IWSR

function unstable_scheduleCallback(priorityLevel, callback, options) {
  /**
   * Get the current time when the function runs, which will later be used to calculate the startTime of the task based on priority and to determine
   * whether the task has expired.
   * */
  var currentTime = exports.unstable_now();
  var startTime;
  /**
   * The external may manually pass in delay to postpone the execution of the task.
   */
  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;
  /**
   * The timeout will be assigned different values based on the priority level.
   * The timeout corresponding to ImmediatePriority is -1, so tasks of this priority can be considered expired upon registration,
   * and will be executed quickly.
   */
  switch (priorityLevel) {
    case ImmediatePriority:
      timeout = IMMEDIATE_PRIORITY_TIMEOUT; // -1
      break;

    case UserBlockingPriority:
      timeout = USER_BLOCKING_PRIORITY_TIMEOUT; // 250
      break;

    case IdlePriority:
      timeout = IDLE_PRIORITY_TIMEOUT; // 1073741823
      break;

    case LowPriority:
      timeout = LOW_PRIORITY_TIMEOUT; // 10000
      break;

    case NormalPriority:
    default:
      timeout = NORMAL_PRIORITY_TIMEOUT; // 5000
      break;
  }
  /**
   * Calculate the expiration time.
   */
  var expirationTime = startTime + timeout;
  var newTask = {
    id: taskIdCounter++,
    callback: callback, // The function registered for the task
    priorityLevel: priorityLevel,
    startTime: startTime, // Task start time
    expirationTime: expirationTime, // Expiration time
    sortIndex: -1 // Basis for min-heap sorting
  };

  if (startTime > currentTime) {
    // This is a delayed task. Because a delay is set, this is a task that needs to set a timer.
    newTask.sortIndex = startTime;
    // Push this task into the timerQueue.
    push(timerQueue, newTask);

    if (peek(taskQueue) === null && newTask === peek(timerQueue)) {
      // All tasks are delayed, and this is the task with the earliest delay.
      // taskQueue is empty, and the current task is the first in timerQueue.
      if (isHostTimeoutScheduled) {
        // Cancel an existing timeout.
        cancelHostTimeout();
      } else {
        isHostTimeoutScheduled = true;
      } // Schedule a timeout.

      // requestHostTimeout is just a wrapped timer.
      /**
       * handleTimeout is a function that starts scheduling, which will be analyzed later.
       * In this code, it can be temporarily understood as starting scheduling to execute the current task.
       * */
      requestHostTimeout(handleTimeout, startTime - currentTime);
    }
  } else {
    /**
     * If no delay is set, directly push the task into taskQueue.
     */
    newTask.sortIndex = expirationTime;
    push(taskQueue, newTask);
    // Wait until the next time we yield.

    if (!isHostCallbackScheduled && !isPerformingWork) {
      isHostCallbackScheduled = true;
      /**
       * flushWork is a relatively important function that involves clearing the taskQueue.
       * requestHostCallback will trigger MessageChannel to execute performWorkUntilDeadline, which will be analyzed in detail later.
       * Here, the following code can be understood as calling flushWork to clear the tasks in taskQueue.
       */
      requestHostCallback(flushWork);
    }
  }

  return newTask;
}

/**
 * After advanceTimers organizes timerQ and taskQ,
 * if there are tasks in taskQ, schedule that task.
 * Otherwise, check if there are tasks in timerQ; if so, set a timer to its startTime and call handleTimeout.
 */
function handleTimeout(currentTime) {
  isHostTimeoutScheduled = false;
  advanceTimers(currentTime); // Function to organize timerQ and taskQ, see analysis below.

  if (!isHostCallbackScheduled) {
    if (peek(taskQueue) !== null) {
      /**
       * When taskQ is not empty, start scheduling.
       */
      isHostCallbackScheduled = true;
      // The same operation exists in unstable_scheduleCallback, which is the entry point for starting scheduling.
      requestHostCallback(flushWork);
    } else {
      /**
       * At this point, there are no tasks in taskQ to schedule.
       * We can only check if there are tasks in timerQ that have reached startTime.
       */
      var firstTimer = peek(timerQueue);

      if (firstTimer !== null) {
        /**
         * Set a timer for the first element in timerQ to wait for it to reach startTime,
         * then use advanceTimers to transfer the task to taskQ and schedule it.
         */
        requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
      }
    }
  }
}

/**
 * Continuously traverse timerQ to take out tasks that have reached startTime and put them into taskQ.
 * Additionally, it is responsible for discarding canceled tasks.
 */
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) { // unstable_cancelCallback cancels the task by setting its callback to null.
      // Timer was cancelled.
      pop(timerQueue); // Here, the canceled task is discarded.
    } else if (timer.startTime <= currentTime) {
      // Timer fired. Transfer to the task queue.
      // The task has reached startTime, move it to taskQ.
      pop(timerQueue);
      timer.sortIndex = timer.expirationTime;
      push(taskQueue, timer);
    } else {
      // Remaining timers are pending.
      return;
    }

    timer = peek(timerQueue);
  }
}

/**
 * Based on requestHostCallback(flushWork)
 * Here, the callback is flushWork.
 */
function requestHostCallback(callback) {
  scheduledHostCallback = callback; // Assign flushWork to scheduledHostCallback.

  if (!isMessageLoopRunning) {
    isMessageLoopRunning = true;
    schedulePerformWorkUntilDeadline(); // Key function.
  }
}

