Swift - implementing repository pattern with generics

Question

I have been doing some research into repository pattern as want to use it new project I am working on. However I am having issues getting it working with generics

I have been following this guide here as an example

https://medium.com/@frederikjacques/repository-design-pattern-in-swift-952061485aa

Which does a fairly good job of explaining it. However, the guide leaves off one important detail.. which is using dependancy injection with generics.

In example code, he shows this

class ArticleFeedViewModel {
  let articleRepo:ArticleRepository
  init( articleRepo:ArticleRepository = WebArticleRepository() ) {

    self.articleRepo = articleRepo
  }
}

Which works fine if you are not using generics. But once you change ArticleRepository to Repository example... so from

protocol ArticleRepository {
    func getAll() -> [Article]
    func get( identifier:Int ) -> Article?
    func create( article:Article ) -> Bool
    func update( article:Article ) -> Bool
    func delete( article:Article ) -> Bool
}

to this

protocol Repository {

  associatedtype T

  func getAll() -> [T]
  func get( identifier:Int ) -> T?
  func create( a:T ) -> Bool
  func update( a:T ) -> Bool
  func delete( a:T ) -> Bool

}

I can no longer get the dependancy injection working. So If I were to try re-creating the model shown above.

class WebArticleRepository: Repository {
    func getAll() -> [Article] {
        return [Article()]
    }

    func get(identifier: Int) -> Article? {
        return Article()
    }

    func create(a: Article) -> Bool {
        return true
    }

    func update(a: Article) -> Bool {
        return false
    }

    func delete(a: Article) -> Bool {
        return true
    }
}

class ArticleFeedViewModel {
    let articleRepo:Repository
    init( articleRepo:Repository = WebArticleRepository() ) {
        self.articleRepo = articleRepo
    }
}

This no longer works. I now get an error saying

Protocol 'Repository' can only be used as a generic constraint because it has Self or associated type requirements

Any ideas on what I am doing wrong here. It seems adding associatedType causes this to stop working. I would really like to get this functionality working as I want to be able to inject either local or web based repository pattern depending on current state of the app

Any help would be much appriecated

Answer 1

You need to make everything else generic as well:

protocol Repository {

    associatedtype RepositoryType

    func getAll() -> [RepositoryType]
    func get( identifier:Int ) -> RepositoryType?
    func create( a:RepositoryType ) -> Bool
    func update( a:RepositoryType ) -> Bool
    func delete( a:RepositoryType ) -> Bool

}

class WebArticle { }

class WebArticleRepository: Repository {
    typealias RepositoryType = WebArticle

    func getAll() -> [WebArticle] {
        return [WebArticle()]
    }

    func get(identifier: Int) -> WebArticle? {
        return WebArticle()
    }

    func create(a: WebArticle) -> Bool {
        return true
    }

    func update(a: WebArticle) -> Bool {
        return false
    }

    func delete(a: WebArticle) -> Bool {
        return true
    }
}

class ArticleFeedViewModel<T : Repository> {
    let articleRepo: T
    init( articleRepo: T) {

        self.articleRepo = articleRepo
    }
}

// you cannot have the optional parameter in the init, instead, you can extract the following line to a method
ArticleFeedViewModel(articleRepo: WebArticleRepository())

In Swift you can't use a protocol with associated types as the type of a property/parameter etc. It's supposed to make your code more type-safe.