js请求并发控制

const delay = function (time) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      console.log(time);
      resolve(time)
    }, time);
  })
}

let tasks = [
  () => delay(1001),
  () => delay(1002),
  () => delay(1005),
  () => delay(1004),
  () => delay(1007),
  () => delay(1003),
  () => delay(1008)
]

// Promise.all(tasks.map(item => item())).then(res => {
//   console.log(res);
// })

// function createRequest(tasks, limit = 3) {
//   let index = 0,
//     results = [],// 存放结果
//     together = new Array(limit).fill(null);//创建工作区
//   together = together.map(_ => {
//     return new Promise((resolve, reject) => {
//       const fun = function () {
//         if (index >= tasks.length) {
//           resolve(results)
//           return
//         }
//         let oldIndex = index
//         const task = tasks[index++]
//         task().then(res => {
//           results[oldIndex] = res
//           fun()
//         }).catch(reason => reject(reason))
//       }
//       fun()
//     })
//   })
//   return Promise.all(together).then(() => results, reason => Promise.reject(reason))
// }
// createRequest(tasks, 2).then(res => {
//   console.log(res);
// }).catch(reason => {
//   console.log(reason);
// })


function createRequest(tasks, limit, cb) {
  class TaskQueue {
    // 执行中的任务个数
    running = 0;
    // 存放任务的工作区
    queue = [];
    // 存放结果
    results = []

    pushTask(task) {
      let self = this
      self.queue.push(task)
      self.next()
    }
    next() {
      let self = this
      // 执行中任务个数小于并发数量并且工作区有值继续拿任务执行
      while (self.running < limit && self.queue.length) {
        self.running++
        let task = self.queue.shift()
        task().then(res => {
          self.results.push(res)
        }).finally(() => {
          // 不论失败成功继续拿任务执行
          self.running--
          self.next()
        })
      }
      // 执行中任务个数=0结束
      if (self.running === 0) {
        cb(self.results)
      }
    }
  }
  let tq = new TaskQueue()
  tasks.forEach(task => tq.pushTask(task))
}

createRequest(tasks, 3, results => {
  console.log(results);
})