Using array of protocol in Swift class' generics

Question

Is there any way to use array of protocol's generic? For example,

/* I want to use protocol like below, 
 * but I can't because protocol is not concrete 
 * so cannot make array of it */
class MyClass<T where T:MyProtocol, T:[MyProtocol]> {
    let result: T
}

protocol MyProtocol {
    init(with: String)
}

class SpecialThing: MyProtocol {
    let appleWatch: AppleWatch

    init(with: String) {
        self.appleWatch = AppleWatch(with)
    }
}

class SampleClass {
    func test {
        typealias listCallback = (MyClass<[SpecialThing]>, NSError) -> ()
        typealias oneCallback = (MyClass<SpecialThing>, NSError) -> ()
    }
}

There can be one object or array of protocol's subclass. I think "typealias" does not help me.

I want to find something more simple way.....

I don't think it is possible to have the generic type T specified as MyProtocol and array of MyProtocol at the same time. — Lachezar
– Lachezar, Commented Mar 29, 2016 at 13:52
I think so. I want to initialize generic object in MyClass. Researched everything but still cannot find solution. — nullmari
– nullmari, Commented Mar 29, 2016 at 14:11
Not possible far as I know. Pretty obvious but the only "solution" I know is class MyClass<T where T:MyProtocol> { let result: [T] } — Will Glück
– Will Glück, Commented Mar 29, 2016 at 14:28
@WillGlück I already solved my problem as exactly what you said. Just I was wondering there is another better way to do this. Thanks anyway! — nullmari
– nullmari, Commented Mar 30, 2016 at 2:20

Good Doug · Accepted Answer · 2016-03-29 14:35:23Z

1

My first issue with this is that the type signature is wrong:

class MyClass<T where T:MyProtocol, T:[MyProtocol]>

That's the same type of thing as doing:

let t: String
let t: [String]
t = String("foo")

The compiler will complain because you are redefining T, once as a MyProtocol and again as an array of MyProtocol. You can't have both, you can only have one.

answered Mar 29, 2016 at 14:35

Good Doug

2,07215 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Vatsal · Accepted Answer · 2016-03-29 14:46:08Z

0

Answer: Use a construct like Either:

enum Either<T, U>
{
    case Left(T)
    case Right(U)

    var leftValue: T?
    {
        if case .Left(let leftValue) = self
        {
            return leftValue
        }

        return nil
    }

    var rightValue: U?
    {
        if case .Right(let rightValue) = self
        {
            return rightValue
        }

        return nil
    }
}

Allowing for:

class MyClass<T: MyProtocol>
{
    let result: Either<T, [MyProtocol]>
}

answered Mar 29, 2016 at 14:46

Vatsal

18.3k11 gold badges48 silver badges83 bronze badges

Collectives™ on Stack Overflow

Using array of protocol in Swift class' generics

2 Answers 2

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related