看着自己写的并发控制，虽然有效果，但。。。太蹩脚了。。。自己都想笑 ^^ ^^ ,你们有什么好的方法让我借鉴一下？

        downloadList.push(url);
        downloadDir.push(dir);
        console.log("待下载数量："+downloadList.length);  
var down=function(){
    console.log("正在下载数量"+streamNumber);
    while(downloadList.length&&(streamNumber<streamLimit)){
        streamNumber++;
        var url=downloadList.pop();
        download(url,downloadDir.pop(),Math.floor(Math.random()*100000) + url.substr(-4,4),function(err){
            if(err){
                console.log(err);
            }
            streamNumber--;
            console.log("下载了"+(downloadAll++)+"张图片");
        });
    }
}

var download = function(url, dir, filename,cb){
        var stream=fs.createWriteStream(dir + "/" + filename);
        stream.on("error",function(err){
            if(err){
                cb(err+"ssssssssssssssssss");
            }
        });     
        var r=request.get({
          url: url,
          headers: {
            'referer': 'http://www.dgtle.com/'
          }
        });
        r.on("error",function(err){
            if(err){
                cb(err+"rrrrrrrrrrrrrrrrrrr");
            }
        });
        r.pipe(stream);
        r.on("end",function(){      
            cb(null)
        });
};


setInterval(function(){
    console.log("--------------------");
    down();
},1000);