Viewing github.com/facebook/react/packages/scheduler/src/forks/Scheduler.js

```
/**
```

 * Copyright (c) Meta Platforms, Inc. and affiliates.

```
 *
```

 * This source code is licensed under the MIT license found in the

 * LICENSE file in the root directory of this source tree.

```
 *
```
```
 * @flow
```
```
 */
```
```
/* eslint-disable no-var */
```

import type {PriorityLevel} from '../SchedulerPriorities';

```
import {
```
```
  enableSchedulerDebugging,
```
```
  enableProfiling,
```
```
  enableIsInputPending,
```
```
  enableIsInputPendingContinuous,
```
```
  frameYieldMs,
```
```
  continuousYieldMs,
```
```
  maxYieldMs,
```
```
} from '../SchedulerFeatureFlags';
```

import {push, pop, peek} from '../SchedulerMinHeap';

```
// TODO: Use symbols?
```
```
import {
```
```
  ImmediatePriority,
```
```
  UserBlockingPriority,
```
```
  NormalPriority,
```
```
  LowPriority,
```
```
  IdlePriority,
```
```
} from '../SchedulerPriorities';
```
```
import {
```
```
  markTaskRun,
```
```
  markTaskYield,
```
```
  markTaskCompleted,
```
```
  markTaskCanceled,
```
```
  markTaskErrored,
```
```
  markSchedulerSuspended,
```
```
  markSchedulerUnsuspended,
```
```
  markTaskStart,
```
```
  stopLoggingProfilingEvents,
```
```
  startLoggingProfilingEvents,
```
```
} from '../SchedulerProfiling';
```

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 =
```
```
  // $FlowFixMe[method-unbinding]
```

  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;

```
}
```

// Max 31 bit integer. The max integer size in V8 for 32-bit systems.

```
// Math.pow(2, 30) - 1
```
```
// 0b111111111111111111111111111111
```
```
var maxSigned31BitInt = 1073741823;
```
```
// Times out immediately
```
```
var IMMEDIATE_PRIORITY_TIMEOUT = -1;
```
```
// Eventually times out
```

var USER_BLOCKING_PRIORITY_TIMEOUT = 250;

```
var NORMAL_PRIORITY_TIMEOUT = 5000;
```
```
var LOW_PRIORITY_TIMEOUT = 10000;
```
```
// Never times out
```

var IDLE_PRIORITY_TIMEOUT = maxSigned31BitInt;

```
// Tasks are stored on a min heap
```
```
var taskQueue: Array<Task> = [];
```
```
var timerQueue: Array<Task> = [];
```

// Incrementing id counter. Used to maintain insertion order.

```
var taskIdCounter = 1;
```

// Pausing the scheduler is useful for debugging.

```
var isSchedulerPaused = false;
```
```
var currentTask = null;
```

var currentPriorityLevel = NormalPriority;

// This is set while performing work, to prevent re-entrance.

```
var isPerformingWork = false;
```
```
var isHostCallbackScheduled = false;
```
```
var isHostTimeoutScheduled = false;
```

// Capture local references to native APIs, in case a polyfill overrides them.

const localSetTimeout = typeof setTimeout === 'function' ? setTimeout : null;

```
const localClearTimeout =
```

  typeof clearTimeout === 'function' ? clearTimeout : null;

```
const localSetImmediate =
```

  typeof setImmediate !== 'undefined' ? setImmediate : null; // IE and Node.js + jsdom

```
const isInputPending =
```
```
  typeof navigator !== 'undefined' &&
```
```
  // $FlowFixMe[prop-missing]
```

  navigator.scheduling !== undefined &&

```
  // $FlowFixMe[incompatible-type]
```

  navigator.scheduling.isInputPending !== undefined

    ? navigator.scheduling.isInputPending.bind(navigator.scheduling)

```
    : null;
```

const continuousOptions = {includeContinuous: enableIsInputPendingContinuous};

function advanceTimers(currentTime: number) {

  // Check for tasks that are no longer delayed and add them to the queue.

```
  let timer = peek(timerQueue);
```
```
  while (timer !== null) {
```
```
    if (timer.callback === null) {
```
```
      // Timer was cancelled.
```
```
      pop(timerQueue);
```

    } else if (timer.startTime <= currentTime) {

      // Timer fired. Transfer to the task queue.

