How to make a dynamic generic? - swift

Question

I'm trying to write a protocol that allows me to version models in my app. In order to do that I wrote the following VersionManager.
class VersionManager<Type: Decodable> {
private var database: Database
init(database: Database) {
self.database = database
}
var versions: [Type] {
return []
}
}
After that I wrote a protocol that I can add to models:
protocol Versionable {
}
extension Versionable {
private var manager: VersionManager<Restaurant> {
return VersionManager<Restaurant>(database: Database.shared)
}
public var versions: [Restaurant] {
return manager.versions
}
}
Now, the problem I'm facing is that I tried passing the type dynamically instead of hardcoded, like I have now for Restaurant.
So I tried changing the protocol to this:
protocol Versionable {
var kind: Decodable.Type { get }
}
Then I wanted to pass kind to VersionManager. However, when I try that Xcode throws this error: Expected '>' to complete generic argument list.
Is there any other way to do this?

Answer 1

If you want to use generics inside a protocol, you need to use an associatedtype
protocol Versionable {
associatedtype Model: Decodable
}
extension Versionable {
private var manager: VersionManager<Model> {
return VersionManager<Model>(database: Database.shared)
}
public var versions: [Model] {
return manager.versions
}
}
The model that is going to implement the Versionable protocol will have to resolve this type:
struct SomeModel: Versionable {
typealias Model = Int
}
SomeModel().versions // [Int]
I'm guessing Restaurant in your example refers to the model that implements Versionable. In that case, you can just use the Self reference inside your protocol extension:
protocol Versionable: Decodable {
}
extension Versionable {
private var manager: VersionManager<Self> {
return VersionManager<Self>(database: Database.shared)
}
public var versions: [Self] {
return manager.versions
}
}
struct SomeModel: Versionable {}
SomeModel().versions // [SomeModel]
Please note that the Versionable protocol now requires the Decodable conformance because of the VersionManager<Type: Decodable> generic constraint.