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

schedulePerformWorkUntilDeadline = function () {
  port.postMessage(null);
};

var performWorkUntilDeadline = function () {
  /**
   * Here, scheduledHostCallback is actually flushWork.
   * See requestHostCallback for details.
   */
  if (scheduledHostCallback !== null) {
    var currentTime = exports.unstable_now();

    startTime = currentTime;
    var hasTimeRemaining = true;

    var hasMoreWork = true;
    try {
      /**
       * Execute flushWork, its return value indicates whether taskQ is empty.
       * If not empty, it means there are still tasks to be scheduled.
       */
      hasMoreWork = scheduledHostCallback(hasTimeRemaining, currentTime);
    } finally {
      /**
       * If there is more work,
       */
      if (hasMoreWork) {
        // If there's more work, schedule the next message event at the end
        // of the preceding one.
        /**
         * Because there are still tasks to schedule, trigger MessageChannel again.
         */
        schedulePerformWorkUntilDeadline();
      } else {
        isMessageLoopRunning = false;
        scheduledHostCallback = null;
      }
    }
  } else {
    isMessageLoopRunning = false;
  } // Yielding to the browser will give it a chance to paint, so we can
};

/**
 * Its purpose is simply to call workLoop.
 */
function flushWork(hasTimeRemaining, initialTime) {
  /**
   * hasTimeRemaining is assigned true.
   * initialTime is the timestamp when performWorkUntilDeadline is called.
   */
  isHostCallbackScheduled = false;

  if (isHostTimeoutScheduled) {
    // We scheduled a timeout but it's no longer needed. Cancel it.
    /**
     * Prioritize clearing taskQ to prevent the contents set by the timer from jumping the queue.
     * Cancel any existing timers.
     */
    isHostTimeoutScheduled = false;
    cancelHostTimeout();
  }

  isPerformingWork = true;
  var previousPriorityLevel = currentPriorityLevel;

  try {
    if (enableProfiling) {
      try {
        /**
         * The focus of the entire flushWork is actually to call workLoop.
         */
        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 code path.
      return workLoop(hasTimeRemaining, initialTime);
    }
  } finally {
    currentTask = null;
    currentPriorityLevel = previousPriorityLevel;
    isPerformingWork = false;
  }
}

/**
 * Loop through taskQ and execute the callback of each task.
 */
function workLoop(hasTimeRemaining, initialTime) {
  var currentTime = initialTime;
  /**
   * Organize timerQ and taskQ.
   */
  advanceTimers(currentTime);
  currentTask = peek(taskQueue); // Get the first task.

  while (currentTask !== null && !(enableSchedulerDebugging)) {
    /**
     * If the task has expired and the time slice has not been used up, continue the while loop; otherwise, break.
     * The content of shouldYieldToHost contains the time slicing, which will be analyzed separately later.
     */
    if (currentTask.expirationTime > currentTime && (!hasTimeRemaining || shouldYieldToHost())) {
      // This currentTask hasn't expired, and we've reached the deadline.
      break;
    }

    var callback = currentTask.callback;

    /**
     * The callback can be either function/null.
     * If it's a function, it's a valid task.
     * If it's null, it's a canceled task.
     */
    if (typeof callback === 'function') {
      currentTask.callback = null;
      currentPriorityLevel = currentTask.priorityLevel;
      var didUserCallbackTimeout = currentTask.expirationTime <= currentTime;

      /**
       * If you use Performance tools to record here,
       * it is not difficult to find that the callback here is actually performConcurrentWorkOnRoot,
       * and in that function, when root.callbackNode === originalCallbackNode is satisfied,
       * it means the original task has not been completed, and it will return to performConcurrentWorkOnRoot itself to resume the interrupted task.
       * 
       * To add, the reason why there may be interruptions here is mainly because the time slice has run out, which is the content of shouldYieldToHost().
       * 
       * Tip: Don't get too caught up in the details; the biggest pitfall in reading source code is getting lost in the details (I will supplement this debugging record later when I have time).
      */
      var continuationCallback = callback(didUserCallbackTimeout);
      currentTime = exports.unstable_now();

      if (typeof continuationCallback === 'function') {
        /**
         * Because it hasn't finished executing, reassign the task's callback so that it can resume the interrupted task on the next call.
        */
        currentTask.callback = continuationCallback;
      } else {
        /**
         * Here, the task has successfully completed, so we can discard this task.
        */
        if (currentTask === peek(taskQueue)) {
          pop(taskQueue);
        }
      }
      /**
       * After analysis, if you forget to scroll up...
      */
      advanceTimers(currentTime);
    } else {
      /**
       * Because it is a canceled task,
       * there is no need to execute it, so pop it off.
      */
      pop(taskQueue);
    }

    currentTask = peek(taskQueue);
  } // Return whether there's additional work.

  /**
   * Even if the while loop is interrupted due to the time slice running out, it will still enter the following judgment logic.
  */
  if (currentTask !== null) {
    // If it was interrupted due to the while loop, then currentTask must not be null.
    // Thus, return true.
    return true;
  } else {
    // If the while loop ends, then currentTask must be null.
    var firstTimer = peek(timerQueue);

    if (firstTimer !== null) {
      // As long as timerQueue is not empty, start the next round of scheduling.
      requestHostTimeout(handleTimeout, firstTimer.startTime - currentTime);
    }
    // Since the task has completed, there are no remaining tasks, so return false.
    return false;
  }
  // The return value of workLoop is actually assigned to hasMoreWork.
}

function shouldYieldToHost() {
  /**
   * performWorkUntilDeadline assigns startTime at the beginning of execution.
  */
  const timeElapsed = getCurrentTime() - startTime;
  /**
   * frameInterval's default value is 5 ms,
   * but it will also be automatically calculated based on the monitor's fps (0, 125),
   * calculated as Math.floor(1000 / fps).
   * This means the duration of a single frame, and this value is the time slice.
  */
  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.
    // Because there's still plenty of time, no need to interrupt.
    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.
  /**
   * In short: we prioritize responding to user input, balabala.
  */
  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.
  // If isInputPending is not available, then yield.
  return true;
}