在oc中为了增强已有类的功能，我们经常使用分类。使用分类，我们可以在不破坏原有类的结构的前提下，对原有类进行模块化的扩展。

但是在swift中没有分类这种写法了。相对应的是swift中只有扩展(Extensions)。

下面是swift中扩展(Extensions)的说明

扩展就是向一个已有的类、结构体、枚举类型或者协议类型添加新功能（functionality）。这包括在没有权限获取原始源代码的情况下扩展类型的能力（即逆向建模）。扩展和 Objective-C 中的分类（categories）类似。（不过与 Objective-C 不同的是，Swift 的扩展没有名字。）

下面是swift实现对UIView的分类：

extension UIView {
    //x
    var x:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.origin.x = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin.x
        }
    }
    //y
    var y:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.origin.y = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin.y
        }
    }
    //height
    var height:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.size.height = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.size.height
        }
    }
    //width
    var width:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.size.width = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.size.width
        }
    }
    
    //top
    var top:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.origin.y = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin.y
        }
    }
    //left
    var left:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.origin.x = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin.x
        }
    }
    //bottom
    var bottom:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.origin.y = newValue - oldFrame.size.height;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin.y + self.frame.size.height
        }
    }
    //right
    var right:CGFloat {
        set{
            var oldFrame = self.frame
            oldFrame.origin.x = newValue+oldFrame.size.width;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin.x + self.frame.size.width
        }
    }
    //size
    var size:CGSize {
        set{
            var oldFrame = self.frame
            oldFrame.size = newValue
            self.frame = oldFrame
        }
        get{
            return self.frame.size
        }
    }
    //origin
    var origin: CGPoint{
        set{
            var oldFrame = self.frame
            oldFrame.origin = newValue;
            self.frame = oldFrame
        }
        get{
            return self.frame.origin
        }
    }
}