scheduler.development.js

/** @license React v0.17.0
 * scheduler.development.js
 *
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 */

'use strict';



if (process.env.NODE_ENV !== "production") {
  (function() {
'use strict';

Object.defineProperty(exports, '__esModule', { value: true });

var enableSchedulerDebugging = false;
var enableIsInputPending = false;
var enableMessageLoopImplementation = true;
var enableProfiling = true;

// works by scheduling a requestAnimationFrame, storing the time for the start
// of the frame, then scheduling a postMessage which gets scheduled after paint.
// Within the postMessage handler do as much work as possible until time + frame
// rate. By separating the idle call into a separate event tick we ensure that
// layout, paint and other browser work is counted against the available time.
// The frame rate is dynamically adjusted.

var requestHostCallback;

var requestHostTimeout;
var cancelHostTimeout;
var shouldYieldToHost;
var requestPaint;



if ( // If Scheduler runs in a non-DOM environment, it falls back to a naive
// implementation using setTimeout.
typeof window === 'undefined' || // Check if MessageChannel is supported, too.
typeof MessageChannel !== 'function') {
  // If this accidentally gets imported in a non-browser environment, e.g. JavaScriptCore,
  // fallback to a naive implementation.
  var _callback = null;
  var _timeoutID = null;

  var _flushCallback = function () {
    if (_callback !== null) {
      try {
        var currentTime = exports.unstable_now();
        var hasRemainingTime = true;

        _callback(hasRemainingTime, currentTime);

        _callback = null;
      } catch (e) {
        setTimeout(_flushCallback, 0);
        throw e;
      }
    }
  };

  var initialTime = Date.now();

  exports.unstable_now = function () {
    return Date.now() - initialTime;
  };

  requestHostCallback = function (cb) {
    if (_callback !== null) {
      // Protect against re-entrancy.
      setTimeout(requestHostCallback, 0, cb);
    } else {
      _callback = cb;
      setTimeout(_flushCallback, 0);
    }
  };

  requestHostTimeout = function (cb, ms) {
    _timeoutID = setTimeout(cb, ms);
  };

  cancelHostTimeout = function () {
    clearTimeout(_timeoutID);
  };

  shouldYieldToHost = function () {
    return false;
  };

  requestPaint = exports.unstable_forceFrameRate = function () {};
} else {
  // Capture local references to native APIs, in case a polyfill overrides them.
  var performance = window.performance;
  var _Date = window.Date;
  var _setTimeout = window.setTimeout;
  var _clearTimeout = window.clearTimeout;
  var requestAnimationFrame = window.requestAnimationFrame;
  var cancelAnimationFrame = window.cancelAnimationFrame;

  if (typeof console !== 'undefined') {
    // TODO: Remove fb.me link
    if (typeof requestAnimationFrame !== 'function') {
      console.error("This browser doesn't support requestAnimationFrame. " + 'Make sure that you load a ' + 'polyfill in older browsers. https://fb.me/react-polyfills');
    }

    if (typeof cancelAnimationFrame !== 'function') {
      console.error("This browser doesn't support cancelAnimationFrame. " + 'Make sure that you load a ' + 'polyfill in older browsers. https://fb.me/react-polyfills');
    }
  }

  if (typeof performance === 'object' && typeof performance.now === 'function') {
    exports.unstable_now = function () {
      return performance.now();
    };
  } else {
    var _initialTime = _Date.now();

    exports.unstable_now = function () {
      return _Date.now() - _initialTime;
    };
  }

  var isRAFLoopRunning = false;
  var isMessageLoopRunning = false;
  var scheduledHostCallback = null;
  var rAFTimeoutID = -1;
  var taskTimeoutID = -1;
  var frameLength = enableMessageLoopImplementation ? // We won't attempt to align with the vsync. Instead we'll yield multiple
  // times per frame, often enough to keep it responsive even at really
  // high frame rates > 120.
  5 : // Use a heuristic to measure the frame rate and yield at the end of the
  // frame. We start out assuming that we run at 30fps but then the
  // heuristic tracking will adjust this value to a faster fps if we get
  // more frequent animation frames.
  33.33;
  var prevRAFTime = -1;
  var prevRAFInterval = -1;
  var frameDeadline = 0;
  var fpsLocked = false; // TODO: Make this configurable
  // TODO: Adjust this based on priority?

