class RealmTodoManager: TodoProvider { ... }

class FirebaseTodoManager: TodoProvider { ... }

extension AppDelegate: UIApplicationDelegate {

    func application(
        _ application: UIApplication,
        didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?
    )
    -> Bool {

        ....

        // Use the database as the provider.
        todoListViewController.todoProvider = RealmTodoManager()

        // Use the cahce as the provider.
        todoListViewController.todoProvider = FirebaseTodoManager()

        ...

    }

}