In Swift, how to cast to protocol with associated type?
[Edited to fix: : SpecialValue
, not = SpecialValue
]
This is not possible. SpecialValueController
is an "incomplete type" conceptually so the compiler cannot know. SpecialValueType
, although it is constrained by SpecialValue
, it is not known until it is determined by any adopting class. So it is a really placeholder with inadequate information. as?
-ness cannot be checked.
You could have a base class that adopts SpecialController
with a concrete type for SpecialValueController
, and have multiple child classes that inherit from the adopting class, if you're still seeking a degree of polymorphism.
Unfortunately, Swift doesn't currently support the use of protocols with associated types as actual types. This however is technically possible for the compiler to do; and it may well be implemented in a future version of the language.
A simple solution in your case is to define a 'shadow protocol' that SpecialController
derives from, and allows you to access currentValue
through a protocol requirement that type erases it:
// This assumes SpecialValue doesn't have associated types – if it does, you can
// repeat the same logic by adding TypeErasedSpecialValue, and then using that.
protocol SpecialValue {
// ...
}
protocol TypeErasedSpecialController {
var typeErasedCurrentValue: SpecialValue? { get }
}
protocol SpecialController : TypeErasedSpecialController {
associatedtype SpecialValueType : SpecialValue
var currentValue: SpecialValueType? { get }
}
extension SpecialController {
var typeErasedCurrentValue: SpecialValue? { return currentValue }
}
extension String : SpecialValue {}
struct S : SpecialController {
var currentValue: String?
}
var x: Any = S(currentValue: "Hello World!")
if let sc = x as? TypeErasedSpecialController {
print(sc.typeErasedCurrentValue as Any) // Optional("Hello World!")
}