func setTitle(_ title: String?,for state: UIControlState) // Use this method to set the title for the button

例：

btn.setTitle(title: "Title",for: .normal)

注意btn控件的默认状态更改为.Normal到.normal.

// prevIoUs 
public static var Normal: UIControlState { get }
 
// Swift 3.0
static var normal: UIControlState { get }