// Singleton
public class Repository {
    private Repository instance = new Repository();

    private Repository() {}

    public static Repository getInstance() {
        return instance == null ? new Repository() : instance;
    }

    /**
     * Creates a {@link Single} which will send a sign in request to remote API, if necessary, and
     * return the appropriate UI transition as {@link Intent} with appropriate action and extras.
     *
     * @param context {@link Context} to create an Intent
     * @param context {@link String} something that authenticates the user from input
     * @return {@link Single<Intent>} for the requested operation
     */
    @NonNull
    public Single<Intent> signInOperation(@NonNull Context context, @NonNull String userInput) {
        if (!tokenFresh() && isConnectionUp()) {
            return remoteSignInSingle(userInput);
        } else if (userAuthenticatedLocally(userInput)) {
            Intent cachedLogIn = Ui.buildSignedInIntent(context);
            cachedLogIn.putExtra(Ui.USER_EXTRA, getAuthenticatedUser());
            cachedLogIn.putExtra(Ui.ONLINE_EXTRA, false);
            return Single.just(cachedLogIn);
        } else {
            Intent errorLogIn = Ui.buildErrorIntent(context);
            errorLogIn.putExtra(Ui.MESSAGE_EXTRA, "No connection!");
            return Single.just(errorLogIn);
        }
    }

    @NonNull
    public Single<List<Stuff> getStuffOperation(Context context) {
        // TODO
    }
}
public class SignInViewModel extends ViewModel {
    /**
     * Handles intention of submit from a sign in screen, calling remote API. This either loads from
     * server, local cache, fails with a proper Intent, or throws an exception if errors unhandled.
     *
     * @param context   context to acquire the next UI Intent
     * @param userInput {@link String} for input to pass to remote API (assume no password required)
     */
    public Single<Intent> clickSignIn(@NonNull final Context context,
                                @NonNull final String userInput) {
        return Repository.getInstance().signInOperation(context, userInput)
                .map(intent -> {
                    // return unmodified, for now
                    return intent;
                });
    }
}
public abstract class SignInScreen extends Fragment {
    /**
     * ViewModel performs submit operations for sign in. This screen uses the resulting
     * {@link Intent} to start next activity, transition to next screen, finish, or show an error.
     *
     * @param context   a context to perform next ui flow call
     * @param userInput input to perform the request with
     */
    protected Disposable onClickSubmitLanding(@NonNull Context context, String userInput) {
        return viewModel.clickSignIn(context, userInput)
                .observeOn(Schedulers.io())
                .subscribeOn(AndroidSchedulers.mainThread())
                .subscribe((Consumer<Intent>) context::startActivity);
    }
}