Android Audio Focus

public int requestAudioFocus(OnAudioFocusChangeListener l, int streamType, int durationHint)

/** The audio stream for phone calls */
public static final int STREAM_VOICE_CALL = AudioSystem.STREAM_VOICE_CALL;
/** The audio stream for system sounds */
public static final int STREAM_SYSTEM = AudioSystem.STREAM_SYSTEM;
/** The audio stream for the phone ring */
public static final int STREAM_RING = AudioSystem.STREAM_RING;
/** The audio stream for music playback */
public static final int STREAM_MUSIC = AudioSystem.STREAM_MUSIC;
/** The audio stream for alarms */
public static final int STREAM_ALARM = AudioSystem.STREAM_ALARM;
/** The audio stream for notifications */
public static final int STREAM_NOTIFICATION = AudioSystem.STREAM_NOTIFICATION;
/** @hide The audio stream for phone calls when connected to bluetooth */
public static final int STREAM_BLUETOOTH_SCO = AudioSystem.STREAM_BLUETOOTH_SCO;
/** @hide The audio stream for enforced system sounds in certain countries (e.g camera in Japan) */
public static final int STREAM_SYSTEM_ENFORCED = AudioSystem.STREAM_SYSTEM_ENFORCED;
/** The audio stream for DTMF Tones */
public static final int STREAM_DTMF = AudioSystem.STREAM_DTMF;
/** @hide The audio stream for text to speech (TTS) */
public static final int STREAM_TTS = AudioSystem.STREAM_TTS;

/**
* @hide
* Used to indicate no audio focus has been gained or lost.
*/
public static final int AUDIOFOCUS_NONE = 0;

/**
* Used to indicate a gain of audio focus, or a request of audio focus, of unknown duration.
* @see OnAudioFocusChangeListener#onAudioFocusChange(int)
* @see #requestAudioFocus(OnAudioFocusChangeListener, int, int)
*/
public static final int AUDIOFOCUS_GAIN = 1;
/**
* Used to indicate a temporary gain or request of audio focus, anticipated to last a short
* amount of time. Examples of temporary changes are the playback of driving directions, or an
* event notification.
* @see OnAudioFocusChangeListener#onAudioFocusChange(int)
* @see #requestAudioFocus(OnAudioFocusChangeListener, int, int)
*/
public static final int AUDIOFOCUS_GAIN_TRANSIENT = 2;
/**
* Used to indicate a temporary request of audio focus, anticipated to last a short
* amount of time, and where it is acceptable for other audio applications to keep playing
* after having lowered their output level (also referred to as "ducking").
* Examples of temporary changes are the playback of driving directions where playback of music
* in the background is acceptable.
* @see OnAudioFocusChangeListener#onAudioFocusChange(int)
* @see #requestAudioFocus(OnAudioFocusChangeListener, int, int)
*/
public static final int AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK = 3;
/**
* Used to indicate a temporary request of audio focus, anticipated to last a short
* amount of time, during which no other applications, or system components, should play
* anything. Examples of exclusive and transient audio focus requests are voice
* memo recording and speech recognition, during which the system shouldn't play any
* notifications, and media playback should have paused.
* @see #requestAudioFocus(OnAudioFocusChangeListener, int, int)
*/
public static final int AUDIOFOCUS_GAIN_TRANSIENT_EXCLUSIVE = 4;
/**
* Used to indicate a loss of audio focus of unknown duration.
* @see OnAudioFocusChangeListener#onAudioFocusChange(int)
*/
public static final int AUDIOFOCUS_LOSS = -1 * AUDIOFOCUS_GAIN;
/**
* Used to indicate a transient loss of audio focus.
* @see OnAudioFocusChangeListener#onAudioFocusChange(int)
*/
public static final int AUDIOFOCUS_LOSS_TRANSIENT = -1 * AUDIOFOCUS_GAIN_TRANSIENT;
/**
* Used to indicate a transient loss of audio focus where the loser of the audio focus can
* lower its output volume if it wants to continue playing (also referred to as "ducking"), as
* the new focus owner doesn't require others to be silent.
* @see OnAudioFocusChangeListener#onAudioFocusChange(int)
*/
public static final int AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK =
-1 * AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK;

public int requestAudioFocus(OnAudioFocusChangeListener l, int streamType, int durationHint) {
        int status = AUDIOFOCUS_REQUEST_FAILED;

        try {
            // status is guaranteed to be either AUDIOFOCUS_REQUEST_FAILED or
            // AUDIOFOCUS_REQUEST_GRANTED as focus is requested without the
            // AUDIOFOCUS_FLAG_DELAY_OK flag
            status = requestAudioFocus(l,
                    new AudioAttributes.Builder()
                            .setInternalLegacyStreamType(streamType).build(),
                    durationHint,
                    0 /* flags, legacy behavior */);
        } catch (IllegalArgumentException e) {
            Log.e(TAG, "Audio focus request denied due to ", e);
        }

        return status;
    }

// fun requestAudioFocus()
int status = AUDIOFOCUS_REQUEST_FAILED;
registerAudioFocusListener(l);
IAudioService service = getService();

// fun registerAudioFocusListener()
public void registerAudioFocusListener(OnAudioFocusChangeListener l) {
    synchronized(mFocusListenerLock) {
    if (mAudioFocusIdListenerMap.containsKey(getIdForAudioFocusListener(l))) {
            return;
        }
        mAudioFocusIdListenerMap.put(getIdForAudioFocusListener(l), l);
    }
}
// fun getIdForAudioFocusListener()
private String getIdForAudioFocusListener(OnAudioFocusChangeListener l) {
    if (l == null) {
        return new String(this.toString());
    } else {
        return new String(this.toString() + l.toString());
    }
}

...
try {
    status = service.requestAudioFocus(requestAttributes, durationHint, mICallBack,
            mAudioFocusDispatcher, getIdForAudioFocusListener(l),
            getContext().getOpPackageName() /* package name */, flags,
            ap != null ? ap.cb() : null);
} catch (RemoteException e) {
    Log.e(TAG, "Can't call requestAudioFocus() on AudioService:", e);
}
...

private final Stack<FocusRequester> mFocusStack = new Stack<FocusRequester>();