```
      pop(timerQueue);
```

      timer.sortIndex = timer.expirationTime;

```
      push(taskQueue, timer);
```
```
      if (enableProfiling) {
```

        markTaskStart(timer, currentTime);

```
        timer.isQueued = true;
```
```
      }
```
```
    } else {
```
```
      // Remaining timers are pending.
```
```
      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) {

```
  if (enableProfiling) {
```

    markSchedulerUnsuspended(initialTime);

```
  }
```

  // We'll need a host callback the next time work is scheduled.

```
  isHostCallbackScheduled = false;
```
```
  if (isHostTimeoutScheduled) {
```

    // We scheduled a timeout but it's no longer needed. Cancel it.

```
    isHostTimeoutScheduled = false;
```
```
    cancelHostTimeout();
```
```
  }
```
```
  isPerformingWork = true;
```

  const previousPriorityLevel = currentPriorityLevel;

```
  try {
```
```
    if (enableProfiling) {
```
```
      try {
```
```
        return workLoop(initialTime);
```
```
      } catch (error) {
```
```
        if (currentTask !== null) {
```

          const currentTime = getCurrentTime();

          // $FlowFixMe[incompatible-call] found when upgrading Flow

          markTaskErrored(currentTask, currentTime);

          // $FlowFixMe[incompatible-use] found when upgrading Flow

          currentTask.isQueued = false;

```
        }
```
```
        throw error;
```
```
      }
```
```
    } else {
```
```
      // No catch in prod code path.
```
```
      return workLoop(initialTime);
```
```
    }
```
```
  } finally {
```
```
    currentTask = null;
```

    currentPriorityLevel = previousPriorityLevel;

```
    isPerformingWork = false;
```
```
    if (enableProfiling) {
```

      const currentTime = getCurrentTime();

      markSchedulerSuspended(currentTime);

```
    }
```
```
  }
```
```
}
```

function workLoop(initialTime: number) {

```
  let currentTime = initialTime;
```
```
  advanceTimers(currentTime);
```
```
  currentTask = peek(taskQueue);
```
```
  while (
```
```
    currentTask !== null &&
```

    !(enableSchedulerDebugging && isSchedulerPaused)

```
  ) {
```

    if (currentTask.expirationTime > currentTime && shouldYieldToHost()) {

      // This currentTask hasn't expired, and we've reached the deadline.

```
      break;
```
```
    }
```

    // $FlowFixMe[incompatible-use] found when upgrading Flow

    const callback = currentTask.callback;

    if (typeof callback === 'function') {

      // $FlowFixMe[incompatible-use] found when upgrading Flow

```
      currentTask.callback = null;
```

      // $FlowFixMe[incompatible-use] found when upgrading Flow

      currentPriorityLevel = currentTask.priorityLevel;

      // $FlowFixMe[incompatible-use] found when upgrading Flow

      const didUserCallbackTimeout = currentTask.expirationTime <= currentTime;

```
      if (enableProfiling) {
```

        // $FlowFixMe[incompatible-call] found when upgrading Flow

        markTaskRun(currentTask, currentTime);

```
      }
```

      const continuationCallback = callback(didUserCallbackTimeout);

```
      currentTime = getCurrentTime();
```

      if (typeof continuationCallback === 'function') {

        // If a continuation is returned, immediately yield to the main thread

        // regardless of how much time is left in the current time slice.

        // $FlowFixMe[incompatible-use] found when upgrading Flow

        currentTask.callback = continuationCallback;

```
        if (enableProfiling) {
```

          // $FlowFixMe[incompatible-call] found when upgrading Flow

          markTaskYield(currentTask, currentTime);

```
        }
```
```
        advanceTimers(currentTime);
```
```
        return true;
```
```
      } else {
```
```
        if (enableProfiling) {
```

          // $FlowFixMe[incompatible-call] found when upgrading Flow

          markTaskCompleted(currentTask, currentTime);

          // $FlowFixMe[incompatible-use] found when upgrading Flow

          currentTask.isQueued = false;

```
        }
```

        if (currentTask === peek(taskQueue)) {

```
          pop(taskQueue);
```
```
        }
```
```
        advanceTimers(currentTime);
```
```
      }
```
```
    } else {
```
```
      pop(taskQueue);
```
```
    }
```
```
    currentTask = peek(taskQueue);
```
```
  }
```