  var maxFrameLength = 300;
  var needsPaint = false;

  if (enableIsInputPending && navigator !== undefined && navigator.scheduling !== undefined && navigator.scheduling.isInputPending !== undefined) {
    var scheduling = navigator.scheduling;

    shouldYieldToHost = function () {
      var currentTime = exports.unstable_now();

      if (currentTime >= frameDeadline) {
        // There's no time left in the frame. 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 (needsPaint || scheduling.isInputPending()) {
          // There is either a pending paint or a pending input.
          return true;
        } // There's no pending input. Only yield if we've reached the max
        // frame length.


        return currentTime >= frameDeadline + maxFrameLength;
      } else {
        // There's still time left in the frame.
        return false;
      }
    };

    requestPaint = function () {
      needsPaint = true;
    };
  } else {
    // `isInputPending` is not available. Since we have no way of knowing if
    // there's pending input, always yield at the end of the frame.
    shouldYieldToHost = function () {
      return exports.unstable_now() >= frameDeadline;
    }; // Since we yield every frame regardless, `requestPaint` has no effect.


    requestPaint = function () {};
  }

  exports.unstable_forceFrameRate = function (fps) {
    if (fps < 0 || fps > 125) {
      console.error('forceFrameRate takes a positive int between 0 and 125, ' + 'forcing framerates higher than 125 fps is not unsupported');
      return;
    }

    if (fps > 0) {
      frameLength = Math.floor(1000 / fps);
      fpsLocked = true;
    } else {
      // reset the framerate
      frameLength = 33.33;
      fpsLocked = false;
    }
  };

  var performWorkUntilDeadline = function () {
    if (enableMessageLoopImplementation) {
      if (scheduledHostCallback !== null) {
        var currentTime = exports.unstable_now(); // Yield after `frameLength` ms, regardless of where we are in the vsync
        // cycle. This means there's always time remaining at the beginning of
        // the message event.

        frameDeadline = currentTime + frameLength;
        var hasTimeRemaining = true;

        try {
          var hasMoreWork = scheduledHostCallback(hasTimeRemaining, currentTime);

          if (!hasMoreWork) {
            isMessageLoopRunning = false;
            scheduledHostCallback = null;
          } else {
            // If there's more work, schedule the next message event at the end
            // of the preceding one.
            port.postMessage(null);
          }
        } catch (error) {
          // If a scheduler task throws, exit the current browser task so the
          // error can be observed.
          port.postMessage(null);
          throw error;
        }
      } else {
        isMessageLoopRunning = false;
      } // Yielding to the browser will give it a chance to paint, so we can
      // reset this.


      needsPaint = false;
    } else {
      if (scheduledHostCallback !== null) {
        var _currentTime = exports.unstable_now();

        var _hasTimeRemaining = frameDeadline - _currentTime > 0;

        try {
          var _hasMoreWork = scheduledHostCallback(_hasTimeRemaining, _currentTime);

          if (!_hasMoreWork) {
            scheduledHostCallback = null;
          }
        } catch (error) {
          // If a scheduler task throws, exit the current browser task so the
          // error can be observed, and post a new task as soon as possible
          // so we can continue where we left off.
          port.postMessage(null);
          throw error;
        }
      } // Yielding to the browser will give it a chance to paint, so we can
      // reset this.


      needsPaint = false;
    }
  };

  var channel = new MessageChannel();
  var port = channel.port2;
  channel.port1.onmessage = performWorkUntilDeadline;

  var onAnimationFrame = function (rAFTime) {
    if (scheduledHostCallback === null) {
      // No scheduled work. Exit.
      prevRAFTime = -1;
      prevRAFInterval = -1;
      isRAFLoopRunning = false;
      return;
    } // Eagerly schedule the next animation callback at the beginning of the
    // frame. If the scheduler queue is not empty at the end of the frame, it
    // will continue flushing inside that callback. If the queue *is* empty,
    // then it will exit immediately. Posting the callback at the start of the
    // frame ensures it's fired within the earliest possible frame. If we
    // waited until the end of the frame to post the callback, we risk the
    // browser skipping a frame and not firing the callback until the frame
    // after that.


    isRAFLoopRunning = true;
    requestAnimationFrame(function (nextRAFTime) {
      _clearTimeout(rAFTimeoutID);

      onAnimationFrame(nextRAFTime);
    }); // requestAnimationFrame is throttled when the tab is backgrounded. We
    // don't want to stop working entirely. So we'll fallback to a timeout loop.
    // TODO: Need a better heuristic for backgrounded work.

