Swift4.0 便利构造函数的写法

extension UIButton {
    
    convenience init(composeForeImageName:String,composeBackImageName:String){
        self.init()
        setImage(UIImage.init(named: composeForeImageName), for: .normal)
        setBackgroundImage(UIImage.init(named: composeBackImageName), for: .normal)
        setBackgroundImage(UIImage.init(named: composeBackImageName+"_highlighted"), for: .selected)
        sizeToFit()
    }
    
}

  //MARK:属性 懒加载
    lazy var composeBtn = UIButton(composeForeImageName: "tabbar_compose_icon_add", composeBackImageName: "tabbar_compose_button")