Android原生音量控制实例详解

  // 1.确定要调整音量的流类型 2.在某些情况下屏蔽FLAG_PLAY_SOUND 3.调用adjustStreamVolume()
  private void adjustSuggestedStreamVolume(int direction, int suggestedStreamType, int flags, String callingPackage, int uid) {
    ......
    //从这一小段代码中可以看出，在AudioService中还有地方可以强行改变音量键控制的流类型。
    //mVolumeControlStream是VolumePanel通过forceVolumeControlStream()设置的,
    //VolumePanel显示时会调用forceVolumeControlStream强制后续的音量键操作固定为促使它显示的那个流类型，
    //并在它关闭时取消这个强制设置，设值为-1
    if (mVolumeControlStream != -1) { 
      streamType = mVolumeControlStream;
    } else {
      //通过getActiveStreamType()函数获取要控制的流类型，这里根据建议的流类型与AudioService的实际情况，返回一个值
      streamType = getActiveStreamType(suggestedStreamType);
    }
    final int resolvedStream = mStreamVolumeAlias[streamType];
    ......
    adjustStreamVolume(streamType, direction, flags, callingPackage, uid);
  }

  private void adjustStreamVolume(int streamType, int direction, int flags,
      String callingPackage, int uid) {
    ......
    ensureValidDirection(direction); //确认一下调整的音量方向
    ensureValidStreamType(streamType); //确认一下调整的流类型 
    int streamTypeAlias = mStreamVolumeAlias[streamType];//获取streamType映射到的流类型
    //VolumeStreamState类，保存与一个流类型所有音量相关的信息
    VolumeStreamState streamState = mStreamStates[streamTypeAlias];
    final int device = getDeviceForStream(streamTypeAlias);
    int aliasIndex = streamState.getIndex(device);//获取当前音量
    ......

      //rescaleIndex用于将音量值的变化量从源流类型变换到目标流类型下，
      //由于不同的流类型的音量调节范围不同，所以这个转换是必需的
      step = rescaleIndex(10, streamType, streamTypeAlias);
    }
    ......

      final int result = checkForRingerModeChange(aliasIndex, direction, step);
      adjustVolume = (result & FLAG_ADJUST_VOLUME) != 0; //布尔变量，用来表示是否有必要继续设置音量值
    ......

    int oldIndex = mStreamStates[streamType].getIndex(device);//取出调整前的音量值。这个值会在sendVolumeUpdate()调用

    if (adjustVolume && (direction != AudioManager.ADJUST_SAME)) {
    ......

      if ((direction == AudioManager.ADJUST_RAISE) &&
          !checkSafeMediaVolume(streamTypeAlias, aliasIndex + step, device)) {
        Log.e(TAG, "adjustStreamVolume() safe volume index = "+oldIndex);
        mVolumeController.postDisplaySafeVolumeWarning(flags);
       //判断streamState.adjustIndex返回值，如果音量值在调整之后并没有发生变化，比如到了最大值，就不需要继续后面的操作了
      } else if (streamState.adjustIndex(direction * step, device)) {
        //这个消息将把音量设置到底层去，并将其存储到Settingsprovider中
        sendMsg(mAudioHandler,
            MSG_SET_DEVICE_VOLUME,
            SENDMSG_QUEUE,
            device,
            0,
            streamState,
            0);
      }
    ......

    int index = mStreamStates[streamType].getIndex(device);
    sendVolumeUpdate(streamType, oldIndex, index, flags);// 通知外界音量值发生了变化
  }

    //更改VolumeStreamState对象中保存的音量值
    public boolean adjustIndex(int deltaIndex, int device) {
      return setIndex(getIndex(device) + deltaIndex, device);// 将现有的音量值加上变化量，然后调用setIndex进行设置
    }

    public boolean setIndex(int index, int device) {
    ......

        mIndex.put(device, index);//保存设置的音量值

        if (oldIndex != index) {
          //同时设置所有映射到当前流类型的其他流的音量
          boolean currentDevice = (device == getDeviceForStream(mStreamType));
          int numStreamTypes = AudioSystem.getNumStreamTypes();
          for (int streamType = numStreamTypes - 1; streamType >= 0; streamType--) {
            if (streamType != mStreamType &&
                mStreamVolumeAlias[streamType] == mStreamType) {
              int scaledIndex = rescaleIndex(index, mStreamType, streamType);
              mStreamStates[streamType].setIndex(scaledIndex,
                                device);
              if (currentDevice) {
                mStreamStates[streamType].setIndex(scaledIndex,
                                  getDeviceForStream(streamType));
              }
            }
          }
          return true;
        } else {
          return false;
        }
      }
    }

case MSG_SET_DEVICE_VOLUME:
   setDeviceVolume((VolumeStreamState) msg.obj, msg.arg1);
   break;

    private void setDeviceVolume(VolumeStreamState streamState, int device) {
      synchronized (VolumeStreamState.class) {   
        streamState.applyDeviceVolume_syncVSS(device);//这个函数会调用AudioSystem.setStreamVolumeIndex()，
        //到这，音量就被设置到底层的AudioFlinger中

