Android源码分析－－Audio设备文件解析(07)

        上篇文章中只是讲到配置文件在哪里，怎么读取文件，但是真正的文件解析还没有开始。这篇就开始分析文件解析，认识这些解析的参数。

 接着上篇Audio设备文件加载继续进行分析，知道主要是这个函数起到解析的作用。

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;
}

也知道audioPolicyXmlConfigFile的配置文件是/system/etc/auido_policy_configuration.xml文件。传入的参数是AudioPolicyConfig，这个类包含很多变量，

private:
HwModuleCollection &mHwModules; /**< Collection of Module, with Profiles, i.e. Mix Ports. */
DeviceVector &mAvailableOutputDevices;
DeviceVector &mAvailableInputDevices;
sp<DeviceDescriptor> &mDefaultOutputDevices;
VolumeCurvesCollection *mVolumeCurves;
bool &mIsSpeakerDrcEnabled;

也就是需要传递进来进行初始化的变量，解析的作用就是初始化这五个变量。

函数里面有个PolicySerializer类

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

只有这个函数有作用

status_t deserialize(const char *str, AudioPolicyConfig &config);

来看这个函数的实现

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;
}
if (xmlXIncludeProcess(doc) < 0) {
ALOGE("%s: libxml failed to resolve XIncludes on %s document.", __FUNCTION__, configFile);
}

if (xmlStrcmp(cur->name, (const xmlChar *) mRootElementName.c_str()))  {
ALOGE("%s: No %s root element found in xml data %s.", __FUNCTION__, mRootElementName.c_str(),
(const char *)cur->name);
xmlFreeDoc(doc);
return BAD_VALUE;
}

string version = getXmlAttribute(cur, versionAttribute);
if (version.empty()) {
ALOGE("%s: No version found in root node %s", __FUNCTION__, mRootElementName.c_str());
return BAD_VALUE;
}
if (version != mVersion) {
ALOGE("%s: Version does not match; expect %s got %s", __FUNCTION__, mVersion.c_str(),
version.c_str());
return BAD_VALUE;
}
// Lets deserialize children
// Modules
ModuleTraits::Collection modules;
deserializeCollection<ModuleTraits>(doc, cur, modules, &config);
config.setHwModules(modules);

// deserialize volume section
VolumeTraits::Collection volumes;
deserializeCollection<VolumeTraits>(doc, cur, volumes, &config);
config.setVolumes(volumes);

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

xmlFreeDoc(doc);
return android::OK;
}

doc是文件节点，cur是文件根节点，可以参考libxml2生成和解析配置树文件文章。这个函数会进一步解析包含的文件

<xi:include href="a2dp_audio_policy_configuration.xml"/>

<?xml version="1.0" enc
e84c
oding="UTF-8"?>
<!-- A2dp Audio HAL Audio Policy Configuration file -->
<module name="a2dp" halVersion="2.0">
<mixPorts>
<mixPort name="a2dp output" role="source">
<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
samplingRates="44100"
channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>
</mixPort>
<mixPort name="a2dp input" role="sink">
<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
samplingRates="44100,48000"
channelMasks="AUDIO_CHANNEL_IN_MONO,AUDIO_CHANNEL_IN_STEREO"/>
</mixPort>
</mixPorts>
<devicePorts>
<devicePort tagName="BT A2DP Out" type="AUDIO_DEVICE_OUT_BLUETOOTH_A2DP" role="sink">
<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
samplingRates="44100"
channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>
</devicePort>
<devicePort tagName="BT A2DP Headphones" type="AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_HEADPHONES" role="sink">
<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
samplingRates="44100"
channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>
</devicePort>
<devicePort tagName="BT A2DP Speaker" type="AUDIO_DEVICE_OUT_BLUETOOTH_A2DP_SPEAKER" role="sink">
<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
samplingRates="44100"
channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>
</devicePort>
<devicePort tagName="BT A2DP In" type="AUDIO_DEVICE_IN_BLUETOOTH_A2DP" role="source">
<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"
samplingRates="44100,48000"
channelMasks="AUDIO_CHANNEL_IN_MONO,AUDIO_CHANNEL_IN_STEREO"/>
</devicePort>
</devicePorts>
<routes>
<route type="mix" sink="BT A2DP Out"
sources="a2dp output"/>
<route type="mix" sink="BT A2DP Headphones"
sources="a2dp output"/>
<route type="mix" sink="BT A2DP Speaker"
sources="a2dp output"/>
<route type="mix" sink="a2dp input"
sources="BT A2DP In"/>
</routes>
</module>

可以看到modules的书写规则是需要包括<mixPorts>,<devicePorts>,<routes>几个节点的，解析完成之后会生成树。

下面这句话是获取xml版本号，值为1.0

string version = getXmlAttribute(cur, versionAttribute);

可以看到libxml2已经做好解析的所有事情，接下来就是向参数赋值。

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