I'm experimenting with Swift protocol extensions and I found this quite confusing behaviour. Could you help me how to get the result I want?
See the comments on the last 4 lines of the code. (You can copy paste it to Xcode7 playground if you want). Thank you!!
protocol Color { }
extension Color { var color : String { return "Default color" } }
protocol RedColor: Color { }
extension RedColor { var color : String { return "Red color" } }
protocol PrintColor {
func getColor() -> String
}
extension PrintColor where Self: Color {
func getColor() -> String {
return color
}
}
class A: Color, PrintColor { }
class B: A, RedColor { }
let colorA = A().color // is "Default color" - OK
let colorB = B().color // is "Red color" - OK
let a = A().getColor() // is "Default color" - OK
let b = B().getColor() // is "Default color" BUT I want it to be "Red color"
The short answer is that protocol extensions don't do class polymorphism. This makes a certain sense, because a protocol can be adopted by a struct or enum, and because we wouldn't want the mere adoption of a protocol to introduce dynamic dispatch where it isn't necessary.
Thus, in getColor()
, the color
instance variable (which may be more accurately written as self.color
) doesn't mean what you think it does, because you are thinking class-polymorphically and the protocol is not. So this works:
let colorB = B().color // is "Red color" - OK
...because you are asking a class to resolve color
, but this doesn't do what you expect:
let b = B().getColor() // is "Default color" BUT I want it to be "Red color"
...because the getColor
method is defined entirely in a protocol extension. You can fix the problem by redefining getColor
in B:
class B: A, RedColor {
func getColor() -> String {
return self.color
}
}
Now the class's getColor
is called, and it has a polymorphic idea of what self
is.