手機(jī)音量轉(zhuǎn)換絕對(duì)音量，此處會(huì)有一個(gè)計(jì)算過程，大致就是手機(jī)側(cè)計(jì)算出當(dāng)前音量和最大音量的百分比然后設(shè)置給耳機(jī)，耳機(jī)端收到百分比去處理。不然每個(gè)手機(jī)的音量范圍不同，不做歸一化就很難適配。

當(dāng)通過音量按鍵調(diào)節(jié)音量時(shí)，會(huì)通過input事件分發(fā)，input把event分發(fā)給mediassesion, mediasession調(diào)用audiomanager的adjustStreamVolume調(diào)節(jié)音量。

當(dāng)通過拖動(dòng)音量條調(diào)節(jié)音量時(shí)，settings app會(huì)調(diào)用audiomanager的setStreamVolume調(diào)節(jié)音量。

adjustStreamVolume和setStreamVolume處理過程類似，接下來只看setStreamVolume。

setStreamVolume調(diào)用

// frameworks/base/media/java/android/media/AudioManager.java
public void setStreamVolume(int streamType, int index, int flags) {
    final IAudioService service = getService();
    try {
        service.setStreamVolume(streamType, index, flags, getContext().getOpPackageName()); // audiomanager調(diào)用的是audioservice的setStreamVolume
    } catch (RemoteException e) {
        throw e.rethrowFromSystemServer();
    }
}

audioservice的調(diào)用過程:

// frameworks/base/services/core/java/com/android/server/audio/AudioService.java
private void setStreamVolume(int streamType, int index, int flags, String callingPackage, String caller, int uid, boolean hasModifyAudioSettings) {
    ensureValidStreamType(streamType);
    int streamTypeAlias = mStreamVolumeAlias[streamType];
    VolumeStreamState streamState = mStreamStates[streamTypeAlias];
    inal int device = getDeviceForStream(streamType); // 獲取streamType對(duì)應(yīng)的device
    int oldIndex;
 
     synchronized (mSafeMediaVolumeStateLock) {
        // reset any pending volume command
        mPendingVolumeCommand = null;
        oldIndex = streamState.getIndex(device);
        android.media.AudioServiceInjector.mOriginalIndexWhenSetStreamVolume = index;
        index = rescaleIndex(index * 10, streamType, streamTypeAlias);
 
        // 設(shè)置絕對(duì)音量
        if (streamTypeAlias == AudioSystem.STREAM_MUSIC && AudioSystem.DEVICE_OUT_ALL_A2DP_SET.contains(device) && (flags & AudioManager.FLAG_BLUETOOTH_ABS_VOLUME) == 0) {
            mDeviceBroker.postSetAvrcpAbsoluteVolumeIndex(index / 10);
        }
 
        if (streamTypeAlias == AudioSystem.STREAM_MUSIC) {
            setSystemAudioVolume(oldIndex, index, getStreamMaxVolume(streamType), flags); // 這里是和hdmi相關(guān)，不用管
        }
 
        flags &= ~AudioManager.FLAG_FIXED_VOLUME;
        if (streamTypeAlias == AudioSystem.STREAM_MUSIC && isFixedVolumeDevice(device)) {
            flags |= AudioManager.FLAG_FIXED_VOLUME;
 
            // volume is either 0 or max allowed for fixed volume devices
            if (index != 0) {
                if (mSafeMediaVolumeState == SAFE_MEDIA_VOLUME_ACTIVE && mSafeMediaVolumeDevices.contains(device)) {
                   index = safeMediaVolumeIndex(device);
                } else {
                   index = streamState.getMaxIndex();
                }
            }
        }
 
        if (!checkSafeMediaVolume(streamTypeAlias, index, device)) {
            mVolumeController.postDisplaySafeVolumeWarning(flags); // 這里應(yīng)該就是安全提醒，比如音量過大損傷聽力的提醒
            mPendingVolumeCommand = new StreamVolumeCommand(streamType, index, flags, device);
        } else {
            onSetStreamVolume(streamType, index, flags, device, caller, hasModifyAudioSettings); // 調(diào)用onSetStreamVolume設(shè)置音量
            index = mStreamStates[streamType].getIndex(device);
        }
    }
    sendVolumeUpdate(streamType, oldIndex, index, flags, device);
 }

onSetStreamVolume 和 sendVolumeUpdate的作用:

onSetStreamVolume -> 設(shè)置音量到底層并處理一些靜音邏輯。

sendVolumeUpdate -> 通知ui更新音量值

這里插入個(gè)當(dāng)音量設(shè)置到0時(shí)自動(dòng)靜音的處理:

