UI组件：AlertView警告

1.定义UIAlertView类

2.实现委托

let stopDownload =UIAlertView(title: "警告", message:"正在下载，您确定要取消么？", delegate:self,cancelButtonTitle: "否", otherButtonTitles:"是")

stopDownload.show()

func alertView(alertView:UIAlertView, clickedButtonAtIndex buttonIndex:Int) {

        //根据buttonIndex进行判断

}