前端面试题之promise的手动封装（Promise面试题详解之控制并发）

var urls = [
'https://www.kkkk1000.com/images/getImgData/getImgDatadata.jpg" alt="前端面试题之promise的手动封装（Promise面试题详解之控制并发）" border="0" />

//省略代码

var count = 0;
//对加载图片的函数做处理，计数器叠加计数
function bao(){
    count++;
    console.log("并发数:",count)
    //条件判断，urls长度大于0继续，小于等于零说明图片加载完成
    if(urls.length>0&&count<=3){
    //shift从数组中取出连接
        loadImg(urls.shift()).then(()=>{
        //计数器递减
            count--
            //递归调用
            }).then(bao)
    }
}
function async1(){
//循环开启三次
    for(var i=0;i<3;i++){
        bao();
    }
}
async1()

var count = 0;

// 封装请求的异步函数,增加计数器功能
function request(){
    count++;
    loadImg(urls.shift()).then(()=>{
            count--
            }).then(diaodu)

    
}
// 负责调度的函数
function diaodu(){
    if(urls.length>0&&count<=3){
        request();
    }
}

function async1(){
    for(var i=0;i<3;i++){
        request();
    }
}
async1()

//省略代码

// 计数器
var count = 0;
// 全局锁
var lock = [];
var l = urls.length;
async function bao(){
    if(count>=3){
        //超过限制利用await和promise进行阻塞;
        let _resolve;
        await new Promise((resolve,reject)=>{
            _resolve=resolve;
            // resolve不执行,将其推入lock数组;
            lock.push(_resolve);
        });
    }
    if(urls.length>0){
        console.log(count);
        count++
        await loadImg(urls.shift());
        count--;
        lock.length&&lock.shift()()
    }
}
for (let i = 0; i < l; i++) {
    bao();
}

// 计数器
var count = 0;
// 全局锁
var lock = [];
var l = urls.length;
// 阻塞函数
function block(){
    let _resolve;
    return  new Promise((resolve,reject)=>{
        _resolve=resolve;
        // resolve不执行,将其推入lock数组;
        lock.push(_resolve);
    });
}
// 叫号机
function next(){
    lock.length&&lock.shift()()
}
async function bao(){
    if(count>=3){
        //超过限制利用await和promise进行阻塞;
        await block();
    }
    if(urls.length>0){
        console.log(count);
        count++
        await loadImg(urls.shift());
        count--;
        next()
    }
}
for (let i = 0; i < l; i++) {
    bao();
}

//省略代码
function limitLoad(urls, handler, limit) {
    // 对数组做一个拷贝
    const sequence = [].concat(urls)
    let promises = [];

    //并发请求到最大数
    promises = sequence.splice(0, limit).map((url, index) => {
        // 这里返回的 index 是任务在 promises 的脚标，
        //用于在 Promise.race 之后找到完成的任务脚标
        return handler(url).then(() => {
            return index
        });
    });

    (async function loop() {
        let p = Promise.race(promises);
        for (let i = 0; i < sequence.length; i++) {
            p = p.then((res) => {
                promises[res] = handler(sequence[i]).then(() => {
                    return res
                });
                return Promise.race(promises)
            })
        }
    })()
}
limitLoad(urls, loadImg, 3)