ServiceLocator.swift

protocol Service {}
Class ServiceLocator {
   func register(_ service: Service, for type: ServiceType)
   func resolve(_ type: ServiceType)
}

ServiceLocator.shared.register(UserSession(), type: UserService.self)

Struct UserService: Service {}

Class LoginViewController {
   func login(_ name: String, password: String) {
       let service = ServiceLocator.shared.resolve(UserService.self)
       service.login(name, password)
   }
}