Robert Nikander wrote:
Cc:"swift-users@swift.org" <swift-users@swift.org>
Subject: Re: [swift-users] wishing I could cast (sort of) to protocol
with associated type
Message-ID:<bf76dee9-aca5-4a74-a4f0-febb84a42...@apple.com>
Content-Type: text/plain; charset="utf-8"
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 }
}
One other option may be to put a typealias in an extension instead of
declaring an associated type:
// The almost-a-generic protocol:
protocol AssProtocol {
var xxx: AssType { get }
}
extension AssProtocol {
typealias AssType = Any
}
// My concrete classes:
class MyClass1: AssProtocol {
typealias AssType = Int
var xxx: AssProtocol.AssType { return 1 }
}
class MyClass2: AssProtocol {
typealias AssType = String
var xxx: AssProtocol.AssType { return "two" }
}
// A generic function that uses the aforementioned protocol:
func doit<T>(_ v: T) where T: AssProtocol.AssType {
print(v, type(of: v))
}
// The rare sweet joy that is 0 compiler errors AND 0 runtime crashes:
let res1 = MyClass1().xxx
let res2 = MyClass2().xxx
doit(res1) // 1 Int.Type
doit(res2) // two String.Type
Needless to say my head is screeching "probable undefined behavior" even
as I type this, so caveat emptor, E&OE, don't blame me when it eats your
cat and marries your wife, etc. [1] But it did finally get me out of an
unspeakably intractable problem with the Swift type system, thus
preserving what precious few last shreds of sanity I still possess. Who
knows, now that I only have a myriad of all-but-intractable problems
left to work through, one day I might even release!
Hope this helps (or at least dies mercifully quickly on you),
has
[1] Blame this guy, 'cos he's who I got it from:
https://medium.com/@sunshinejr/what-do-you-really-know-about-typealias-and-associatedtype-5ba25d45848e#.wqpume3n1
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