  // Return whether there's additional work

```
  if (currentTask !== null) {
```
```
    return true;
```
```
  } else {
```

    const firstTimer = peek(timerQueue);

```
    if (firstTimer !== null) {
```

      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);

```
    }
```
```
    return false;
```
```
  }
```
```
}
```
```
function unstable_runWithPriority<T>(
```
```
  priorityLevel: PriorityLevel,
```
```
  eventHandler: () => T,
```
```
): T {
```
```
  switch (priorityLevel) {
```
```
    case ImmediatePriority:
```
```
    case UserBlockingPriority:
```
```
    case NormalPriority:
```
```
    case LowPriority:
```
```
    case IdlePriority:
```
```
      break;
```
```
    default:
```
```
      priorityLevel = NormalPriority;
```
```
  }
```

  var previousPriorityLevel = currentPriorityLevel;

  currentPriorityLevel = priorityLevel;

```
  try {
```
```
    return eventHandler();
```
```
  } finally {
```

    currentPriorityLevel = previousPriorityLevel;

```
  }
```
```
}
```

function unstable_next<T>(eventHandler: () => T): T {

```
  var priorityLevel;
```
```
  switch (currentPriorityLevel) {
```
```
    case ImmediatePriority:
```
```
    case UserBlockingPriority:
```
```
    case NormalPriority:
```
```
      // Shift down to normal priority
```
```
      priorityLevel = NormalPriority;
```
```
      break;
```
```
    default:
```

      // Anything lower than normal priority should remain at the current level.

      priorityLevel = currentPriorityLevel;

```
      break;
```
```
  }
```

  var previousPriorityLevel = currentPriorityLevel;

  currentPriorityLevel = priorityLevel;

```
  try {
```
```
    return eventHandler();
```
```
  } finally {
```

    currentPriorityLevel = previousPriorityLevel;

```
  }
```
```
}
```

function unstable_wrapCallback<T: (...Array<mixed>) => mixed>(callback: T): T {

  var parentPriorityLevel = currentPriorityLevel;

```
  // $FlowFixMe[incompatible-return]
```
```
  // $FlowFixMe[missing-this-annot]
```
```
  return function () {
```

    // This is a fork of runWithPriority, inlined for performance.

    var previousPriorityLevel = currentPriorityLevel;

    currentPriorityLevel = parentPriorityLevel;

```
    try {
```

      return callback.apply(this, arguments);

```
    } finally {
```

      currentPriorityLevel = previousPriorityLevel;

```
    }
```
```
  };
```
```
}
```
```
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 = IMMEDIATE_PRIORITY_TIMEOUT;

```
      break;
```
```
    case UserBlockingPriority:
```

      timeout = USER_BLOCKING_PRIORITY_TIMEOUT;

```
      break;
```
```
    case IdlePriority:
```
```
      timeout = IDLE_PRIORITY_TIMEOUT;
```
```
      break;
```
```
    case LowPriority:
```
```
      timeout = LOW_PRIORITY_TIMEOUT;
```
```
      break;
```
```
    case NormalPriority:
```
```
    default:
```

      timeout = NORMAL_PRIORITY_TIMEOUT;

```
      break;
```
```
  }
```

  var expirationTime = startTime + timeout;

```
  var newTask: Task = {
```
```
    id: taskIdCounter++,
```
```
    callback,
```
```
    priorityLevel,
```
```
    startTime,
```
```
    expirationTime,
```
```
    sortIndex: -1,
```
```
  };
```
```
  if (enableProfiling) {
```
```
    newTask.isQueued = false;
```
```
  }
```
```
  if (startTime > currentTime) {
```
```
    // This is a delayed task.
```
```
    newTask.sortIndex = startTime;
```
```
    push(timerQueue, newTask);
```

    if (peek(taskQueue) === null && newTask === peek(timerQueue)) {

      // All tasks are delayed, and this is the task with the earliest delay.

```
      if (isHostTimeoutScheduled) {
```
```
        // Cancel an existing timeout.
```
```
        cancelHostTimeout();
```
```
      } else {
```
```
        isHostTimeoutScheduled = true;
```
```
      }
```
```
      // Schedule a timeout.
```

      requestHostTimeout(handleTimeout, startTime - currentTime);

```
    }
```
```
  } else {
```

    newTask.sortIndex = expirationTime;

```
    push(taskQueue, newTask);
```
```
    if (enableProfiling) {
```

      markTaskStart(newTask, currentTime);

```
      newTask.isQueued = true;
```
```
    }
```

    // Schedule a host callback, if needed. If we're already performing work,

    // wait until the next time we yield.

    if (!isHostCallbackScheduled && !isPerformingWork) {

```
      isHostCallbackScheduled = true;
```
```
      requestHostCallback();
```
```
    }
```
```
  }
```
```
  return newTask;
```
```
}
```
```
function unstable_pauseExecution() {
```
```
  isSchedulerPaused = true;
```
```
}
```

function unstable_continueExecution() {

