RepositoryListCoordinator.swift

override func start() -> Observable<Void> {
  
    ...
    // Observe request to show Language List screen
    viewModel.showLanguageList
        .flatMap { [weak self] _ -> Observable<String?> in
            guard let `self` = self else { return .empty() }
            // Start next coordinator and subscribe on it's result
            return self.showLanguageList(on: viewController)
        }
        // Ignore nil results which means that Language List screen was dismissed by cancel button.
        .filter { $0 != nil }
        .map { $0! }
        // Bind selected language to the `setCurrentLanguage` observer of the View Model
        .bind(to: viewModel.setCurrentLanguage)
        .disposed(by: disposeBag)

    ...
  
    // We return `Observable.never()` here because RepositoryListViewController is always on screen.
    return Observable.never()
}

// Starts the LanguageListCoordinator
// Emits nil if LanguageListCoordinator resulted with `cancel` or selected language
private func showLanguageList(on rootViewController: UIViewController) -> Observable<String?> {
    let languageListCoordinator = LanguageListCoordinator(rootViewController: rootViewController)
    return coordinate(to: languageListCoordinator)
        .map { result in
            switch result {
            case .language(let language): return language
            case .cancel: return nil
            }
        }
}