Problems with Swift Declaration - ios

Question

I just ask myself why I can't do something like this directly under my Class Declaration in Swift:
let width = 200.0
let height = 30.0
let widthheight = width-height
I can not create a constant with 2 other constants. If I use this inside a function/method everything works fine.
Thanks

Answer 1

When you write let widthheight = width - height, this implicitly means let widthheight = self.width - self.height. In Swift, you're simply not allowed to use self until all its members have been initialised — here, including widthheight.
You have a little bit more flexibility in an init method, though, where you can write things like this:
class Rect {
let width = 200.0
let height = 30.0
let widthheight: Double
let widthheightInverse: Double
init() {
// widthheightInverse = 1.0 / widthheight // widthheight not usable yet
widthheight = width - height
widthheightInverse = 1.0 / widthheight // works
}
}

Answer 2

This is a candidate for a computed property as such:
class Foo {
let width = 200.0
let height = 30.0
var widthheight : Double { return width - height }
}
You might raise an issue of 'but it is computed each time'; perhaps your application will depend on a single subtraction done repeatedly - but not likely. If the subtraction is an issue, set widthheight in init()

Answer 3

For things like that, you could make use of class variables. The code would look like this:
class var width = 200.0
class var height = 30.0
class var widthheight = width - height
But when you try it, you will see a compiler error:
Class variables are not yet supported
I guess they haven't implemented that feature yet. But there is a solution for now. Just move your declarations outside the class declaration, like following:
let width = 200.0
let height = 30.0
let widthheight = width - height
class YourClass { ...