The only real way to do this today is to have two layers of protocol:
protocol SpecialControllerBase {
var currentValueBase: SpecialValue? { get }
}
protocol SpecialController: SpecialControllerBase {
associatedtype SpecialValueType : SpecialValue
var currentValue: SpecialValueType? { get }
}
extension SpecialController {
var currentValueBase: SpecialValue? { return self.currentValue }
}
Supporting this natively is the feature called generalized existentials,
described in the “Generics Manifesto
<https://github.com/apple/swift/blob/master/docs/GenericsManifesto.md#generalized-existentials>”
of potential future Swift features. This has a lot of design and
implementation considerations, so it’s not planned to happen right away, but
it’s definitely a heavily-requested feature.
Jordan
> On Nov 2, 2016, at 12:31, Robert Nikander via swift-users
> <swift-users@swift.org> wrote:
>
> Hi,
>
> In the following code, I want to test if x is a `SpecialController`. If it
> is, I want to get the `currentValue` as a `SpecialValue`. How do you do this?
> If not with a cast, then some other technique.
>
> I understand the error, and that SpecialController by itself is not a simple
> type to cast to. But it seems like what I’m saying is logically consistent
> and not that complicated. Is there really no way to *say* it in Swift?
>
> protocol SpecialController {
> associated type SpecialValueType : SpecialValue
> var currentValue: SpecialValueType? { get }
> }
> ...
> var x: AnyObject = ...
> if let sc = x as? SpecialController { // does not compile
>
> Rob
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