Android: How to create fade-in/fade-out sound effects for any music file that my app plays?

Here is my simplified adaptation of stock Android Alarm Clock's fade in implementation.

Rather than defining the number of steps/increments and then increasing the volume step by step (as in other answers to this question), it adjusts volume every 50ms (configurable value) working out steps/increments on the scale between -40dB (near silent) and 0dB (max; relative to the stream volume) based on:

• Preset effect duration (can be hard-coded or set by user)
• Elapsed time since the playback started

See computeVolume() below for the juicy bits.

Full original code can be found here: Google Source

private MediaPlayer mMediaPlayer;
private long mCrescendoDuration = 0;
private long mCrescendoStopTime = 0;

// Default settings
private static final boolean DEFAULT_CRESCENDO = true;
private static final int CRESCENDO_DURATION = 1;

// Internal message codes
private static final int EVENT_VOLUME = 3;


// Create a message Handler
@SuppressLint("HandlerLeak")
private Handler mHandler = new Handler() {
    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
            ...

            case EVENT_VOLUME:
            if (adjustVolume()) {
                scheduleVolumeAdjustment();
            }
            break;

            ...
        }
    }
};


// Obtain user preferences
private void getPrefs() {
    SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);
    ...

    final boolean crescendo = prefs.getBoolean(SettingsActivity.KEY_CRESCENDO, DEFAULT_CRESCENDO);
    if (crescendo) {
        // Convert mins to millis
        mCrescendoDuration = CRESCENDO_DURATION * 1000 * 60;
    } else {
        mCrescendoDuration = 0;
    }

    ...
}


// Start the playback
private void play(Alarm alarm) {
    ...

    // Check to see if we are already playing
    stop();

    // Obtain user preferences
    getPrefs();

    // Check if crescendo is enabled. If it is, set alarm volume to 0.
    if (mCrescendoDuration > 0) {
        mMediaPlayer.setVolume(0, 0);
    }

    mMediaPlayer.setDataSource(this, alarm.alert);
    startAlarm(mMediaPlayer);

    ...
}


// Do the common stuff when starting the alarm.
private void startAlarm(MediaPlayer player) throws java.io.IOException, IllegalArgumentException, IllegalStateException {

    final AudioManager audioManager = (AudioManager)getSystemService(Context.AUDIO_SERVICE);

    // Do not play alarms if stream volume is 0
    // (typically because ringer mode is silent).
    if (audioManager.getStreamVolume(AudioManager.STREAM_ALARM) != 0) {
        player.setAudioStreamType(AudioManager.STREAM_ALARM);
        player.setLooping(true);
        player.prepare();
        player.start();

        // Schedule volume adjustment
        if (mCrescendoDuration > 0) {
            mCrescendoStopTime = System.currentTimeMillis() + mCrescendoDuration;
            scheduleVolumeAdjustment();
        }
    }
}


// Stop the playback
public void stop() {
    ...

    if (mMediaPlayer != null) {
        mMediaPlayer.stop();
        mMediaPlayer.release();
        mMediaPlayer = null;
    }

    mCrescendoDuration = 0;
    mCrescendoStopTime = 0;

    ...
}


// Schedule volume adjustment 50ms in the future.
private void scheduleVolumeAdjustment() {
    // Ensure we never have more than one volume adjustment queued.
    mHandler.removeMessages(EVENT_VOLUME);
    // Queue the next volume adjustment.
    mHandler.sendMessageDelayed( mHandler.obtainMessage(EVENT_VOLUME, null), 50);
}


// Adjusts the volume of the ringtone being played to create a crescendo effect.
private boolean adjustVolume() {
    // If media player is absent or not playing, ignore volume adjustment.
    if (mMediaPlayer == null || !mMediaPlayer.isPlaying()) {
        mCrescendoDuration = 0;
        mCrescendoStopTime = 0;
        return false;
    }
    // If the crescendo is complete set the volume to the maximum; we're done.
    final long currentTime = System.currentTimeMillis();
    if (currentTime > mCrescendoStopTime) {
        mCrescendoDuration = 0;
        mCrescendoStopTime = 0;
        mMediaPlayer.setVolume(1, 1);
        return false;
    }
    // The current volume of the crescendo is the percentage of the crescendo completed.
    final float volume = computeVolume(currentTime, mCrescendoStopTime, mCrescendoDuration);
    mMediaPlayer.setVolume(volume, volume);

    // Schedule the next volume bump in the crescendo.
    return true;
}


/**
 * @param currentTime current time of the device
 * @param stopTime time at which the crescendo finishes
 * @param duration length of time over which the crescendo occurs
 * @return the scalar volume value that produces a linear increase in volume (in decibels)
 */
private static float computeVolume(long currentTime, long stopTime, long duration) {
    // Compute the percentage of the crescendo that has completed.
    final float elapsedCrescendoTime = stopTime - currentTime;
    final float fractionComplete = 1 - (elapsedCrescendoTime / duration);
    // Use the fraction to compute a target decibel between -40dB (near silent) and 0dB (max).
    final float gain = (fractionComplete * 40) - 40;
    // Convert the target gain (in decibels) into the corresponding volume scalar.
    final float volume = (float) Math.pow(10f, gain/20f);
    //LOGGER.v("Ringtone crescendo %,.2f%% complete (scalar: %f, volume: %f dB)", fractionComplete * 100, volume, gain);
    return volume;
}