swift----UIButton的状态

static var Normal: UIControlState { get }   // 正常状态

    static var Highlighted: UIControlState { get } // used when UIControl isHighlighted is set   按钮按下

    static var Disabled: UIControlState { get } // 不可用

    static var Selected: UIControlState { get } // flag usable by app (see below)

    static var Application: UIControlState { get } // additional flags available for application use

    static var Reserved: UIControlState { get } // flags reserved for internal framework use