android audio volume 知识点

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frameworks/av/services/audiopolicy/common/managerdefinitions/include/VolumeCurve.h

class VolumeCurvesForStream : public KeyedVector<device_category, sp<VolumeCurve> >
{
private:
    KeyedVector<device_category, sp<VolumeCurve> > mOriginVolumeCurves;
    KeyedVector<audio_devices_t, int> mIndexCur; /**< current volume index per device. */
    int mIndexMin; /**< min volume index. */
    int mIndexMax; /**< max volume index. */
    bool mCanBeMuted; /**< true is the stream can be muted. */
};
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VolumeCurve代表一个音量曲线，即音量值与分贝值或者增益的对应关系。
mCanBeMuted 表示该流是否可以被Mute
int mIndexMin; 最小
int mIndexMax; 最大
可以看出 一个devices 对应 一个音量值
一个 device_category 对应 一个VolumeCurve

device_category是把设备分成了四类DEVICE_CATEGORY_HEADSET, DEVICE_CATEGORY_SPEAKER,DEVICE_CATEGORY_EARPIECE,DEVICE_CATEGORY_EXT_MEDIA. getDeviceCategory(audio_devices_t device)函数可以获得设备的分类。

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frameworks/av/services/audiopolicy/common/managerdefinitions/include/VolumeCurve.h

class VolumeCurvesCollection : public KeyedVector<audio_stream_type_t, VolumeCurvesForStream>,
                               public IVolumeCurvesCollection
{
public:
    VolumeCurvesCollection()
    {
        // Create an empty collection of curves
        for (ssize_t i = 0 ; i < AUDIO_STREAM_CNT; i++) {
            audio_stream_type_t stream = static_cast<audio_stream_type_t>(i);
            KeyedVector::add(stream, VolumeCurvesForStream());
        }
    }
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从中可以得到 每个 stream 对应一个 VolumeCurvesForStream

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frameworks/av/services/audiopolicy/common/managerdefinitions/include/AudioPolicyConfig.h
class AudioPolicyConfig 描述

class AudioPolicyConfig
{
	private:
    		HwModuleCollection &mHwModules; /**< Collection of Module, with Profiles, i.e. Mix Ports. */
    		DeviceVector &mAvailableOutputDevices;
    		DeviceVector &mAvailableInputDevices;
    		sp<DeviceDescriptor> &mDefaultOutputDevices;
    		VolumeCurvesCollection *mVolumeCurves;
    		bool &mIsSpeakerDrcEnabled;
};
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AudioPolicyConfig类 成员声音类型（但是个指针，需要给他赋予内存）
当然还包括其他重要的成员变量

class AudioPolicyConfig 构造函数

class AudioPolicyConfig
{
public:
    AudioPolicyConfig(HwModuleCollection &hwModules,
                      DeviceVector &availableOutputDevices,
                      DeviceVector &availableInputDevices,
                      sp<DeviceDescriptor> &defaultOutputDevices,
                      bool &isSpeakerDrcEnabled,
                      VolumeCurvesCollection *volumes = nullptr)
        : mHwModules(hwModules),
          mAvailableOutputDevices(availableOutputDevices),
          mAvailableInputDevices(availableInputDevices),
          mDefaultOutputDevices(defaultOutputDevices),
          mVolumeCurves(volumes),
          mIsSpeakerDrcEnabled(isSpeakerDrcEnabled)
    {}
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AudioPolicyManager 构造函数

#ifdef USE_XML_AUDIO_POLICY_CONF
    mVolumeCurves = new VolumeCurvesCollection();
    AudioPolicyConfig config(mHwModules, mAvailableOutputDevices, mAvailableInputDevices,
                             mDefaultOutputDevice, speakerDrcEnabled,
                             static_cast<VolumeCurvesCollection *>(mVolumeCurves));
    if (deserializeAudioPolicyXmlConfig(config) != NO_ERROR) {
#else
    mVolumeCurves = new StreamDescriptorCollection();
    AudioPolicyConfig config(mHwModules, mAvailableOutputDevices, mAvailableInputDevices,
                             mDefaultOutputDevice, speakerDrcEnabled);
    if ((ConfigParsingUtils::loadConfig(AUDIO_POLICY_VENDOR_CONFIG_FILE, config) != NO_ERROR) &&
            (ConfigParsingUtils::loadConfig(AUDIO_POLICY_CONFIG_FILE, config) != NO_ERROR)) {
#endif
        ALOGE("could not load audio policy configuration file, setting defaults");
        config.setDefault();
    }
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//构造函数 mVolumeCurves(volumes), 音量赋值给 mVolumeCurves。
//把申请的空间给了 AudioPolicyConfig类 mVolumeCurves有成员

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frameworks/av/services/audiopolicy/managerdefault/AudioPolicyManager.cpp

#define AUDIO_POLICY_XML_CONFIG_FILE_NAME "audio_policy_configuration.xml"

#ifdef USE_XML_AUDIO_POLICY_CONF
// Treblized audio policy xml config will be located in /odm/etc or /vendor/etc.
static const char *kConfigLocationList[] =
        {"/odm/etc", "/vendor/etc", "/system/etc"};
static const int kConfigLocationListSize =
        (sizeof(kConfigLocationList) / sizeof(kConfigLocationList[0]));

static status_t deserializeAudioPolicyXmlConfig(AudioPolicyConfig &config) {
    char audioPolicyXmlConfigFile[AUDIO_POLICY_XML_CONFIG_FILE_PATH_MAX_LENGTH];
    status_t ret;

    for (int i = 0; i < kConfigLocationListSize; i++) {
        PolicySerializer serializer;
        snprintf(audioPolicyXmlConfigFile,
                 sizeof(audioPolicyXmlConfigFile),
                 "%s/%s",
                 kConfigLocationList[i],
                 AUDIO_POLICY_XML_CONFIG_FILE_NAME);
        ret = serializer.deserialize(audioPolicyXmlConfigFile, config);
        if (ret == NO_ERROR) {
            break;
        }
    }
    return ret;
}
#endif
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//解析 audio_policy_configuration.xml 文件，此xml 文件还会指定 volume的xml 文件是哪个。

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frameworks/av/services/audiopolicy/common/managerdefinitions/src/Serializer.cpp

status_t PolicySerializer::deserialize(const char *configFile, AudioPolicyConfig &config)
{
    xmlDocPtr doc;
    doc = xmlParseFile(configFile);
    if (doc == NULL) {
        ALOGE("%s: Could not parse %s document.", __FUNCTION__, configFile);
        return BAD_VALUE;
    }
    xmlNodePtr cur = xmlDocGetRootElement(doc);
    if (cur == NULL) {
        ALOGE("%s: Could not parse %s document: empty.", __FUNCTION__, configFile);
        xmlFreeDoc(doc);
        return BAD_VALUE;
    }

    // deserialize volume section  解析声音文件
    VolumeTraits::Collection volumes;
    deserializeCollection<VolumeTraits>(doc, cur, volumes, &config); //解析音量相关的xml
    config.setVolumes(volumes); //并把得到值赋给 config

    // Global Configuration
    GlobalConfigTraits::deserialize(cur, config);

    xmlFreeDoc(doc);
    return android::OK;
}
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参考AudioPolicy–音量的控制