iOS Swift: Protocol and Types

Question

I'm having problems writing a protocol and doing a default implementation of it.

The protocol is RealmServiceCompatible and the Object should conform on it. In the extension RealmServiceCompatible I gave a default implementation of it, but still when I'm conforming Object to it, Xcdoe still asks me to compile protocol stubs..

I can't figure the correct syntax.

protocol RealmServiceCompatible {

    associatedtype CompatibleType

    var rs: RealmService<CompatibleType> { get }
    static var rs: RealmServiceStatic<CompatibleType> { get }
}

extension RealmServiceCompatible {

    public var rs: RealmService<Self> {
        get {
            return RealmService(self)
        }
    }
    public static var rs: RealmServiceStatic<Self> {
        get {
            return RealmServiceStatic(Self.self)
        }
    }
}


extension Object: RealmServiceCompatible {

    var rs: RealmService<NSObject> {
        <#code#>
    }


    static var rs: RealmServiceStatic<NSObject> {
        <#code#>
    }

}

Xcode has accepted this:

protocol RealmServiceCompatible: AnyObject where CompatibleType: Object {

    associatedtype CompatibleType

    var rs: RealmService<CompatibleType> { get }
    static var rs: RealmServiceStatic<CompatibleType> { get }
}

extension RealmServiceCompatible {

    var rs: RealmService<Self> {
            return RealmService(self)
    }
    static var rs: RealmServiceStatic<Self> {
            return RealmServiceStatic(Self.self)
    }
}


extension Object: RealmServiceCompatible {
    typealias CompatibleType = Object
}

Answer 1

You didn't provide a default implementation that applies:

public var rs: RealmService<Self> { ... }

This is only relevant if CompatibleType == Self (the type of the class that is implementing the protocol). But your protocol applies over all possible CompatibleType values.

That said, last time I checked, you can't do this in any case (even if you made sure to handle all possible CompatibleType values), since you get in a circular definition. The default implementation only applies to things that already conform, but they don't conform unless they have implementations. In principle, it might be possible for a type checker to take a step back and say (as humans do), "let's just pretend it does conform, even though it doesn't yet, and see if by pretending to conform it actually magically does," but I don't think the type checker does that today. (That's actually a pretty hard problem in the general case and would likely increase compile times quite a bit to chase all the ways things might work out.)

It's not really clear what you're trying to do here, since RealmServiceCompatible doesn't seem to be pulling any weight. I suspect you're trying to do something like make SomeClass().rs return RealmService<SomeClass> , while SomeSubClass().rs returns RealmService<SomeSubClass> , but that's not possible in Swift. RealmService<SomeSubClass> is not a subtype of RealmService<SomeClass> because Swift lacks a way to express covariance, so this would break inheritance.

It feels like you're trying to make something magical here that isn't going to work.