class TodoManager {

    ...

    enum CreateTodoError: Error {

        case noStore

    }

    func createTodo(
        title: String,
        completionHandler: @escaping (Result<Todo>) -> Void
    ) {

        guard
            let container = persistentContainer
        else {

            let error: CreateTodoError = .noStore

            completionHandler(
                .failure(error: error)
            )

            return

        }

        container.performBackgroundTask { backgroundContext in

            guard
                let description = NSEntityDescription.entity(
                    forEntityName: "Todo",
                    in: backgroundContext
                )
            else { fatalError("CANNOT find the entity description for Todo.") }

            backgroundContext.perform {

                let todoEntity = TodoEntity(
                    entity: description,
                    insertInto: backgroundContext
                )

                todoEntity.title = title

                todoEntity.createdAtDate = Date()

                do {

                    try backgroundContext.save()

                    let todo = try Todo(entity: todoEntity)

                    completionHandler(
                        .success(value: todo)
                    )

                }
                catch {

                    completionHandler(
                        .failure(error: error)
                    )

                }

            }

        }

    }

    ...

}