好家伙 我这个上浮就跟放风筝一样，只能下移埋地雷

fruitObj.prototype.draw = function(){

    for(var i = 0; i < this.num; i++){

        if (this.alive[i] == true){

            if (this.l[i] < 20){

                this.l[i]+=this.speed[i]*0.3*dateTime;

            }else{

                this.y[i]+=(this.speed[i]*0.4*dateTime);

            }

            ctx2.drawImage(this.orange, this.x[i] - this.l[i]*0.5, this.y[i] - this.l[i]*0.5, this.l[i], this.l[i] )

            // 检测，y坐标改变alive状态

            if (this.y[i] < 20){

                this.alive = false;

            }

        }

    }

}