前端算法题-01

JS实现一个带并发限制的异步调度器Scheduler，保证同时运行的任务最多有两个。完善代码中Scheduler类，使得以下程序能正确输出

class Scheduler {
  constructor() {
  }
}

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

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

addTask(1000, '1');
addTask(500, '2');
addTask(300, '3');
addTask(200, '5');
addTask(400, '4');
// output: 2 3 1 4

// 一开始，1、2两个任务进入队列
// 500ms时，2完成，输出2，任务3进队
// 800ms时，3完成，输出3，任务4进队
// 1000ms时，1完成，输出1
// 1200ms时，4完成，输出4

答案

class Scheduler {
  constructor() {
    this.promises = [];
    this.doingJobs = 0;
  }

  add(promiseCreater) {
    return new Promise(
      (resolve, reject) => {
        promiseCreater.resolve =
          resolve;
        promiseCreater.reject = reject;
        this.promises.push(
          promiseCreater
        );
        this.run();
      }
    );
  }

  run() {
    if (
      this.promises.length &&
      this.doingJobs < 2
    ) {
      const promise =
        this.promises.shift();
      this.doingJobs++;
      promise()
        .then(() => {
          promise.resolve();
        })
        .catch(() => {
          promise.reject();
        })
        .finally(() => {
          this.doingJobs -= 1;
          this.run();
        });
    }
  }
}

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

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

addTask(1000, '1');
addTask(500, '2');
addTask(300, '3');
addTask(200, '5');
addTask(400, '4');