直接对label的cornerRadius进行设置
var text = UILabel(frame: CGRectMake(10,30,UIScreen.mainScreen().bounds.size.width - 20,200))
text.text = "背景为圆形 "
text.backgroundColor = self.view.backgroundColor
text.textColor = UIColor.whiteColor()
text.font = UIFont.systemFontOfSize(22)
text.textAlignment = NSTextAlignment.Center
label.layer.cornerRadius = label1.bounds.size.width/2
self.view.addSubview(text)