快速宣言的问题

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 { ...