Swift protocols with generic collections-CodePudding

I want to have a protocol that contains a collection of Item. The goal is for it to not matter if the collection of items is an array/set/etc, as long as it conforms to Collection.

protocol Content {
    var items: Collection<Item> // This doesn't work, swift doesn't support generic protocol parameters
}

An example use case, where I use Realm to persist Content, but would like to be able to mock data without needing to import Realm:

// Typical usage with Alamofire and Realm

struct RealmArticle: Object, Content {
   var items = List<Item>()
}


func fetchContent(completion: (Content?, Error) -> ())) {
   session.request(...).responseDecodable { (content: RealmArticle) in 
      completion(content, nil)
   }
}

// Mock without needing to include Realm

func mockFetchContent(completion: (Content?, Error) -> ()) {
   completion(MockArticle(items: (0...5).map { Item() }), nil)
}

struct MockArticle: Content {
    var items: [Item]
}

My current solution is to use an array in the protocol, and modify the realm model:

protocol Content {
    var items: [Item]
}

struct RealmArticle: Object, Content {
   var _items = List<Item>()

   var items: [Item] { Array(_items) }
}

But this isn't ideal because it loads the entire List into memory whenever items is accessed. Is there a better solution or should I take a different approach?

CodePudding user response：

If you can live without the requirement that the element of the collection must be Item then you can use associatedtype to make it work with Collection

protocol Content {
    associatedtype T: Collection
    var items: T { get set }
}

This should be enough to be able to mock as you want.

CodePudding user response：

Why do you need a stand-alone protocol when you could just make this method generic?

func fetchContent<C: Collection>(completion: (Result<C, Error>) -> Void) where C.Iterator.Element == Object {
   …
   }