    var onTimeout = function () {
      frameDeadline = exports.unstable_now() + frameLength / 2;
      performWorkUntilDeadline();
      rAFTimeoutID = _setTimeout(onTimeout, frameLength * 3);
    };

    rAFTimeoutID = _setTimeout(onTimeout, frameLength * 3);

    if (prevRAFTime !== -1 && // Make sure this rAF time is different from the previous one. This check
    // could fail if two rAFs fire in the same frame.
    rAFTime - prevRAFTime > 0.1) {
      var rAFInterval = rAFTime - prevRAFTime;

      if (!fpsLocked && prevRAFInterval !== -1) {
        // We've observed two consecutive frame intervals. We'll use this to
        // dynamically adjust the frame rate.
        //
        // If one frame goes long, then the next one can be short to catch up.
        // If two frames are short in a row, then that's an indication that we
        // actually have a higher frame rate than what we're currently
        // optimizing. For example, if we're running on 120hz display or 90hz VR
        // display. Take the max of the two in case one of them was an anomaly
        // due to missed frame deadlines.
        if (rAFInterval < frameLength && prevRAFInterval < frameLength) {
          frameLength = rAFInterval < prevRAFInterval ? prevRAFInterval : rAFInterval;

          if (frameLength < 8.33) {
            // Defensive coding. We don't support higher frame rates than 120hz.
            // If the calculated frame length gets lower than 8, it is probably
            // a bug.
            frameLength = 8.33;
          }
        }
      }

      prevRAFInterval = rAFInterval;
    }

    prevRAFTime = rAFTime;
    frameDeadline = rAFTime + frameLength; // We use the postMessage trick to defer idle work until after the repaint.

    port.postMessage(null);
  };

  requestHostCallback = function (callback) {
    scheduledHostCallback = callback;

    if (enableMessageLoopImplementation) {
      if (!isMessageLoopRunning) {
        isMessageLoopRunning = true;
        port.postMessage(null);
      }
    } else {
      if (!isRAFLoopRunning) {
        // Start a rAF loop.
        isRAFLoopRunning = true;
        requestAnimationFrame(function (rAFTime) {
          onAnimationFrame(rAFTime);
        });
      }
    }
  };

  requestHostTimeout = function (callback, ms) {
    taskTimeoutID = _setTimeout(function () {
      callback(exports.unstable_now());
    }, ms);
  };

  cancelHostTimeout = function () {
    _clearTimeout(taskTimeoutID);

    taskTimeoutID = -1;
  };
}

function push(heap, node) {
  var index = heap.length;
  heap.push(node);
  siftUp(heap, node, index);
}
function peek(heap) {
  var first = heap[0];
  return first === undefined ? null : first;
}
function pop(heap) {
  var first = heap[0];

  if (first !== undefined) {
    var last = heap.pop();

    if (last !== first) {
      heap[0] = last;
      siftDown(heap, last, 0);
    }

    return first;
  } else {
    return null;
  }
}

function siftUp(heap, node, i) {
  var index = i;

  while (true) {
    var parentIndex = Math.floor((index - 1) / 2);
    var parent = heap[parentIndex];

    if (parent !== undefined && compare(parent, node) > 0) {
      // The parent is larger. Swap positions.
      heap[parentIndex] = node;
      heap[index] = parent;
      index = parentIndex;
    } else {
      // The parent is smaller. Exit.
      return;
    }
  }
}

function siftDown(heap, node, i) {
  var index = i;
  var length = heap.length;

  while (index < length) {
    var leftIndex = (index + 1) * 2 - 1;
    var left = heap[leftIndex];
    var rightIndex = leftIndex + 1;
    var right = heap[rightIndex]; // If the left or right node is smaller, swap with the smaller of those.

    if (left !== undefined && compare(left, node) < 0) {
      if (right !== undefined && compare(right, left) < 0) {
        heap[index] = right;
        heap[rightIndex] = node;
        index = rightIndex;
      } else {
        heap[index] = left;
        heap[leftIndex] = node;
        index = leftIndex;
      }
    } else if (right !== undefined && compare(right, node) < 0) {
      heap[index] = right;
      heap[rightIndex] = node;
      index = rightIndex;
    } else {
      // Neither child is smaller. Exit.
      return;
    }
  }
}

function compare(a, b) {
  // Compare sort index first, then task id.
  var diff = a.sortIndex - b.sortIndex;
  return diff !== 0 ? diff : a.id - b.id;
}

