Swift - implementing repository pattern with generics - swift

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.