promise并发

class Scheduler{
    //正在执行的顺序
    constructor(max){
        //当前正在执行的任务个数
        this._count = 0;
        //最大上限数
        this._max = max;
        this._taskQueue = [];
    }

    add(promiseCreator){
        const task = this._createTask(promiseCreator);
        if(this._count >= this._max){//正在执行任务已满，放入任务队列中
            this._taskQueue.push(task);
        }else{
            task();//立即执行
        }
    }
    _createTask(promiseCreator){
        return ()=>{//返回一个函数
            promiseCreator().then(()=> {//立刻执行promiseCreator
                    this._count--;
                    if (this._taskQueue.length) {
                        let task = this._taskQueue.shift();
                        task();
                    }
                }
            );
            this._count++;
        }
    }

}
const timeout = (time) => new Promise(resolve => {
    setTimeout(resolve,time);
});
const scheduler = new Scheduler(2);

const addTask = (time,order)=>{
    scheduler.add(()=>timeout(time).then(()=>console.log(order)));
};
addTask(1000,"1");
addTask(500,"2");
addTask(300,"3");
addTask(400,"4");