swift 计算型属性

计算型的属性是通过类的其它属性赋值的过程自动赋值，先看一下代码

struct Size {    var width = 0.0    var height = 0.0}struct Point {    var x = 0.0    var y = 0.0}class Rectangle{    var origin = Point()    var size = Size()    var center: Point {            let centerX = origin.x + size.width/2            let centerY = origin.y + size.height/2            return Point(x: centerX, y: centerY)    }//计算型的属性    init( origin: Point , size: Size){        self.origin = origin;        self.size = size    }}

这里的center属性就是计算型的属性，这个属性通过矩形的x、y、width、height属性赋值后自动赋值

此时给center属性赋值会报错

因为此时center属性是只读的，此时要想赋值，需要重写center属性的Set方法

class Rectangle{    var origin = Point()    var size = Size()    var center: Point {        get{            let centerX = origin.x + size.width/2            let centerY = origin.y + size.height/2            return Point(x: centerX, y: centerY)        }        set(newCenter){            origin.x = newCenter.x - size.width/2            origin.y = newCenter.y - size.height/2        }    }//计算型的属性    init( origin: Point , size: Size){        self.origin = origin;        self.size = size    }}

此时赋值不会报错，并且会把x、y属性值进行修改

Set方法的优化写法，swift定义了好了newValue变量，可以重写Set方法，代码如下

class Rectangle{    var origin = Point()    var size = Size()    var center: Point {        get{            let centerX = origin.x + size.width/2            let centerY = origin.y + size.height/2            return Point(x: centerX, y: centerY)        }        set{            origin.x = newValue.x - size.width/2            origin.y = newValue.y - size.height/2        }    }//计算型的属性    init( origin: Point , size: Size){        self.origin = origin;        self.size = size    }}

苹果官方定义矩形——CGRect

//苹果官方var rect2 = CGRect(origin: CGPoint(x: 0,y: 0),size:CGSize(width: 10,height: 5))rect2.originrect2.size