带有并发限制的异步调度器

tip

考察event loop的高级用法

一、题意

JS实现一个带有并发限制的异步调度器Scheduler，保证同时运行的任务最多有十个，完善代码中Scheduler类：

Class Scheduler{
    constructor(max){}
    run(callback){/*...*/}
}

let s = new Scheduler(10);
s.run(async () => {
  /* some async operations */
});
s.run(async () => {
  /* some async operations */
});
s.run(async () => {
  /* some async operations */
});

二、解法

Class Scheduler{
    COUNT = 0; // 记录调试器接收的执行任务个数
    LIST = []; // 记录resolve方法
    constructor(max){
        this.LIMIT = max;
    }
    async run(callback){
        if(this.COUNT >= this.LIMIT){
            // 通过await，只要不resolve，代码运行就会阻塞在这里
            await new Promise((resolve) => {
                this.LIST.push(resolve);
            });
            
        }
        this.COUNT++;

        const result = await callback();

        this.COUNT--;
        
        if(this.LIST.length > 0){
            this.LIST.shift()(); // 解锁代码阻塞 (注意是双括号，目的是执行resolve())
        }
        
        return result;
    }
}

let s = new Scheduler(10);
s.run(async () => {
  /* some async operations */
});
s.run(async () => {
  /* some async operations */
});
s.run(async () => {
  /* some async operations */
});