// 1.確定要調(diào)整音量的流類型 2.在某些情況下屏蔽FLAG_PLAY_SOUND 3.調(diào)用adjustStreamVolume()
  private void adjustSuggestedStreamVolume(int direction, int suggestedStreamType, int flags, String callingPackage, int uid) {
    ......
    //從這一小段代碼中可以看出，在AudioService中還有地方可以強行改變音量鍵控制的流類型。
    //mVolumeControlStream是VolumePanel通過forceVolumeControlStream()設(shè)置的,
    //VolumePanel顯示時會調(diào)用forceVolumeControlStream強制后續(xù)的音量鍵操作固定為促使它顯示的那個流類型，
    //并在它關(guān)閉時取消這個強制設(shè)置，設(shè)值為-1
    if (mVolumeControlStream != -1) { 
      streamType = mVolumeControlStream;
    } else {
      //通過getActiveStreamType()函數(shù)獲取要控制的流類型，這里根據(jù)建議的流類型與AudioService的實際情況，返回一個值
      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); //確認(rèn)一下調(diào)整的音量方向
    ensureValidStreamType(streamType); //確認(rèn)一下調(diào)整的流類型 
    int streamTypeAlias = mStreamVolumeAlias[streamType];//獲取streamType映射到的流類型
    //VolumeStreamState類，保存與一個流類型所有音量相關(guān)的信息
    VolumeStreamState streamState = mStreamStates[streamTypeAlias];
    final int device = getDeviceForStream(streamTypeAlias);
    int aliasIndex = streamState.getIndex(device);//獲取當(dāng)前音量
    ......

      //rescaleIndex用于將音量值的變化量從源流類型變換到目標(biāo)流類型下，
      //由于不同的流類型的音量調(diào)節(jié)范圍不同，所以這個轉(zhuǎn)換是必需的
      step = rescaleIndex(10, streamType, streamTypeAlias);
    }
    ......

      final int result = checkForRingerModeChange(aliasIndex, direction, step);
      adjustVolume = (result & FLAG_ADJUST_VOLUME) != 0; //布爾變量，用來表示是否有必要繼續(xù)設(shè)置音量值
    ......

    int oldIndex = mStreamStates[streamType].getIndex(device);//取出調(diào)整前的音量值。這個值會在sendVolumeUpdate()調(diào)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);
       //判斷streamState.adjustIndex返回值，如果音量值在調(diào)整之后并沒有發(fā)生變化，比如到了最大值，就不需要繼續(xù)后面的操作了
      } else if (streamState.adjustIndex(direction * step, device)) {
        //這個消息將把音量設(shè)置到底層去，并將其存儲到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);// 通知外界音量值發(fā)生了變化
  }

    //更改VolumeStreamState對象中保存的音量值
    public boolean adjustIndex(int deltaIndex, int device) {
      return setIndex(getIndex(device) + deltaIndex, device);// 將現(xiàn)有的音量值加上變化量，然后調(diào)用setIndex進(jìn)行設(shè)置
    }

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

        mIndex.put(device, index);//保存設(shè)置的音量值

        if (oldIndex != index) {
          //同時設(shè)置所有映射到當(dāng)前流類型的其他流的音量
          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);//這個函數(shù)會調(diào)用AudioSystem.setStreamVolumeIndex()，
        //到這，音量就被設(shè)置到底層的AudioFlinger中

        // 對所有流應(yīng)用更改，使用此別名作為別名。處理流音量映射的情況
        int numStreamTypes = AudioSystem.getNumStreamTypes();
        for (int streamType = numStreamTypes - 1; streamType >= 0; streamType--) {
          ......

      }
      //發(fā)送消息，其處理函數(shù)將會調(diào)用persitVolume()函數(shù)，這將會把音量的設(shè)置信息存儲到SettingsProvide中。
      //Audioservice在初始化時，將會從SettingsProvide中將音量設(shè)置讀取出來并進(jìn)行設(shè)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) {
    //判斷設(shè)備是否擁有通話功能。對沒有通話能力的設(shè)備來說，RING流類型自然也就沒有意義了。這句話應(yīng)該算是一種從語義操作上進(jìn)行的保護(hù)
    if (!isPlatformVoice() && (streamType == AudioSystem.STREAM_RING)) { 
      streamType = AudioSystem.STREAM_NOTIFICATION;          
    }

    if (streamType == AudioSystem.STREAM_MUSIC) {
      flags = updateFlagsForSystemAudio(flags);
    }
    mVolumeController.postVolumeChanged(streamType, flags);//最后將顯示系統(tǒng)音量條的提示框

    if ((flags & AudioManager.FLAG_FIXED_VOLUME) == 0) {
      oldIndex = (oldIndex + 5) / 10; //+5的意義是實現(xiàn)四舍五入；除以10是因為存儲時先乘了10，轉(zhuǎn)換過程中保留小數(shù)點后一位的精度
      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);//先判斷一下流類型這個參數(shù)的有效性
    int streamTypeAlias = mStreamVolumeAlias[streamType];//對這個數(shù)組進(jìn)行流類型的轉(zhuǎn)換
    VolumeStreamState streamState = mStreamStates[streamTypeAlias];

    final int device = getDeviceForStream(streamType);//獲取當(dāng)前流將使用哪一個音頻設(shè)備進(jìn)行播放。最終會被調(diào)用到AudioPolicyService中
    ......

      oldIndex = streamState.getIndex(device);//獲取當(dāng)前流的音量

      index = rescaleIndex(index * 10, streamType, streamTypeAlias);//將原流類型下的音量值映射到目標(biāo)流類型下的音量值
      ......

      if (!checkSafeMediaVolume(streamTypeAlias, index, device)) {
        mVolumeController.postDisplaySafeVolumeWarning(flags);
        mPendingVolumeCommand = new StreamVolumeCommand(
                          streamType, index, flags, device);
      } else {
        onSetStreamVolume(streamType, index, flags, device);//將調(diào)用setStreamVolumeInt()方法
        index = mStreamStates[streamType].getIndex(device);//獲取設(shè)置結(jié)果
      }
    }
    sendVolumeUpdate(streamType, oldIndex, index, flags);//通知外界音量發(fā)生了變化
  }

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

    if (streamState.setIndex(index, device) || force) { //調(diào)用streamState.setIndex()，更改VolumeStreamState對象中保存的音量值
      //這個消息將把音量設(shè)置到底層去，并將其存儲到Settingsprovider中
      sendMsg(mAudioHandler,
          MSG_SET_DEVICE_VOLUME,
          SENDMSG_QUEUE,
          device,
          0,
          streamState,
          0);
    }
  }