SOURCE

class PromisePool {
    constructor(max, fn) {
        this.max = max; //最大并发量
        this.fn = fn; //自定义的请求函数
        this.pool = []; //并发池
        this.urls = []; //剩余的请求地址
    }
    start(urls) {
        this.urls = urls; //先循环把并发池塞满
        while (this.pool.length < this.max) {
            let url = this.urls.shift();
            this.setTask(url);
        }
        //利用Promise.race方法来获得并发池中某任务完成的信号
        let race = Promise.race(this.pool);
        return this.run(race);
    }
    run(race) {
        race
            .then(res => {
                //每当并发池跑完一个任务，就再塞入一个任务
                let url = this.urls.shift();
                this.setTask(url);
                return this.run(Promise.race(this.pool));
            })
    }
    setTask(url) {
        if (!url) return
        let task = this.fn(url);
        this.pool.push(task); //将该任务推入pool并发池中
        console.log(`${url} 开始，当前并发数：${this.pool.length}`)
        task.then(res => {
            //请求结束后将该Promise任务从并发池中移除
            this.pool.splice(this.pool.indexOf(task), 1);
            console.log(`${url} 结束，当前并发数：${this.pool.length}`);
        })
    }
}
//test
const URLS = [
    'bytedance.com',
    'tencent.com',
    'alibaba.com',
    'microsoft.com',
    'apple.com',
    'hulu.com',
    'amazon.com'
]
//自定义请求函数
var requestFn = url => {
    return new Promise(resolve => {
        setTimeout(() => {
            resolve(`任务${url}完成`)
        }, 1000)
    }).then(res => {
        console.log('外部逻辑', res);
    })
}
const pool = new PromisePool(5, requestFn); //并发数为5
pool.start(URLS)