```
  isSchedulerPaused = false;
```

  if (!isHostCallbackScheduled && !isPerformingWork) {

```
    isHostCallbackScheduled = true;
```
```
    requestHostCallback();
```
```
  }
```
```
}
```

function unstable_getFirstCallbackNode(): Task | null {

```
  return peek(taskQueue);
```
```
}
```

function unstable_cancelCallback(task: Task) {

```
  if (enableProfiling) {
```
```
    if (task.isQueued) {
```

      const currentTime = getCurrentTime();

      markTaskCanceled(task, currentTime);

```
      task.isQueued = false;
```
```
    }
```
```
  }
```

  // Null out the callback to indicate the task has been canceled. (Can't

  // remove from the queue because you can't remove arbitrary nodes from an

  // array based heap, only the first one.)

```
  task.callback = null;
```
```
}
```

function unstable_getCurrentPriorityLevel(): PriorityLevel {

```
  return currentPriorityLevel;
```
```
}
```
```
let isMessageLoopRunning = false;
```

let taskTimeoutID: TimeoutID = (-1: any);

// Scheduler periodically yields in case there is other work on the main

// thread, like user events. By default, it yields multiple times per frame.

// It does not attempt to align with frame boundaries, since most tasks don't

// need to be frame aligned; for those that do, use requestAnimationFrame.

```
let frameInterval = frameYieldMs;
```

const continuousInputInterval = continuousYieldMs;

```
const maxInterval = maxYieldMs;
```
```
let startTime = -1;
```
```
let needsPaint = false;
```

function shouldYieldToHost(): boolean {

  const timeElapsed = getCurrentTime() - startTime;

```
  if (timeElapsed < frameInterval) {
```

    // The main thread has only been blocked for a really short amount of time;

    // smaller than a single frame. Don't yield yet.

```
    return false;
```
```
  }
```

  // The main thread has been blocked for a non-negligible amount of time. We

  // may want to yield control of the main thread, so the browser can perform

  // high priority tasks. The main ones are painting and user input. If there's

  // a pending paint or a pending input, then we should yield. But if there's

  // neither, then we can yield less often while remaining responsive. We'll

  // eventually yield regardless, since there could be a pending paint that

  // wasn't accompanied by a call to `requestPaint`, or other main thread tasks

```
  // like network events.
```
```
  if (enableIsInputPending) {
```
```
    if (needsPaint) {
```

      // There's a pending paint (signaled by `requestPaint`). Yield now.

```
      return true;
```
```
    }
```

    if (timeElapsed < continuousInputInterval) {

      // We haven't blocked the thread for that long. Only yield if there's a

      // pending discrete input (e.g. click). It's OK if there's pending

      // continuous input (e.g. mouseover).

```
      if (isInputPending !== null) {
```
```
        return isInputPending();
```
```
      }
```

    } else if (timeElapsed < maxInterval) {

      // Yield if there's either a pending discrete or continuous input.

```
      if (isInputPending !== null) {
```

        return isInputPending(continuousOptions);

```
      }
```
```
    } else {
```

      // We've blocked the thread for a long time. Even if there's no pending

      // input, there may be some other scheduled work that we don't know about,

      // like a network event. Yield now.

```
      return true;
```
```
    }
```
```
  }
```

  // `isInputPending` isn't available. Yield now.

```
  return true;
```
```
}
```
```
function requestPaint() {
```
```
  if (
```
```
    enableIsInputPending &&
```
```
    navigator !== undefined &&
```
```
    // $FlowFixMe[prop-missing]
```

    navigator.scheduling !== undefined &&

```
    // $FlowFixMe[incompatible-type]
```

    navigator.scheduling.isInputPending !== undefined

```
  ) {
```
```
    needsPaint = true;
```
```
  }
```

  // Since we yield every frame regardless, `requestPaint` has no effect.

```
}
```
```
function forceFrameRate(fps: number) {
```
```
  if (fps < 0 || fps > 125) {
```

    // Using console['error'] to evade Babel and ESLint

```
    console['error'](
```

      'forceFrameRate takes a positive int between 0 and 125, ' +

        'forcing frame rates higher than 125 fps is not supported',

```
    );
```
```
    return;
```
```
  }
```
```
  if (fps > 0) {
```

    frameInterval = Math.floor(1000 / fps);

```
  } else {
```
```
    // reset the framerate
```
```
    frameInterval = frameYieldMs;
```
```
  }
```
```
}
```

const performWorkUntilDeadline = () => {

```
  if (isMessageLoopRunning) {
```

    const currentTime = getCurrentTime();

    // Keep track of the start time so we can measure how long the main thread

```
    // has been blocked.
```
```
    startTime = currentTime;
```

    // If a scheduler task throws, exit the current browser task so the

```
    // error can be observed.
```
```
    //
```

    // Intentionally not using a try-catch, since that makes some debugging

    // techniques harder. Instead, if `flushWork` errors, then `hasMoreWork` will

    // remain true, and we'll continue the work loop.

