在android系统中，处理了音源逻辑的切换。

在车载项目中，进行导航播报时，需要暂停或者降低多媒体的音量。保证导航的声音优先级高和正常输出。

代码如下：

public class MainActivity extends AppCompatActivity {
    private static final String TAG = "Audio Sink";
    private static final boolean DBG = true;

    private AudioPlayer mMusicPlayer;

    private Handler mHandler;
    private Context mContext;
    private AudioAttributes mMusicAudioAttrib;
    private AudioAttributes mNavAudioAttrib;
    private AudioAttributes mVrAudioAttrib;
    private AudioAttributes mRadioAudioAttrib;
    private AudioAttributes mSystemSoundAudioAttrib;

    private AudioManager mAudioManager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.audio);
        mContext = this;
        mAudioManager = (AudioManager) mContext.getSystemService(
                Context.AUDIO_SERVICE);

        mMusicAudioAttrib = new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_MEDIA)
                .build();

        mMusicPlayer = new AudioPlayer(mContext, R.raw.well_worth_the_wait,
                mMusicAudioAttrib);

        mNavAudioAttrib = new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_ASSISTANCE_NAVIGATION_GUIDANCE)
                .build();
        mVrAudioAttrib = new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_ASSISTANCE_NAVIGATION_GUIDANCE)
                .build();
        mRadioAudioAttrib = new AudioAttributes.Builder()
                .setUsage(AudioAttributes.USAGE_MEDIA)
                .build();

        findViewById(R.id.button_media_play_start).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                boolean requestFocus = true;
                boolean repeat = true;
                mMusicPlayer.start(requestFocus, repeat, AudioManager.AUDIOFOCUS_GAIN);
            }
        });

        findViewById(R.id.button_media_play_stop).setOnClickListener(new View.OnClickListener() {
             @Override
             public void onClick(View view) {
                 mMusicPlayer.stop();
             }
         });

        findViewById(R.id.button_nav_start).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                handleNavStart();
            }
        });
        findViewById(R.id.button_nav_end).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                handleNavEnd();
            }
        });
    }

    private void handleNavStart() {
        if (DBG) {
            Log.i(TAG, "Nav start");
        }
        try {
            AudioFocusRequest naviFocusRequest = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN)
                    .setOnAudioFocusChangeListener(mNavFocusListener)
                    .setAudioAttributes(mNavAudioAttrib)
                    .setFocusGain(AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK).build();
            mAudioManager.requestAudioFocus(naviFocusRequest);
        } catch (Exception e) {
            Log.e(TAG, "Failed to set active focus", e);
        }
    }

    private void handleNavEnd() {
        if (DBG) {
            Log.i(TAG, "Nav end");
        }
        mAudioManager.abandonAudioFocus(mNavFocusListener);
    }

    private final AudioManager.OnAudioFocusChangeListener mNavFocusListener = new AudioManager.OnAudioFocusChangeListener() {
        @Override
        public void onAudioFocusChange(int focusChange) {
            Log.i(TAG, "Nav focus change:" + focusChange);
        }
    };

    private final AudioManager.OnAudioFocusChangeListener mVrFocusListener = new AudioManager.OnAudioFocusChangeListener() {
        @Override
        public void onAudioFocusChange(int focusChange) {
            Log.i(TAG, "VR focus change:" + focusChange);
        }
    };

    private final AudioManager.OnAudioFocusChangeListener mRadioFocusListener = new AudioManager.OnAudioFocusChangeListener() {
        @Override
        public void onAudioFocusChange(int focusChange) {
            Log.i(TAG, "Radio focus change:" + focusChange);
        }
    };
}

关注点：
1.导航播报开始

private void handleNavStart() {
    if (DBG) {
        Log.i(TAG, "Nav start");
    }
    try {
        AudioFocusRequest naviFocusRequest = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN)
                .setOnAudioFocusChangeListener(mNavFocusListener)
                .setAudioAttributes(mNavAudioAttrib)
                .setFocusGain(AudioManager.AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK).build();
        mAudioManager.requestAudioFocus(naviFocusRequest);
    } catch (Exception e) {
        Log.e(TAG, "Failed to set active focus", e);
    }
}

2.导航播报结束

private void handleNavEnd() {
    if (DBG) {
        Log.i(TAG, "Nav end");
    }
    mAudioManager.abandonAudioFocus(mNavFocusListener);
}

重点：
官网解释：https://developer.android.com/reference/android/media/AudioManager
AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK

Added in API level 8
public static final int AUDIOFOCUS_GAIN_TRANSIENT_MAY_DUCK
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 also:

AudioManager.OnAudioFocusChangeListener.onAudioFocusChange(int)
requestAudioFocus(OnAudioFocusChangeListener, int, int)
Constant Value: 3 (0x00000003)

即：用于指示音频焦点的临时请求，预计持续很短的时间，以及其他音频应用程序在降低其输出电平（也称为“闪避”）后继续播放的情况。 临时改变的示例是可以接受背景中的音乐回放的驾驶方向的回放。

在进行导航播报的时候，会降低其他音频的输出电平（闪避），持续短的时间，释放后，其他音频继续播放。