callback = continuationCallback;
        advanceTimers(currentTime);
        return true;
      } else {
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
        advanceTimers(currentTime);
      }
    } else {
      pop(taskQueue);
    }
    currentTask = peek(taskQueue);
  }
 
  if (currentTask !== null) {
    return true;
  } else {
    const firstTimer = peek(timerQueue);
    if (firstTimer !== null) {
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }
    return false;
  }
}
 
function unstable_scheduleCallback(
  priorityLevel: PriorityLevel,
  callback: Callback,
  options?: {delay: number},
): Task {
  var currentTime = getCurrentTime();
 
  var startTime;
  if (typeof options === 'object' && options !== null) {
    var delay = options.delay;
    if (typeof delay === 'number' && delay > 0) {
      startTime = currentTime + delay;
    } else {
      startTime = currentTime;
    }
  } else {
    startTime = currentTime;
  }
 
  var timeout;
  switch (priorityLevel) {
    case ImmediatePriority:
      timeout = -1;
      break;
    case UserBlockingPriority:
      timeout = userBlockingPriorityTimeout;
      break;
    case IdlePriority:
      timeout = maxSigned31BitInt;
      break;
    case LowPriority:
      timeout = lowPriorityTimeout;
      break;
    case NormalPriority:
    default:
      timeout = normalPriorityTimeout;
      break;
  }
 
  var expirationTime = startTime + timeout;
 
  var newTask: Task = {
    id: taskIdCounter++,
    callback,
    priorityLevel,
    startTime,
    expirationTime,
    sortIndex: -1,
  };
 
  if (startTime > currentTime) {
    newTask.sortIndex = startTime;
    push(timerQueue, newTask);
    if (peek(taskQueue) === null && newTask === peek(timerQueue)) {
      if (isHostTimeoutScheduled) {
        cancelHostTimeout();
      } else {
        isHostTimeoutScheduled = true;
      }
      requestHostTimeout(handleTimeout, startTime - currentTime);
    }
  } else {
    newTask.sortIndex = expirationTime;
    push(taskQueue, newTask);
 
    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      requestHostCallback();
    }
  }
 
  return newTask;
}
 
function unstable_cancelCallback(task: Task) {
  task.callback = null;
}
 
function unstable_getCurrentPriorityLevel(): PriorityLevel {
  return currentPriorityLevel;
}
 
let isMessageLoopRunning = false;
let taskTimeoutID: TimeoutID = (-1: any);
let frameInterval = frameYieldMs;
let startTime = -1;
 
function shouldYieldToHost(): boolean {
  const timeElapsed = getCurrentTime() - startTime;
  return timeElapsed >= frameInterval;
}
 
const performWorkUntilDeadline = () => {
  if (isMessageLoopRunning) {
    const currentTime = getCurrentTime();
    startTime = currentTime;
    let hasMoreWork = true;
 
    try {
      hasMoreWork = flushWork(currentTime);
    } finally {
      if (hasMoreWork) {
        schedulePerformWorkUntilDeadline();
      } else {
        isMessageLoopRunning = false;
      }
    }
  }
};
 
let schedulePerformWorkUntilDeadline;
if (typeof localSetImmediate === 'function') {
  schedulePerformWorkUntilDeadline = () => {
    localSetImmediate(performWorkUntilDeadline);
  };
} else if (typeof MessageChannel !== 'undefined') {
  const channel = new MessageChannel();
  const port = channel.port2;
  channel.port1.onmessage = performWorkUntilDeadline;
  schedulePerformWorkUntilDeadline = () => {
    port.postMessage(null);
  };
} else {
  schedulePerformWorkUntilDeadline = () => localSetTimeout(performWorkUntilDeadline, 0);
}
 
function requestHostCallback() {
  if (!isMessageLoopRunning) {
    isMessageLoopRunning = true;
    schedulePerformWorkUntilDeadline();
  }
}
 
function requestHostTimeout(
  callback: (currentTime: number) => void,
  ms: number,
) {
  taskTimeoutID = localSetTimeout(() => {
    callback(getCurrentTime());
  }, ms);
}
 
function cancelHostTimeout() {
  localClearTimeout(taskTimeoutID);
  taskTimeoutID = ((-1: any): TimeoutID);
}