// TODO: Use symbols?
var NoPriority = 0;
var ImmediatePriority = 1;
var UserBlockingPriority = 2;
var NormalPriority = 3;
var LowPriority = 4;
var IdlePriority = 5;

var runIdCounter = 0;
var mainThreadIdCounter = 0;
var profilingStateSize = 4;
var sharedProfilingBuffer = enableProfiling ? // $FlowFixMe Flow doesn't know about SharedArrayBuffer
typeof SharedArrayBuffer === 'function' ? new SharedArrayBuffer(profilingStateSize * Int32Array.BYTES_PER_ELEMENT) : // $FlowFixMe Flow doesn't know about ArrayBuffer
typeof ArrayBuffer === 'function' ? new ArrayBuffer(profilingStateSize * Int32Array.BYTES_PER_ELEMENT) : null // Don't crash the init path on IE9
: null;
var profilingState = enableProfiling && sharedProfilingBuffer !== null ? new Int32Array(sharedProfilingBuffer) : []; // We can't read this but it helps save bytes for null checks

var PRIORITY = 0;
var CURRENT_TASK_ID = 1;
var CURRENT_RUN_ID = 2;
var QUEUE_SIZE = 3;

if (enableProfiling) {
  profilingState[PRIORITY] = NoPriority; // This is maintained with a counter, because the size of the priority queue
  // array might include canceled tasks.

  profilingState[QUEUE_SIZE] = 0;
  profilingState[CURRENT_TASK_ID] = 0;
} // Bytes per element is 4


var INITIAL_EVENT_LOG_SIZE = 131072;
var MAX_EVENT_LOG_SIZE = 524288; // Equivalent to 2 megabytes

var eventLogSize = 0;
var eventLogBuffer = null;
var eventLog = null;
var eventLogIndex = 0;
var TaskStartEvent = 1;
var TaskCompleteEvent = 2;
var TaskErrorEvent = 3;
var TaskCancelEvent = 4;
var TaskRunEvent = 5;
var TaskYieldEvent = 6;
var SchedulerSuspendEvent = 7;
var SchedulerResumeEvent = 8;

function logEvent(entries) {
  if (eventLog !== null) {
    var offset = eventLogIndex;
    eventLogIndex += entries.length;

    if (eventLogIndex + 1 > eventLogSize) {
      eventLogSize *= 2;

      if (eventLogSize > MAX_EVENT_LOG_SIZE) {
        console.error("Scheduler Profiling: Event log exceeded maximum size. Don't " + 'forget to call `stopLoggingProfilingEvents()`.');
        stopLoggingProfilingEvents();
        return;
      }

      var newEventLog = new Int32Array(eventLogSize * 4);
      newEventLog.set(eventLog);
      eventLogBuffer = newEventLog.buffer;
      eventLog = newEventLog;
    }

    eventLog.set(entries, offset);
  }
}

function startLoggingProfilingEvents() {
  eventLogSize = INITIAL_EVENT_LOG_SIZE;
  eventLogBuffer = new ArrayBuffer(eventLogSize * 4);
  eventLog = new Int32Array(eventLogBuffer);
  eventLogIndex = 0;
}
function stopLoggingProfilingEvents() {
  var buffer = eventLogBuffer;
  eventLogSize = 0;
  eventLogBuffer = null;
  eventLog = null;
  eventLogIndex = 0;
  return buffer;
}
function markTaskStart(task, ms) {
  if (enableProfiling) {
    profilingState[QUEUE_SIZE]++;

    if (eventLog !== null) {
      // performance.now returns a float, representing milliseconds. When the
      // event is logged, it's coerced to an int. Convert to microseconds to
      // maintain extra degrees of precision.
      logEvent([TaskStartEvent, ms * 1000, task.id, task.priorityLevel]);
    }
  }
}
function markTaskCompleted(task, ms) {
  if (enableProfiling) {
    profilingState[PRIORITY] = NoPriority;
    profilingState[CURRENT_TASK_ID] = 0;
    profilingState[QUEUE_SIZE]--;

    if (eventLog !== null) {
      logEvent([TaskCompleteEvent, ms * 1000, task.id]);
    }
  }
}
function markTaskCanceled(task, ms) {
  if (enableProfiling) {
    profilingState[QUEUE_SIZE]--;

