将从另一个协议继承的协议设置为关联类型会产生错误

Question

I'm trying to learn protocols and associatedtypes. I have couple of protocols which declare associatedtypes, starting with:

protocol MasterViewModel {
    associatedtype Item: AWMediaItem
...
}

AWMediaItem is another protocol

protocol AWMediaItem {
    var name: String { get }
    var source: AWMediaSource { get }
}

And AWAlbum is yet another protocol which inherits from AWMediaItem

protocol AWAlbum: AWMediaItem {
    var albumName: String { get }
...
}

For some reason, in a class implementing MasterViewModel protocol, I cannot set the AWAlbum to be the Item .

final class AlbumsMasterViewModel: MasterViewModel {
    typealias Item = AWAlbum // Error
...
}

The warning I get is

1. Possibly intended match 'AlbumsMasterViewModel.Item' (aka 'AWAlbum') does not conform to 'AWMediaItem'

If I understand correctly, all AWAlbum's will implement AWMediaItem so why is this not working?

Answer 1

I think you meant to write

final class AlbumsMasterViewModel<Item: AWAlbum>: MasterViewModel {
    
}

I assume that when you write:

    typealias Item = AWAlbum // Error

you want AlbumsMasterViewModel 's item to conform to your AWAlbum protocol but you just create a typelias meaning that Item is just an alias for AWAlbum .

If you want to use a type alias you need a concrete type conforming to AWMediaItem , not a protocol inheriting from it. eg:

class ConcreteAlbum: AWAlbum {
    var albumName: String
    var name: String
    var source: AWMediaSource
    ...    
}

final class AlbumsMasterViewModel: MasterViewModel {
    typealias Item = ConcreteAlbum // No Error
}

Edit

if you want to use AlbumsMasterViewModel with multiple Item types you can also declare it that way:

final class AlbumsMasterViewModel<Item: AWMediaItem>: MasterViewModel {
    
}