Audio系列之通用的音量设置函数setStreamVolume（）

系统设置界面中就是使用AudioManager.setStreamVolume（），

源码：

/**     * Sets the volume index for a particular stream.     * <p>This method has no effect if the device implements a fixed volume policy     * as indicated by {@link #isVolumeFixed()}.     * <p>From N onward, volume adjustments that would toggle Do Not Disturb are not allowed unless     * the app has been granted Do Not Disturb access.     * See {@link NotificationManager#isNotificationPolicyAccessGranted()}.     * @param streamType The stream whose volume index should be set.     * @param index The volume index to set. See     *            {@link #getStreamMaxVolume(int)} for the largest valid value.     * @param flags One or more flags.     * @see #getStreamMaxVolume(int)     * @see #getStreamVolume(int)     * @see #isVolumeFixed()     */    public void setStreamVolume(int streamType, int index, int flags) {        IAudioService service = getService();        try {            service.setStreamVolume(streamType, index, flags, getContext().getOpPackageName());        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }    }只是简单的调用了AudioService的setStreamVolume（）方法，下面是AudioService#setStreamVolume（）源码：
PRivate void setStreamVolume(int streamType, int index, int flags, String callingPackage,            String caller, int uid) {        if (mUseFixedVolume) {            return;        }        ensureValidStreamType(streamType);//下面两句是进行流类型转换，后面会详细介绍。mStreamVolumeAlias保存了源类型//和目标类型的映射关系。mStreamStates是保存了各种流的对应的VolumeStreamState//VolumeStreamState保存与一个流类型所有音量相关的信息，每种流分配了一个VolumeStreamState对象          int streamTypeAlias = mStreamVolumeAlias[streamType];        VolumeStreamState streamState = mStreamStates[streamTypeAlias];//获取当前流使用哪一个音频设备进行播放，最终会被调用到AudioPolicyService中        final int device = getDeviceForStream(streamType);        int oldIndex;        // skip a2dp absolute volume control request when the device        // is not an a2dp device        if ((device & AudioSystem.DEVICE_OUT_ALL_A2DP) == 0 &&            (flags & AudioManager.FLAG_BLUETOOTH_ABS_VOLUME) != 0) {            return;        }        // If we are being called by the system (e.g. hardware keys) check for current user        // so we handle user restrictions correctly.        if (uid == android.os.Process.SYSTEM_UID) {            uid = UserHandle.getUid(getCurrentUserId(), UserHandle.getAppId(uid));        }        if (mAppOps.noteOp(STREAM_VOLUME_OPS[streamTypeAlias], uid, callingPackage)                != AppOpsManager.MODE_ALLOWED) {            return;        }        if (isAndroidNPlus(callingPackage)                && wouldToggleZenMode(getNewRingerMode(streamTypeAlias, index, flags))                && !mNm.isNotificationPolicyAccessGrantedForPackage(callingPackage)) {            throw new SecurityException("Not allowed to change Do Not Disturb state");        }        if (!volumeAdjustmentAllowedByDnd(streamTypeAlias, flags)) {            return;        }        synchronized (mSafeMediaVolumeState) {            // reset any pending volume command            mPendingVolumeCommand = null;
          //获取当前流的音量 oldIndex = streamState.getIndex(device);//由于流类型转换需要转换设置的音量值 index = rescaleIndex(index * 10, streamType, streamTypeAlias); if (streamTypeAlias == AudioSystem.STREAM_MUSIC && (device & AudioSystem.DEVICE_OUT_ALL_A2DP) != 0 && (flags & AudioManager.FLAG_BLUETOOTH_ABS_VOLUME) == 0) { synchronized (mA2dpAvrcpLock) {//和蓝牙耳机协议有关 if (mA2dp != null && mAvrcpAbsVolSupported) { mA2dp.setAvrcpAbsoluteVolume(index / 10); } } } if (streamTypeAlias == AudioSystem.STREAM_MUSIC) { setSystemAudioVolume(oldIndex, index, getStreamMaxVolume(streamType), flags); } flags &= ~AudioManager.FLAG_FIXED_VOLUME; if ((streamTypeAlias == AudioSystem.STREAM_MUSIC) && ((device & mFixedVolumeDevices) != 0)) { 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 && (device & mSafeMediaVolumeDevices) != 0) { index = mSafeMediaVolumeIndex; } else { index = streamState.getMaxIndex(); } } } if (!checkSafeMediaVolume(streamTypeAlias, index, device)) { mVolumeController.postDisplaySafeVolumeWarning(flags); mPendingVolumeCommand = new StreamVolumeCommand( streamType, index, flags, device); } else { onSetStreamVolume(streamType, index, flags, device, caller);//获取设置的结果 index = mStreamStates[streamType].getIndex(device); } }//广播通知 sendVolumeUpdate(streamType, oldIndex, index, flags); }
当VolumeStreamState#muteCount() ！= 0则是静音。
应该是调用VolumeStreamState#setIndext（）设置音量值的，但是API24的AudioService的setStreamVolume（）代码中找不到这句。
int streamTypeAlias = mStreamVolumeAlias[streamType];
VolumeStreamState streamState = mStreamStates[streamTypeAlias];
以上两句涉及到满足所谓的“将铃声音量作为通知音量”这种需求，如，实现“以铃声音量用作音乐音量”代码如下：
mStreamVolumeAlias[AudioSystem.STREAM_MUSIC] = AudioSystem.STREAM_RING;
这样，当传入AudioSystem.STREAM_MUSIC参数时，实际操作的回事AudioSystem.STREAM_RING这个流对应的VolumeStreamState。