    if (eventLog !== null) {
      logEvent([TaskCancelEvent, ms * 1000, task.id]);
    }
  }
}
function markTaskErrored(task, ms) {
  if (enableProfiling) {
    profilingState[PRIORITY] = NoPriority;
    profilingState[CURRENT_TASK_ID] = 0;
    profilingState[QUEUE_SIZE]--;

    if (eventLog !== null) {
      logEvent([TaskErrorEvent, ms * 1000, task.id]);
    }
  }
}
function markTaskRun(task, ms) {
  if (enableProfiling) {
    runIdCounter++;
    profilingState[PRIORITY] = task.priorityLevel;
    profilingState[CURRENT_TASK_ID] = task.id;
    profilingState[CURRENT_RUN_ID] = runIdCounter;

    if (eventLog !== null) {
      logEvent([TaskRunEvent, ms * 1000, task.id, runIdCounter]);
    }
  }
}
function markTaskYield(task, ms) {
  if (enableProfiling) {
    profilingState[PRIORITY] = NoPriority;
    profilingState[CURRENT_TASK_ID] = 0;
    profilingState[CURRENT_RUN_ID] = 0;

    if (eventLog !== null) {
      logEvent([TaskYieldEvent, ms * 1000, task.id, runIdCounter]);
    }
  }
}
function markSchedulerSuspended(ms) {
  if (enableProfiling) {
    mainThreadIdCounter++;

    if (eventLog !== null) {
      logEvent([SchedulerSuspendEvent, ms * 1000, mainThreadIdCounter]);
    }
  }
}
function markSchedulerUnsuspended(ms) {
  if (enableProfiling) {
    if (eventLog !== null) {
      logEvent([SchedulerResumeEvent, ms * 1000, mainThreadIdCounter]);
    }
  }
}

/* eslint-disable no-var */
// Math.pow(2, 30) - 1
// 0b111111111111111111111111111111

var maxSigned31BitInt = 1073741823; // Times out immediately

var IMMEDIATE_PRIORITY_TIMEOUT = -1; // Eventually times out

var USER_BLOCKING_PRIORITY = 250;
var NORMAL_PRIORITY_TIMEOUT = 5000;
var LOW_PRIORITY_TIMEOUT = 10000; // Never times out

var IDLE_PRIORITY = maxSigned31BitInt; // Tasks are stored on a min heap

var taskQueue = [];
var timerQueue = []; // 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-entrancy.

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

function advanceTimers(currentTime) {
  // Check for tasks that are no longer delayed and add them to the queue.
  var 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) {
  isHostTimeoutScheduled = false;
  advanceTimers(currentTime);

  if (!isHostCallbackScheduled) {
    if (peek(taskQueue) !== null) {
      isHostCallbackScheduled = true;
      requestHostCallback(flushWork);
    } else {
      var firstTimer = peek(timerQueue);

      if (firstTimer !== null) {
        requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
      }
    }
  }
}

function flushWork(hasTimeRemaining, initialTime) {
  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;
  var previousPriorityLevel = currentPriorityLevel;

  try {
    if (enableProfiling) {
      try {
        return workLoop(hasTimeRemaining, initialTime);
      } catch (error) {
        if (currentTask !== null) {
          var currentTime = exports.unstable_now();
          markTaskErrored(currentTask, currentTime);
          currentTask.isQueued = false;
        }

        throw error;
      }
    } else {
      // No catch in prod codepath.
      return workLoop(hasTimeRemaining, initialTime);
    }
  } finally {
    currentTask = null;
    currentPriorityLevel = previousPriorityLevel;
    isPerformingWork = false;

    if (enableProfiling) {
      var _currentTime = exports.unstable_now();

      markSchedulerSuspended(_currentTime);
    }
  }
}

function workLoop(hasTimeRemaining, initialTime) {
  var currentTime = initialTime;
  advanceTimers(currentTime);
  currentTask = peek(taskQueue);

  while (currentTask !== null && !(enableSchedulerDebugging && isSchedulerPaused)) {
    if (currentTask.expirationTime > currentTime && (!hasTimeRemaining || shouldYieldToHost())) {
      // This currentTask hasn't expired, and we've reached the deadline.
      break;
    }

    var callback = currentTask.callback;

    if (callback !== null) {
      currentTask.callback = null;
      currentPriorityLevel = currentTask.priorityLevel;
      var didUserCallbackTimeout = currentTask.expirationTime <= currentTime;
      markTaskRun(currentTask, currentTime);
      var continuationCallback = callback(didUserCallbackTimeout);
      currentTime = exports.unstable_now();

