摘自https://juejin.im/post/5d4a1d30f265da03f564cbf1
class Limit {
constructor (n) {
this.limit = n
this.count = 0
this.queue = []
}
enqueue (fn) {
// 关键代码: fn, resolve, reject 统一管理
return new Promise((resolve, reject) => {
this.queue.push({ fn, resolve, reject })
})
}
dequeue () {
if (this.count < this.limit && this.queue.length) {
// 等到 Promise 计数器小于阈值时，则出队执行
const { fn, resolve, reject } = this.queue.shift()
this.run(fn).then(resolve).catch(reject)
}
}
// async/await 简化错误处理
async run (fn) {
this.count++
// 维护一个计数器
const value = await fn()
this.count--
// 执行完，看看队列有东西没
this.dequeue()
return value
}
build (fn) {
if (this.count < this.limit) {
// 如果没有到达阈值，直接执行
return this.run(fn)
} else {
// 如果超出阈值，则先扔到队列中，等待有空闲时执行
return this.enqueue(fn)
}
}
}
Promise.map = function (list, fn, { concurrency }) {
const limit = new Limit(concurrency)
return Promise.all(list.map((...args) => {
return limit.build(() => fn(...args))
}))
}
