音视频基础能力之 Android 音频篇 （四）：音频路由

case AUDIO_SOURCE_VOICE_COMMUNICATION:
    
    //如果是 in_call 状态，指定可用设备列表 (primary)。
    if ((getPhoneState() == AUDIO_MODE_IN_CALL) &&
            (availableOutputDevices.getDevice(AUDIO_DEVICE_OUT_TELEPHONY_TX,
                    String8(""), AUDIO_FORMAT_DEFAULT)) == nullptr) {
        LOG_ALWAYS_FATAL_IF(availablePrimaryDevices.isEmpty(), "Primary devices not found");
        availableDevices = availablePrimaryDevices;
    }

		//优先选择带有语音通信的蓝牙
    if (audio_is_bluetooth_out_sco_device(commDeviceType)) {
        // if SCO device is requested but no SCO device is available, fall back to default case
        device = availableDevices.getDevice(
                AUDIO_DEVICE_IN_BLUETOOTH_SCO_HEADSET, String8(""), AUDIO_FORMAT_DEFAULT);
        if (device != nullptr) {
            break;
        }
    }
    
    switch (commDeviceType) {
    //支持低功耗协议的蓝牙设备
    case AUDIO_DEVICE_OUT_BLE_HEADSET:
        device = availableDevices.getDevice(
                AUDIO_DEVICE_IN_BLE_HEADSET, String8(""), AUDIO_FORMAT_DEFAULT);
        break;
    //外放(喇叭)
    case AUDIO_DEVICE_OUT_SPEAKER:
        device = availableDevices.getFirstExistingDevice({
                AUDIO_DEVICE_IN_BACK_MIC, AUDIO_DEVICE_IN_BUILTIN_MIC,
                AUDIO_DEVICE_IN_USB_DEVICE, AUDIO_DEVICE_IN_USB_HEADSET});
        break;
    default:    // FORCE_NONE
        //如果是其他的 audiosource，则按照以下顺序来选择
        device = availableDevices.getFirstExistingDevice({
                AUDIO_DEVICE_IN_WIRED_HEADSET, AUDIO_DEVICE_IN_USB_HEADSET,
                AUDIO_DEVICE_IN_USB_DEVICE, AUDIO_DEVICE_IN_BLUETOOTH_BLE,
                AUDIO_DEVICE_IN_BUILTIN_MIC});
        break;

    }
    break;

  //系统底层会对 streamType 再一次的分类，
  //STRATEGY_PHONE 是 STREAM_VOICE_CALL 和 STREAM_BLUETOOTH_SCO 的集合
  //这里在不赘述
  case STRATEGY_PHONE: {
      //为了用户体验，会优先选择最后使用过的可插拔音频输出设备。
      devices = availableOutputDevices.getFirstDevicesFromTypes(
                      getLastRemovableMediaDevices(GROUP_NONE, {
                          //要排除这两种设备，因为 Dialer 系统应用会显示的使用它们
                          AUDIO_DEVICE_OUT_HEARING_AID,
                          AUDIO_DEVICE_OUT_BLE_HEADSET
                          }));
      if (!devices.isEmpty()) break;
      
      //如果最近没有使用过可插拔的音频输出设备，按照以下顺序来选择输出设备。
      devices = availableOutputDevices.getFirstDevicesFromTypes({
              AUDIO_DEVICE_OUT_DGTL_DOCK_HEADSET, AUDIO_DEVICE_OUT_EARPIECE,
              AUDIO_DEVICE_OUT_SPEAKER});
  } break;

AudioManager audioManager = (AudioManager) context.getSystemService(Context.AUDIO_SERVICE);
AudioManager.OnAudioFocusChangeListener afChangeListener;

...
// Request audio focus for playback
int result = audioManager.requestAudioFocus(afChangeListener,
                             // Use the music stream.
                             AudioManager.STREAM_MUSIC,
                             // Request permanent focus.
                             AudioManager.AUDIOFOCUS_GAIN);

if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
    // Start playback
}

private Handler handler = new Handler();
AudioManager.OnAudioFocusChangeListener afChangeListener =
  new AudioManager.OnAudioFocusChangeListener() {
    public void onAudioFocusChange(int focusChange) {
      if (focusChange == AudioManager.AUDIOFOCUS_LOSS) {
        // Permanent loss of audio focus
        // Pause playback immediately
        mediaController.getTransportControls().pause();
        // Wait 30 seconds before stopping playback
        handler.postDelayed(delayedStopRunnable,
          TimeUnit.SECONDS.toMillis(30));
      }
      else if (focusChange == AudioManager.AUDIOFOCUS_LOSS_TRANSIENT) {
        // Pause playback
      } else if (focusChange == AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK) {
        // Lower the volume, keep playing
      } else if (focusChange == AudioManager.AUDIOFOCUS_GAIN) {
        // Your app has been granted audio focus again
        // Raise volume to normal, restart playback if necessary
      }
    }
  };

audioManager.abandonAudioFocus(afChangeListener);

// initializing variables for audio focus and playback management
audioManager = (AudioManager) Context.getSystemService(Context.AUDIO_SERVICE);
playbackAttributes = new AudioAttributes.Builder()
        .setUsage(AudioAttributes.USAGE_GAME)
        .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
        .build();
focusRequest = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN)
        .setAudioAttributes(playbackAttributes)
        .setAcceptsDelayedFocusGain(true)
        .setOnAudioFocusChangeListener(afChangeListener, handler)
        .build();
final Object focusLock = new Object();

boolean playbackDelayed = false;
boolean playbackNowAuthorized = false;

// requesting audio focus and processing the response
int res = audioManager.requestAudioFocus(focusRequest);
synchronized(focusLock) {
    if (res == AudioManager.AUDIOFOCUS_REQUEST_FAILED) {
        playbackNowAuthorized = false;
    } else if (res == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
        playbackNowAuthorized = true;
        playbackNow();
    } else if (res == AudioManager.AUDIOFOCUS_REQUEST_DELAYED) {
        playbackDelayed = true;
        playbackNowAuthorized = false;
    }
}

// implementing OnAudioFocusChangeListener to react to focus changes
@Override
public void onAudioFocusChange(int focusChange) {
    switch (focusChange) {
        case AudioManager.AUDIOFOCUS_GAIN:
            if (playbackDelayed || resumeOnFocusGain) {
                synchronized(focusLock) {
                    playbackDelayed = false;
                    resumeOnFocusGain = false;
                }
                playbackNow();
            }
            break;
        case AudioManager.AUDIOFOCUS_LOSS:
            synchronized(focusLock) {
                resumeOnFocusGain = false;
                playbackDelayed = false;
            }
            pausePlayback();
            break;
        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT:
            synchronized(focusLock) {
                // only resume if playback is being interrupted
                resumeOnFocusGain = isPlaying();
                playbackDelayed = false;
            }
            pausePlayback();
            break;
        case AudioManager.AUDIOFOCUS_LOSS_TRANSIENT_CAN_DUCK:
            // ... pausing or ducking depends on your app
            break;
        }
    }
}