        // 对所有流应用更改，使用此别名作为别名。处理流音量映射的情况
        int numStreamTypes = AudioSystem.getNumStreamTypes();
        for (int streamType = numStreamTypes - 1; streamType >= 0; streamType--) {
          ......

      }
      //发送消息，其处理函数将会调用persitVolume()函数，这将会把音量的设置信息存储到SettingsProvide中。
      //Audioservice在初始化时，将会从SettingsProvide中将音量设置读取出来并进行设置
      sendMsg(mAudioHandler,
          MSG_PERSIST_VOLUME,
          SENDMSG_QUEUE,
          device,
          0,
          streamState,
          PERSIST_DELAY);

    }

  // UI update and Broadcast Intent
  private void sendVolumeUpdate(int streamType, int oldIndex, int index, int flags) {
    //判断设备是否拥有通话功能。对没有通话能力的设备来说，RING流类型自然也就没有意义了。这句话应该算是一种从语义操作上进行的保护
    if (!isPlatformVoice() && (streamType == AudioSystem.STREAM_RING)) { 
      streamType = AudioSystem.STREAM_NOTIFICATION;          
    }

    if (streamType == AudioSystem.STREAM_MUSIC) {
      flags = updateFlagsForSystemAudio(flags);
    }
    mVolumeController.postVolumeChanged(streamType, flags);//最后将显示系统音量条的提示框

    if ((flags & AudioManager.FLAG_FIXED_VOLUME) == 0) {
      oldIndex = (oldIndex + 5) / 10; //+5的意义是实现四舍五入；除以10是因为存储时先乘了10，转换过程中保留小数点后一位的精度
      index = (index + 5) / 10;
      Intent intent = new Intent(AudioManager.VOLUME_CHANGED_ACTION);
      intent.putExtra(AudioManager.EXTRA_VOLUME_STREAM_TYPE, streamType);
      intent.putExtra(AudioManager.EXTRA_VOLUME_STREAM_VALUE, index);
      intent.putExtra(AudioManager.EXTRA_PREV_VOLUME_STREAM_VALUE, oldIndex);
      sendBroadcastToAll(intent);
    }
  }

  private void setStreamVolume(int streamType, int index, int flags, String callingPackage,
      int uid) {
    ......

    ensureValidStreamType(streamType);//先判断一下流类型这个参数的有效性
    int streamTypeAlias = mStreamVolumeAlias[streamType];//对这个数组进行流类型的转换
    VolumeStreamState streamState = mStreamStates[streamTypeAlias];

    final int device = getDeviceForStream(streamType);//获取当前流将使用哪一个音频设备进行播放。最终会被调用到AudioPolicyService中
    ......

      oldIndex = streamState.getIndex(device);//获取当前流的音量

      index = rescaleIndex(index * 10, streamType, streamTypeAlias);//将原流类型下的音量值映射到目标流类型下的音量值
      ......

      if (!checkSafeMediaVolume(streamTypeAlias, index, device)) {
        mVolumeController.postDisplaySafeVolumeWarning(flags);
        mPendingVolumeCommand = new StreamVolumeCommand(
                          streamType, index, flags, device);
      } else {
        onSetStreamVolume(streamType, index, flags, device);//将调用setStreamVolumeInt()方法
        index = mStreamStates[streamType].getIndex(device);//获取设置结果
      }
    }
    sendVolumeUpdate(streamType, oldIndex, index, flags);//通知外界音量发生了变化
  }

  private void setStreamVolumeInt(int streamType, int index, int device, boolean force) {
    VolumeStreamState streamState = mStreamStates[streamType];

    if (streamState.setIndex(index, device) || force) { //调用streamState.setIndex()，更改VolumeStreamState对象中保存的音量值
      //这个消息将把音量设置到底层去，并将其存储到Settingsprovider中
      sendMsg(mAudioHandler,
          MSG_SET_DEVICE_VOLUME,
          SENDMSG_QUEUE,
          device,
          0,
          streamState,
          0);
    }
  }