```
    let hasMoreWork = true;
```
```
    try {
```

      hasMoreWork = flushWork(currentTime);

```
    } finally {
```
```
      if (hasMoreWork) {
```

        // If there's more work, schedule the next message event at the end

```
        // of the preceding one.
```

        schedulePerformWorkUntilDeadline();

```
      } else {
```
```
        isMessageLoopRunning = false;
```
```
      }
```
```
    }
```
```
  }
```

  // Yielding to the browser will give it a chance to paint, so we can

```
  // reset this.
```
```
  needsPaint = false;
```
```
};
```
```
let schedulePerformWorkUntilDeadline;
```

if (typeof localSetImmediate === 'function') {

```
  // Node.js and old IE.
```

  // There's a few reasons for why we prefer setImmediate.

```
  //
```

  // Unlike MessageChannel, it doesn't prevent a Node.js process from exiting.

  // (Even though this is a DOM fork of the Scheduler, you could get here

  // with a mix of Node.js 15+, which has a MessageChannel, and jsdom.)

  // https://github.com/facebook/react/issues/20756

```
  //
```

  // But also, it runs earlier which is the semantic we want.

  // If other browsers ever implement it, it's better to use it.

  // Although both of these would be inferior to native scheduling.

  schedulePerformWorkUntilDeadline = () => {

    localSetImmediate(performWorkUntilDeadline);

```
  };
```

} else if (typeof MessageChannel !== 'undefined') {

```
  // DOM and Worker environments.
```

  // We prefer MessageChannel because of the 4ms setTimeout clamping.

  const channel = new MessageChannel();

```
  const port = channel.port2;
```

  channel.port1.onmessage = performWorkUntilDeadline;

  schedulePerformWorkUntilDeadline = () => {

```
    port.postMessage(null);
```
```
  };
```
```
} else {
```

  // We should only fallback here in non-browser environments.

  schedulePerformWorkUntilDeadline = () => {

    // $FlowFixMe[not-a-function] nullable value

    localSetTimeout(performWorkUntilDeadline, 0);

```
  };
```
```
}
```
```
function requestHostCallback() {
```
```
  if (!isMessageLoopRunning) {
```
```
    isMessageLoopRunning = true;
```

    schedulePerformWorkUntilDeadline();

```
  }
```
```
}
```
```
function requestHostTimeout(
```

  callback: (currentTime: number) => void,

```
  ms: number,
```
```
) {
```

  // $FlowFixMe[not-a-function] nullable value

  taskTimeoutID = localSetTimeout(() => {

```
    callback(getCurrentTime());
```
```
  }, ms);
```
```
}
```
```
function cancelHostTimeout() {
```

  // $FlowFixMe[not-a-function] nullable value

```
  localClearTimeout(taskTimeoutID);
```

  taskTimeoutID = ((-1: any): TimeoutID);

```
}
```
```
export {
```

  ImmediatePriority as unstable_ImmediatePriority,

  UserBlockingPriority as unstable_UserBlockingPriority,

  NormalPriority as unstable_NormalPriority,

  IdlePriority as unstable_IdlePriority,

```
  LowPriority as unstable_LowPriority,
```
```
  unstable_runWithPriority,
```
```
  unstable_next,
```
```
  unstable_scheduleCallback,
```
```
  unstable_cancelCallback,
```
```
  unstable_wrapCallback,
```
```
  unstable_getCurrentPriorityLevel,
```

  shouldYieldToHost as unstable_shouldYield,

  requestPaint as unstable_requestPaint,

```
  unstable_continueExecution,
```
```
  unstable_pauseExecution,
```
```
  unstable_getFirstCallbackNode,
```
```
  getCurrentTime as unstable_now,
```

  forceFrameRate as unstable_forceFrameRate,

```
};
```
```
export const unstable_Profiling: {
```
```
  startLoggingProfilingEvents(): void,
```

  stopLoggingProfilingEvents(): ArrayBuffer | null,

```
} | null = enableProfiling
```
```
  ? {
```
```
      startLoggingProfilingEvents,
```
```
      stopLoggingProfilingEvents,
```
```
    }
```
```
  : null;
```