有趣又有用的面试题

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。完善下面代码中的 Scheduler 类，使得以下程序能正确输出。

class Scheduler {
  add(promiseCreator) { ... }
  // ...
}

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(400, '4')

代码如下：

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

  add = (promiseCreator) => {
    return new Promise((resolve, reject) => {
      // 关键是给传过来的函数加个回调属性，当resolved的时候，就能返回对应的结果了。
      promiseCreator.resolve = resolve
      promiseCreator.reject = reject
      this.pendingState.push(promiseCreator)
      this.doJob()
    })
    
  }

  doJob = () => {

    if (this.doingJobs < 2) {
      if (this.pendingState.length) {
        this.doingJobs += 1
        const job = this.pendingState.shift()

        job().then(res => {
          job.resolve(res)
        }).catch(e => {
          job.reject(e)
        }).finally(() => {
          this.doingJobs -= 1
          this.doJob()
        })

      }
    }
  }
}