<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:padding="16dp">
    
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        android:hint="Enter text here!"
        android:id="@+id/textToSpeak"/>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" 
        android:layout_centerHorizontal="true"
        android:layout_below="@id/textToSpeak"
        android:id="@+id/btnSpeak"/>

</RelativeLayout>

public class AndroidTextToSpeechActivity extends Activity implements
        TextToSpeech.OnInitListener {

    EditText textToSpeak = null;
    Button btnSpeak = null;
    TextToSpeech tts;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        textToSpeak = findViewById(R.id.textToSpeak);
        btnSpeak = findViewById(R.id.btnSpeak);
        btnSpeak.setEnabled(false);
        tts = new TextToSpeech(this, this);
        btnSpeak.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    speakOut();
                }
            });
    }

    @Override
    public void onDestroy() {
        // Don't forget to shutdown tts!
        if (tts != null) {
            tts.stop();
            tts.shutdown();
        }
        super.onDestroy();
    }

    @Override
    public void onInit(int status) {
        if (status == TextToSpeech.SUCCESS) {
            int result = tts.setLanguage(Locale.US);

            if (result == TextToSpeech.LANG_MISSING_DATA
                    || result == TextToSpeech.LANG_NOT_SUPPORTED) {
                Log.e("TTS", "This Language is not supported");
            } else {
                btnSpeak.setEnabled(true);
                speakOut();
            }
        } else {
            Log.e("TTS", "Initilization Failed!");
        }
    }

    private void speakOut() {
        String text = textToSpeak.getText().toString();
        if(text == null || text.isEmpty())
            return;

        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
            String utteranceId=this.hashCode() + "";
            tts.speak(text, TextToSpeech.QUEUE_FLUSH, null, utteranceId);
        } else {
            tts.speak(text, TextToSpeech.QUEUE_FLUSH, null);
        }
    }
}

tts.setLanguage(Locale.CHINESE); // Chinese language

tts.isLanguageAvailable(Locale.CHINESE);

tts.setPitch(0.6);

tts.setSpeechRate(2.0);

public class RxTextToSpeech {

@Nullable RxTTSObservableOnSubscribe audio;

WeakReference<Context> contextRef;

public RxTextToSpeech(Context context) {
    this.contextRef = new WeakReference<>(context);
}

public void requestTTS(FragmentActivity activity, int requestCode) {
    Intent checkTTSIntent = new Intent();
    checkTTSIntent.setAction(TextToSpeech.Engine.ACTION_CHECK_TTS_DATA);
    activity.startActivityForResult(checkTTSIntent, requestCode);
}

public void cancelCurrent() {
    if (audio != null) {
        audio.dispose();
        audio = null;
    }
}

public Observable<Boolean> speak(String textToRead) {
    audio = new RxTTSObservableOnSubscribe(contextRef.get(), textToRead, Locale.GERMANY);
    return Observable.create(audio);
}


public static class RxTTSObservableOnSubscribe extends UtteranceProgressListener
        implements ObservableOnSubscribe<Boolean>,
        Disposable, Cancellable, TextToSpeech.OnInitListener {

    volatile boolean disposed;
    ObservableEmitter<Boolean> emitter;
    TextToSpeech textToSpeech;
    String text = "";
    Locale selectedLocale;
    Context context;

    public RxTTSObservableOnSubscribe(Context context, String text, Locale locale) {
        this.selectedLocale = locale;
        this.context = context;
        this.text = text;
    }

    @Override public void subscribe(ObservableEmitter<Boolean> e) throws Exception {
        this.emitter = e;
        if (context == null) {
            this.emitter.onError(new Throwable("nullable context, cannot execute " + text));
        } else {
            this.textToSpeech = new TextToSpeech(context, this);
        }
    }

    @Override @DebugLog public void dispose() {
        if (textToSpeech != null) {
            textToSpeech.setOnUtteranceProgressListener(null);
            textToSpeech.stop();
            textToSpeech.shutdown();
            textToSpeech = null;
        }
        disposed = true;
    }

    @Override public boolean isDisposed() {
        return disposed;
    }

    @Override public void cancel() throws Exception {
        dispose();
    }

    @Override public void onInit(int status) {

        int languageCode = textToSpeech.setLanguage(selectedLocale);

        if (languageCode == android.speech.tts.TextToSpeech.LANG_COUNTRY_AVAILABLE) {
            textToSpeech.setPitch(1);
            textToSpeech.setSpeechRate(1.0f);
            textToSpeech.setOnUtteranceProgressListener(this);
            performSpeak();
        } else {
            emitter.onError(new Throwable("language " + selectedLocale.getCountry() + " is not supported"));
        }
    }

    @Override public void onStart(String utteranceId) {
        //no-op
    }

    @Override public void onDone(String utteranceId) {
        this.emitter.onNext(true);
        this.emitter.onComplete();
    }

    @Override public void onError(String utteranceId) {
        this.emitter.onError(new Throwable("error TTS " + utteranceId));
    }

    void performSpeak() {

        if (isAtLeastApiLevel(21)) {
            speakWithNewApi();
        } else {
            speakWithOldApi();
        }
    }

    @RequiresApi(api = 21) void speakWithNewApi() {
        Bundle params = new Bundle();
        params.putString(TextToSpeech.Engine.KEY_PARAM_UTTERANCE_ID, "");
        textToSpeech.speak(text, TextToSpeech.QUEUE_ADD, params, uniqueId());
    }

    void speakWithOldApi() {
        HashMap<String, String> map = new HashMap<>();
        map.put(TextToSpeech.Engine.KEY_PARAM_UTTERANCE_ID, uniqueId());
        textToSpeech.speak(text, TextToSpeech.QUEUE_ADD, map);
    }

    private String uniqueId() {
        return UUID.randomUUID().toString();
    }
}

public static boolean isAtLeastApiLevel(int apiLevel) {
    return Build.VERSION.SDK_INT >= apiLevel;
}