swfit扩展

//扩展，typealias别名 , mutating 修改了成员变量就用这个修饰
struct Point{
    var x = 0.0
    var y = 0.0
}

struct Size {
    var width = 0.0
    var height = 0.0
    
    mutating func push(){
        width = 2
    }
}


class Rectangle {
    var origin = Point()
    var size = Size()
    
    init(origin:Point,size:Size){
        self.origin = origin
        self.size = size
    }
}
extension Rectangle{
    func translate(x x:Double,y:Double){
        self.origin.x += x
        self.origin.y += y
    }
    //便利的构造函数,必须用self.init 才行
    convenience init(center:Point,size:Size){
        self.init(origin:center,size: size)
    }
}

let rect = Rectangle(origin: Point(), size: Size(width: 4, height: 3))
rect.translate(x: 1, y: 2)