rxjava observable edittext debouncing

helper_methods.java

    private boolean isUsernameValid(String username) {
        boolean match = username.matches("^(?=.{4,12}$)(?![_.])(?!.*[_.]{2})[a-zA-Z0-9._]+(?<![_.])$");
        if (!match) showMessage(getActivity(), R.string.profile_dialog_username_hint_invalid, true);
        return match;
    }

    private void resetUsernameChecks(Activity activity) {
        searchDisposables.clear();
        activity.runOnUiThread(() -> {
            claimButton.setEnabled(false);
            usernameInputLayout.setError(null);
        });
    }

    private void showMessage(Activity activity, @StringRes int message, boolean error) {
        showMessage(activity, getString(message), error);
    }

    private void showMessage(Activity activity, String message, boolean error) {
        activity.runOnUiThread(() -> {
            if (error) {
                usernameInputLayout.setErrorTextAppearance(R.style.AppTheme_RedMessage);
            } else {
                usernameInputLayout.setErrorTextAppearance(R.style.AppTheme_GreenMessage);
            }

            usernameInputLayout.setError(message);
        });
    }

search.java

        Observable<String> usernameObservable = RxTextView.textChanges(usernameEditText)
                .filter(charSequence -> !TextUtils.isEmpty(charSequence))
                .doOnNext(charSequence -> resetUsernameChecks(getActivity()))
                .map(CharSequence::toString)
                .filter(this::isUsernameValid)
                .debounce(400, TimeUnit.MILLISECONDS);

        Disposable usernameSubscription = usernameObservable
                .subscribe(username -> {
                    searchDisposables.clear();
                    Disposable d = viewModel.checkUsername(username)
                            .subscribe(available -> {
                                if (available) {
                                    claimButton.setEnabled(true);
                                    showMessage(getActivity(), R.string.profile_dialog_username_hint_available, false);
                                } else {
                                    showMessage(getActivity(), R.string.profile_dialog_username_hint_unavailable, true);
                                }
                            });
                    searchDisposables.add(d);
                });

        disposables.add(usernameSubscription);