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.origin
rect2.size