Can't set volume, volume control is not forwarded to the system

Question

I tried to use the Android MediaPlayer framework to play a mp3 file (see this question).

After I managed to make it work, I quickly recognized,

Answer 1

While I hate to constantly answer my own questions, I found a solution after a couple of hours playing with the Android API, digging through some documentations and so on.

My solution is partially based the answer, given by @josé-pereda on the topic "javafxports how to call android native Media Player".

I created an interface for the tasks "volumeUp", "volumeDown" and "mute":

public interface NativeVolumeService {
    void volumeUp();
    void volumeDown();
    void mute();
}

Then, based on the following answer on how to set the system volume on Android, I came up with the following implementation on Android:

import java.util.ArrayList;
import java.util.logging.Logger;

import android.content.Context;
import android.media.AudioManager;
import android.view.KeyEvent;
import android.view.View;
import android.view.ViewGroup;
import my.package.platform.NativeVolumeService;
import javafxports.android.FXActivity;

public class NativeVolumeServiceAndroid implements NativeVolumeService {

    private static final Logger LOG = Logger.getLogger(NativeVolumeServiceAndroid.class.getName());

    private final AudioManager audioManager;

    private final int maxVolume;
    private int preMuteVolume = 0;
    private int currentVolume = 0;

    public NativeVolumeServiceAndroid() {
        audioManager = (AudioManager) FXActivity.getInstance().getSystemService(Context.AUDIO_SERVICE);
        maxVolume = audioManager.getStreamMaxVolume(AudioManager.STREAM_MUSIC);
    }

    @Override
    public void volumeUp() {
        LOG.info("dispatch volume up event");
        KeyEvent event = new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_VOLUME_UP);
        dispatchEvent(event, true, false);
    }

    @Override
    public void volumeDown() {
        LOG.info("dispatch volume down event");
        KeyEvent event = new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_VOLUME_DOWN);
        dispatchEvent(event, false, false);
    }

    @Override
    public void mute() {
        LOG.info("dispatch volume mute event");
        KeyEvent event = new KeyEvent(KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_VOLUME_MUTE);
        dispatchEvent(event, false, true);
    }

    private void dispatchEvent(KeyEvent event, boolean up, boolean mute) {

        // hardware key events (amongst others) get caught by the JavaFXPorts engine (or better: the Dalvik impl from Oracle)
        // to circumvent this, we need to do the volume adjustment the hard way: via the AudioManager

        // see: https://developer.android.com/reference/android/media/AudioManager.html
        // see: https://stackoverflow.com/questions/9164347/setting-the-android-system-volume?rq=1

        // reason: 
        // FXActivity registers a FXDalvikEntity, which etends the surface view and passing a key processor 
        // called KeyEventProcessor - this one catches all key events and matches them to JavaFX representations.
        // Unfortunately, we cannot bypass this, so we need the AudioManager

        currentVolume = audioManager.getStreamVolume(AudioManager.STREAM_MUSIC);
        if (mute) {
            if (currentVolume > 0) {
                preMuteVolume = currentVolume;
                audioManager.adjustStreamVolume(AudioManager.STREAM_MUSIC, AudioManager.ADJUST_SAME,
                        AudioManager.FLAG_REMOVE_SOUND_AND_VIBRATE | AudioManager.FLAG_SHOW_UI);
            } else {
                preMuteVolume = 0;
                audioManager.adjustStreamVolume(AudioManager.STREAM_MUSIC, preMuteVolume, AudioManager.FLAG_SHOW_UI);
            }
        } else if (up) {
            audioManager.adjustStreamVolume(AudioManager.STREAM_MUSIC,
                    (currentVolume + 1) <= maxVolume ? AudioManager.ADJUST_RAISE : AudioManager.ADJUST_SAME, AudioManager.FLAG_SHOW_UI);
        } else if (!up) {
            audioManager.adjustStreamVolume(AudioManager.STREAM_MUSIC,
                    (currentVolume - 1) >= 0 ? AudioManager.ADJUST_LOWER : AudioManager.ADJUST_SAME, AudioManager.FLAG_SHOW_UI);
        }
    }
}

to integrate it, I created the appropriate instance in my main class (I need this globally - you will see, why)

private void instantiateNativeVolumeService() {
    String serviceName = NativeVolumeService.class.getName();
    if (Platform.isDesktop()) {
        serviceName += "Desktop";
    } else if (Platform.isAndroid()) {
        serviceName += "Android";
    }
    try {
        volumeService = (NativeVolumeService) Class.forName(serviceName).newInstance();
    } catch (ClassNotFoundException | InstantiationException | IllegalAccessException e) {
        LOG.log(Level.SEVERE, "Could not get an instance of NativeAudioService for platform " + Platform.getCurrent(), e);
    }
}

volumeService is a class variable.

Then I registered an event handler on my Stages Scene:

@Override
public void start(Stage stage) throws Exception {
    // initiate everything
    scene.addEventFilter(KeyEvent.ANY, this::handleGlobalKeyEvents);
    // do more stuff, if needed
}

And finally, the method handleGlobalKeyEvents looks like this:

private void handleGlobalKeyEvents(KeyEvent event) {
    // use a more specific key event type like
    // --> KeyEvent.KEY_RELEASED == event.getEventType()
    // --> KeyEvent.KEY_PRESSED == event.getEventType()
    // without it, we would react on both events, thus doing one operation too much
    if (event.getCode().equals(KeyCode.VOLUME_UP) && KeyEvent.KEY_PRESSED == event.getEventType()) {
        if (volumeService != null) {
            volumeService.volumeUp();
            event.consume();
        }
    }
    if (event.getCode().equals(KeyCode.VOLUME_DOWN) && KeyEvent.KEY_PRESSED == event.getEventType()) {
        if (volumeService != null) {
            volumeService.volumeDown();
            event.consume();
        }
    }
}

In the end, the solution is as clean as it gets and not too complicated. Only the way until it worked was a bit nasty.

@JoséPereda: If you want to integrate this solution as a charm down plugin or so, please feel free, but it would be nice to be mentioned and notified, if you do.

Regards, Daniel