xterm.js/src/common/TaskQueue.ts

/**
 * Copyright (c) 2022 The xterm.js authors. All rights reserved.
 * @license MIT
 */

import { isNode } from 'common/Platform';

interface ITaskQueue {
  /**
   * Adds a task to the queue which will run in a future idle callback.
   * To avoid perceivable stalls on the mainthread, tasks with heavy workload
   * should split their work into smaller pieces and return `true` to get
   * called again until the work is done (on falsy return value).
   */
  enqueue(task: () => boolean | void): void;

  /**
   * Flushes the queue, running all remaining tasks synchronously.
   */
  flush(): void;

  /**
   * Clears any remaining tasks from the queue, these will not be run.
   */
  clear(): void;
}

interface ITaskDeadline {
  timeRemaining(): number;
}
type CallbackWithDeadline = (deadline: ITaskDeadline) => void;

abstract class TaskQueue implements ITaskQueue {
  private _tasks: (() => boolean | void)[] = [];
  private _idleCallback?: number;
  private _i = 0;

  protected abstract _requestCallback(callback: CallbackWithDeadline): number;
  protected abstract _cancelCallback(identifier: number): void;

  public enqueue(task: () => boolean | void): void {
    this._tasks.push(task);
    this._start();
  }

  public flush(): void {
    while (this._i < this._tasks.length) {
      if (!this._tasks[this._i]()) {
        this._i++;
      }
    }
    this.clear();
  }

  public clear(): void {
    if (this._idleCallback) {
      this._cancelCallback(this._idleCallback);
      this._idleCallback = undefined;
    }
    this._i = 0;
    this._tasks.length = 0;
  }

  private _start(): void {
    if (!this._idleCallback) {
      this._idleCallback = this._requestCallback(this._process.bind(this));
    }
  }

  private _process(deadline: ITaskDeadline): void {
    this._idleCallback = undefined;
    let taskDuration = 0;
    let longestTask = 0;
    let lastDeadlineRemaining = deadline.timeRemaining();
    let deadlineRemaining = 0;
    while (this._i < this._tasks.length) {
      taskDuration = performance.now();
      if (!this._tasks[this._i]()) {
        this._i++;
      }
      // other than performance.now, performance.now might not be stable (changes on wall clock
      // changes), this is not an issue here as a clock change during a short running task is very
      // unlikely in case it still happened and leads to negative duration, simply assume 1 msec
      taskDuration = Math.max(1, performance.now() - taskDuration);
      longestTask = Math.max(taskDuration, longestTask);
      // Guess the following task will take a similar time to the longest task in this batch, allow
      // additional room to try avoid exceeding the deadline
      deadlineRemaining = deadline.timeRemaining();
      if (longestTask * 1.5 > deadlineRemaining) {
        // Warn when the time exceeding the deadline is over 20ms, if this happens in practice the
        // task should be split into sub-tasks to ensure the UI remains responsive.
        if (lastDeadlineRemaining - taskDuration < -20) {
          console.warn(`task queue exceeded allotted deadline by ${Math.abs(Math.round(lastDeadlineRemaining - taskDuration))}ms`);
        }
        this._start();
        return;
      }
      lastDeadlineRemaining = deadlineRemaining;
    }
    this.clear();
  }
}

/**
 * A queue of that runs tasks over several tasks via setTimeout, trying to maintain above 60 frames
 * per second. The tasks will run in the order they are enqueued, but they will run some time later,
 * and care should be taken to ensure they're non-urgent and will not introduce race conditions.
 */
export class PriorityTaskQueue extends TaskQueue {
  protected _requestCallback(callback: CallbackWithDeadline): number {
    return setTimeout(() => callback(this._createDeadline(16)));
  }

  protected _cancelCallback(identifier: number): void {
    clearTimeout(identifier);
  }

  private _createDeadline(duration: number): ITaskDeadline {
    const end = performance.now() + duration;
    return {
      timeRemaining: () => Math.max(0, end - performance.now())
    };
  }
}

class IdleTaskQueueInternal extends TaskQueue {
  protected _requestCallback(callback: IdleRequestCallback): number {
    return requestIdleCallback(callback);
  }

  protected _cancelCallback(identifier: number): void {
    cancelIdleCallback(identifier);
  }
}

/**
 * A queue of that runs tasks over several idle callbacks, trying to respect the idle callback's
 * deadline given by the environment. The tasks will run in the order they are enqueued, but they
 * will run some time later, and care should be taken to ensure they're non-urgent and will not
 * introduce race conditions.
 *
 * This reverts to a {@link PriorityTaskQueue} if the environment does not support idle callbacks.
 */
// eslint-disable-next-line @typescript-eslint/naming-convention
export const IdleTaskQueue = (!isNode && 'requestIdleCallback' in window) ? IdleTaskQueueInternal : PriorityTaskQueue;

/**
 * An object that tracks a single debounced task that will run on the next idle frame. When called
 * multiple times, only the last set task will run.
 */
export class DebouncedIdleTask {
  private _queue: ITaskQueue;

  constructor() {
    this._queue = new IdleTaskQueue();
  }

  public set(task: () => boolean | void): void {
    this._queue.clear();
    this._queue.enqueue(task);
  }

  public flush(): void {
    this._queue.flush();
  }
}