UILabel居中显示的方法

1  @IBOutlet weak var myView: UIView!

        var label = UILabel()
        label.text = "你好，朋友！"
        label.backgroundColor = UIColor.blueColor()
        label.sizeToFit()
        label.center = CGPointMake(CGRectGetMidX(myView.bounds), CGRectGetMidY(myView.bounds))
        myView.addSubview(label)

1         var label = UILabel()
2         label.text = "你好，朋友！"
3         label.backgroundColor = UIColor.blueColor()
4         label.sizeToFit()
5         label.center = myView.convertPoint(myView.center, fromView: myView.superview)
6         myView.addSubview(label)

 1     func setAlignCenter(subView: UIView,superView:UIView) {
 2         subView.setTranslatesAutoresizingMaskIntoConstraints(false)
 3         var dic = ["superView":superView, "subView":subView]
 4         superView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat(
 5             "V:[superView]-(<=0)-[subView]",
 6             options: NSLayoutFormatOptions.AlignAllCenterX,
 7             metrics: nil,
 8             views: dic))
 9         superView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat(
10             "H:[superView]-(<=0)-[subView]",
11             options: NSLayoutFormatOptions.AlignAllCenterY,
12             metrics: nil,
13             views: dic))
14     }