// frameworks/base/services/core/java/com/android/server/audio/AudioService.java
private void onSetStreamVolume(int streamType, int index, int flags, int device, String caller, boolean hasModifyAudioSettings) {
    final int stream = mStreamVolumeAlias[streamType];
    setStreamVolumeInt(stream, index, device, false, caller, hasModifyAudioSettings); // 設(shè)置音量到底層
    // setting volume on ui sounds stream type also controls silent mode
    if (((flags & AudioManager.FLAG_ALLOW_RINGER_MODES) != 0) || (stream == getUiSoundsStreamType())) {
       int ringerMode = getNewRingerMode(stream, index, flags); // 獲取ringmode
        miuiRingerMode = miui.util.AudioManagerHelper.getValidatedRingerMode(mContext, miuiRingerMode); // 獲取ringmode
        setRingerMode(ringerMode, TAG + ".onSetStreamVolume", false /*external*/); // 設(shè)置ringmode
    }
}
    
private int getNewRingerMode(int stream, int index, int flags) {
    if (((flags & AudioManager.FLAG_ALLOW_RINGER_MODES) != 0) || (stream == getUiSoundsStreamType())) {
        int newRingerMode;
        if (index == 0) { // 如果設(shè)置的音量是0
        // 如果有震動(dòng)就設(shè)置ringmode是震動(dòng)模式，否則判斷volumeDownToEnterSilent決定設(shè)置靜音還是不靜音
            newRingerMode = mHasVibrator ? AudioManager.RINGER_MODE_VIBRATE : mVolumePolicy.volumeDownToEnterSilent ? AudioManager.RINGER_MODE_SILENT : AudioManager.RINGER_MODE_NORMAL;
        } else {
            newRingerMode = AudioManager.RINGER_MODE_NORMAL; // 如果音量不是0，ringmode設(shè)置為非靜音
        }
        return newRingerMode;
    }
    return getRingerModeExternal();
}

設(shè)置絕對(duì)音量: postSetAvrcpAbsoluteVolumeIndex

// frameworks/base/services/core/java/com/android/server/audio/AudioDeviceBroker.java
void postSetAvrcpAbsoluteVolumeIndex(int index) {
    sendIMsgNoDelay(MSG_I_SET_AVRCP_ABSOLUTE_VOLUME, SENDMSG_REPLACE, index);
}
 
// 收到消息MSG_I_SET_AVRCP_ABSOLUTE_VOLUME，調(diào)用mBtHelper.setAvrcpAbsoluteVolumeIndex(msg.arg1);
 
// frameworks/base/services/core/java/com/android/server/audio/BtHelper.java
synchronized void setAvrcpAbsoluteVolumeIndex(int index) {
    mA2dp.setAvrcpAbsoluteVolume(index);
}

藍(lán)牙側(cè)絕對(duì)音量處理:

// vendor/qcom/opensource/commonsys/packages/apps/Bluetooth/src/com/android/bluetooth/a2dp/A2dpService.java
public void setAvrcpAbsoluteVolume(int volume) {
    if (mFactory.getAvrcpTargetService() != null) {
        mFactory.getAvrcpTargetService().sendVolumeChanged(volume);
        return;
    }
    if(ApmConstIntf.getLeAudioEnabled()) {
        VolumeManagerIntf mVolumeManager = VolumeManagerIntf.get();
        mVolumeManager.setMediaAbsoluteVolume(volume);
        return;
    }
    synchronized(mBtAvrcpLock) {
        if (mAvrcp_ext != null) {
           mAvrcp_ext.setAbsoluteVolume(volume);
           return;
        }
        if (mAvrcp != null) {
           mAvrcp.setAbsoluteVolume(volume);
        }
    }
}
 
// vendor/qcom/opensource/commonsys/bluetooth_ext/packages_apps_bluetooth_ext/src/avrcp/Avrcp_ext.java
public void setAbsoluteVolume(int volume) {
    Message msg = mHandler.obtainMessage(MSG_SET_ABSOLUTE_VOLUME, volume, 0);
    mHandler.sendMessage(msg);
}
 
收到消息MSG_SET_ABSOLUTE_VOLUME之后調(diào)用如下
case MSG_SET_ABSOLUTE_VOLUME: {
    int avrcpVolume = convertToAvrcpVolume(msg.arg1);
    ......
}
 
// 計(jì)算avrcpVolume
private int convertToAvrcpVolume(int volume) {
   if(mAudioStreamMax == 150) {
       return (int) Math.round((double) volume*AVRCP_MAX_VOL/mAudioStreamMax);
   }
   return (int) Math.ceil((double) volume*AVRCP_MAX_VOL/mAudioStreamMax);
}

從這里可以看到，藍(lán)牙側(cè)傳給耳機(jī)的音量其實(shí)就是百分比。

從相對(duì)音量切換到絕對(duì)音量的處理:

藍(lán)牙APP有一個(gè)NotificationReceiver，當(dāng)絕對(duì)音量的開關(guān)發(fā)生變化NotificationReceiver就會(huì)收到信息，然后做如下處理:

private class NotificationReceiver extends BroadcastReceiver {
    public void onReceive(Context context, Intent intent) {
        if(ABS_VOLUME_ACTION.equals(intent.getAction())) {
            handleDeviceAbsVolume(mac, value);
        }
    }
}
// handleDeviceAbsVolume就會(huì)做對(duì)應(yīng)的處理，把音量值發(fā)送給耳機(jī)側(cè)。

此處每次切換的時(shí)候會(huì)有l(wèi)og打印，log mask: handleDeviceAbsVolume|setVolumeNative

切換之后播放聲音就會(huì)調(diào)用checkAndSetVolume把音量值設(shè)置到最大，保證手機(jī)側(cè)無音量處理。