便利构造器

类方法

extension UIBarButtonItem {
     class func createItem(imageName:String,highImageName : String,size : CGSize) -> UIBarButtonItem {
     let btn = UIButton()
     btn.setImage(UIImage.init(named: imageName), for: .normal)
     btn.setImage(UIImage.init(named: highImageName), for: .highlighted)
     btn.frame = CGRect(origin: CGPoint(), size: size)
     return UIBarButtonItem(customView: btn);
     }

• 调用方式

 let size = CGSize(width: 40, height: 40)
 let historyItem = UIBarButtonItem.createItem(imageName: "image_my_history", highImageName: "Image_my_history_click", size: size)

构造方法

• 便利构造函数（推荐使用）
  1. convenience开头
  2. 在构造函数中必须明确调用一个设计的构造函数（self调用）

 convenience init(imageName:String,highImageName : String = "",size : CGSize) {
        let btn = UIButton()
        btn.setImage(UIImage.init(named: imageName), for: .normal)
        if highImageName != "" {
            /// 设置默认值
            btn.setImage(UIImage.init(named: highImageName), for: .highlighted)
        }
        btn.frame = CGRect(origin: CGPoint(), size: size)
        self.init(customView:btn)
    }

• 调用方式

 let size = CGSize(width: 40, height: 40)
 let historyItem = UIBarButtonItem(imageName: "image_my_history", highImageName: "Image_my_history_click", size: size)