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I'm struggling with protocols in Swift. I have defined a protocol like this:

protocol AProtocol {
    var property : BProtocol {get set}
}

And I would like to conform to AProtocol in a class with a property that also conform to another protocol. I've tried in these two ways:

class AClass: AProtocol {
   var property = BClass()
}

and: 

class AClass: AProtocol {
   var property: BProtocol & MyBClassType = BProtocol()
}

but none of them seems to work (BClass itself confirm to BProtocol) This issue is a bit difficult to explain, I hope it was clear.

Is it a limitation of the Swift language? Do you know a work around to this?

Martijn Pieters
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Alberto Scampini
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  • See https://stackoverflow.com/q/40410884/2976878 – because `AProperty` is both gettable and settable, allowing a conforming implementation to use any type other than `BProtocol` would be unsound. – Hamish Oct 25 '18 at 15:05

1 Answers1

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You have two issues: firstly, the property name must match that declared in the protocol, secondly you need to type annotate the variable to be of type BProtocol as Hamish explained in the comment.

protocol AProtocol {
    var aProperty : BProtocol {get set}
}

protocol BProtocol {}
class BClass: BProtocol {}

class AClass: AProtocol {
    var aProperty: BProtocol = BClass()
}

You should also conform to the Swift naming convention, which is lowerCamelCase for variable names, so I changed AProperty to its correct form, aProperty.

Dávid Pásztor
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  • Thanks for the answare :) ... the problem is that if I set " aProperty: BProtocol = BClass() " than I cannot access to the class as BClass type but only as BProtocol without casting it all the time inside AClass – Alberto Scampini Oct 25 '18 at 15:14
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    @AlbertoScampini you can conditionally cast the property to test if its also of type `BClass`, but there's no way to declare it as `BClass` and still conform to `AProtocol` – Dávid Pásztor Oct 25 '18 at 15:16