Promise实现的Scheduler

class Scheduler {
  constructor() {
    super();
    this.queue = [];
    this.maxCount = 2; // 最大并发数
    this.runningCount = 0; // 当前正在运行的任务数
  }

  add(promiseCreator) {
    this.queue.push(promiseCreator);
  }

  start() {
    for (let i = 0; i < this.maxCount; i++) {
      this.run();
    }
  }

  run() {
    if (this.queue.length === 0 || this.runningCount >= this.maxCount) {
      return;
    }

    this.runningCount++;
    const promiseCreator = this.queue.shift();
    const promise = promiseCreator();
    promise
      .then(() => {
        this.runningCount--;
        this.run();
      })
      .catch(() => {
        this.runningCount--;
        this.run();
      });
  }
}

const timeout = (time) => {
  return new Promise((resolve) => {
    setTimeout(resolve, time);
  });
};

const scheduler = new Scheduler();

const addTask = (time, order) => {
  scheduler.add(() => {
    return timeout(time).then(() => console.log(order));
  });
};

addTask(1000, 1);
addTask(500, 2);
addTask(300, 3);
addTask(400, 4);

scheduler.start();

class Scheduler {
  constructor(maxCount) {
    super()
    this.maxCount = maxCount;
    this.queue = [];
    this.runningCount = 0;
  }

  add(fn) {
    this.queue.push(fn);
  }

  async run() {
    while (this.runningCount < this.maxCount && this.queue.length > 0) {
      this.runningCount++;
      const fn = this.queue.shift();
      await fn();
      this.runningCount--;
    }
  }

  async start() {
    while (this.queue.length > 0) {
      await this.run();
    }
  }
}

const timeout = (time) => {
  return new Promise((resolve) => {
    setTimeout(resolve, time);
  });
};

const scheduler = new Scheduler(2);

const addTask = (time, order) => {
  scheduler.add(async () => {
    await timeout(time);
    console.log(order);
  });
};

addTask(1000, 1);
addTask(500, 2);
addTask(300, 3);
addTask(400, 4);

scheduler.start();