      if (typeof continuationCallback === 'function') {
        currentTask.callback = continuationCallback;
        markTaskYield(currentTask, currentTime);
      } else {
        if (enableProfiling) {
          markTaskCompleted(currentTask, currentTime);
          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 {
    var firstTimer = peek(timerQueue);

    if (firstTimer !== null) {
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }

    return false;
  }
}

function unstable_runWithPriority(priorityLevel, eventHandler) {
  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(eventHandler) {
  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(callback) {
  var parentPriorityLevel = currentPriorityLevel;
  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 timeoutForPriorityLevel(priorityLevel) {
  switch (priorityLevel) {
    case ImmediatePriority:
      return IMMEDIATE_PRIORITY_TIMEOUT;

    case UserBlockingPriority:
      return USER_BLOCKING_PRIORITY;

    case IdlePriority:
      return IDLE_PRIORITY;

    case LowPriority:
      return LOW_PRIORITY_TIMEOUT;

    case NormalPriority:
    default:
      return NORMAL_PRIORITY_TIMEOUT;
  }
}

function unstable_scheduleCallback(priorityLevel, callback, options) {
  var currentTime = exports.unstable_now();
  var startTime;
  var timeout;

  if (typeof options === 'object' && options !== null) {
    var delay = options.delay;

    if (typeof delay === 'number' && delay > 0) {
      startTime = currentTime + delay;
    } else {
      startTime = currentTime;
    }

    timeout = typeof options.timeout === 'number' ? options.timeout : timeoutForPriorityLevel(priorityLevel);
  } else {
    timeout = timeoutForPriorityLevel(priorityLevel);
    startTime = currentTime;
  }

  var expirationTime = startTime + timeout;
  var newTask = {
    id: taskIdCounter++,
    callback: callback,
    priorityLevel: priorityLevel,
    startTime: startTime,
    expirationTime: 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(flushWork);
    }
  }

  return newTask;
}

function unstable_pauseExecution() {
  isSchedulerPaused = true;
}

function unstable_continueExecution() {
  isSchedulerPaused = false;

  if (!isHostCallbackScheduled && !isPerformingWork) {
    isHostCallbackScheduled = true;
    requestHostCallback(flushWork);
  }
}

function unstable_getFirstCallbackNode() {
  return peek(taskQueue);
}

function unstable_cancelCallback(task) {
  if (enableProfiling) {
    if (task.isQueued) {
      var currentTime = exports.unstable_now();
      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() {
  return currentPriorityLevel;
}

function unstable_shouldYield() {
  var currentTime = exports.unstable_now();
  advanceTimers(currentTime);
  var firstTask = peek(taskQueue);
  return firstTask !== currentTask && currentTask !== null && firstTask !== null && firstTask.callback !== null && firstTask.startTime <= currentTime && firstTask.expirationTime < currentTask.expirationTime || shouldYieldToHost();
}

var unstable_requestPaint = requestPaint;
var unstable_Profiling = enableProfiling ? {
  startLoggingProfilingEvents: startLoggingProfilingEvents,
  stopLoggingProfilingEvents: stopLoggingProfilingEvents,
  sharedProfilingBuffer: sharedProfilingBuffer
} : null;

exports.unstable_ImmediatePriority = ImmediatePriority;
exports.unstable_UserBlockingPriority = UserBlockingPriority;
exports.unstable_NormalPriority = NormalPriority;
exports.unstable_IdlePriority = IdlePriority;
exports.unstable_LowPriority = LowPriority;
exports.unstable_runWithPriority = unstable_runWithPriority;
exports.unstable_next = unstable_next;
exports.unstable_scheduleCallback = unstable_scheduleCallback;
exports.unstable_cancelCallback = unstable_cancelCallback;
exports.unstable_wrapCallback = unstable_wrapCallback;
exports.unstable_getCurrentPriorityLevel = unstable_getCurrentPriorityLevel;
exports.unstable_shouldYield = unstable_shouldYield;
exports.unstable_requestPaint = unstable_requestPaint;
exports.unstable_continueExecution = unstable_continueExecution;
exports.unstable_pauseExecution = unstable_pauseExecution;
exports.unstable_getFirstCallbackNode = unstable_getFirstCallbackNode;
exports.unstable_Profiling = unstable_Profiling;
  })();
}