调度器

2024年04月08日

柏拉文

越努力，越幸运

一、认识

---

在调度器中，我们需要维护一个任务队列，当任务完成后，调度器从队列中取出下一个任务并执行。如果同时允许多个任务并行执行，我们可以使用一个计数器来控制并发数。

二、实现

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class Scheduler {
  constructor(parallelism) {
    this.queue = [];
    this.runningTask = 0;
    this.parallelism = parallelism;
  }

  add(task, callback) {
    return new Promise((resolve, reject) => {
      const taskItem = {
        reject,
        resolve,
        callback,
        processor: () => Promise.resolve().then(() => task())
      };

      this.queue.push(taskItem);
      this.schedule();
    });
  }

  schedule() {
    while (this.runningTask < this.parallelism && this.queue.length) {
      this.runningTask++;
      const taskItem = this.queue.shift();
      const { processor, resolve, reject, callback } = taskItem;

      processor()
        .then(res => {
          resolve && resolve(res);
          callback && callback(null, res);
        })
        .catch(error => {
          reject && reject(error);
          callback && callback(error, null);
        })
        .finally(() => {
          this.runningTask--;
          this.schedule();
        });
    }
  }
}

三、测试

---

const scheduler = new Scheduler(2);

function request(timeout) {
  return new Promise(resolve => {
    setTimeout(() => {
      resolve(timeout);
    }, timeout);
  });
}

function addTask(timeout) {
  scheduler
    .add(
      () => request(timeout),
      (error, result) => {
        console.log('result', result);
        if (result === 3000) {
          console.timeEnd('timer');
        }
      }
    )
    .then(result => {
      // console.log('result', result);
      // if (result === 3000) {
      //   console.timeEnd('timer');
      // }
    });
}

console.time('timer');
addTask(1000);
addTask(2000);
addTask(3000);
addTask(4000);