UILabel居中显示的方法

在IB中拖出一个UIView

 @IBOutlet weak var myView: UIView!

下面创建的UILabel是在myView中居中显示

方法1:

        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)

方法2:

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

注意：以上方法中第四行代码label.sizeToFit()务必写到第五行代码之前执行，否则将不会居中显示

方法3:

通过VFL布局约束的方式

    func setAlignCenter(subView: UIView,superView:UIView) {
        subView.setTranslatesAutoresizingMaskIntoConstraints(false)
        var dic = ["superView":superView, "subView":subView]
        superView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat(
            "V:[superView]-(<=0)-[subView]",
            options: NSLayoutFormatOptions.AlignAllCenterX,
            metrics: nil,
            views: dic))
        superView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat(
            "H:[superView]-(<=0)-[subView]",
            options: NSLayoutFormatOptions.AlignAllCenterY,
            metrics: nil,
            views: dic))
    }