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（），

说下比较难理解的flag参数在AudioManager在handlerKeyDown()中设置了两个flags,分别是FLAG_SHOW_UI和FLAG_VIBRATE.从字面上可知,前者告诉AudioService需要弹出一个音量控制面板,后者可能,只是可能是设置在为0时震动一下.而设置了FLAG_PLAY_SOUND是松开音量键时有个提示音.

源码：

/**
     * Sets the volume index for a particular stream.
     * 
This method has no effect if the device implements a fixed volume policy
     * as indicated by {@link #isVolumeFixed()}.
     * 
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。