前言
说audio_policy_configuration.xml的解析之前，先熟悉下audiopolicy的启动过程，开机时会通过init.rc启动audioservice，audioservice会启动AudioPolicyService，而AudiopolicyService会创建AudioPolicyManager，这样AudioPolicyManager就被初始化了。感兴趣的可看下这个博客有具体的讲解https://blog.csdn.net/Qidi_Hu...
正文
回到AudioPolicyService的onFirstRef()函数中有两句代码
mAudioPolicyClient = new AudioPolicyClient(this); mAudioPolicyManager =createAudioPolicyManager(mAudioPolicyClient); 在createAudioPolicyManager函数中会 new AudioPolicyManager(clientInterface)。此刻正式开始了AudioPolicyManager的初始化。
我们看下frameworks/av/services/audiopolicy/managerdefault/AudioPolicyManager.cpp 的源码
AudioPolicyManager::AudioPolicyManager(AudioPolicyClientInterface *clientInterface): AudioPolicyManager(clientInterface, false /*forTesting*/){ loadConfig(); initialize();}void AudioPolicyManager::loadConfig() {//Android7.0之后便使用此宏#ifdef USE_XML_AUDIO_POLICY_CONF if (deserializeAudioPolicyXmlConfig(getConfig()) != NO_ERROR) {#else if ((ConfigParsingUtils::loadConfig(AUDIO_POLICY_VENDOR_CONFIG_FILE, getConfig()) != NO_ERROR) && (ConfigParsingUtils::loadConfig(AUDIO_POLICY_CONFIG_FILE, getConfig()) != NO_ERROR)) {#endif ALOGE("could not load audio policy configuration file, setting defaults"); getConfig().setDefault(); }}deserializeAudioPolicyXmlConfig函数的getConfig()即AudioPolicyConfig,函数声明在AudioPolicyManager.h文中中
AudioPolicyConfig& getConfig() { return mConfig; }static status_t deserializeAudioPolicyXmlConfig(AudioPolicyConfig &config) { char audioPolicyXmlConfigFile[AUDIO_POLICY_XML_CONFIG_FILE_PATH_MAX_LENGTH]; std::vector<const char*> fileNames; status_t ret; if (property_get_bool("ro.bluetooth.a2dp_offload.supported", false) && property_get_bool("persist.bluetooth.a2dp_offload.disabled", false)) { // A2DP offload supported but disabled: try to use special XML file fileNames.push_back(AUDIO_POLICY_A2DP_OFFLOAD_DISABLED_XML_CONFIG_FILE_NAME); } //文件名#define AUDIO_POLICY_XML_CONFIG_FILE_NAME "audio_policy_configuration.xml"，位于frameworks/av/services/audiopolicy/config/目录下。 fileNames.push_back(AUDIO_POLICY_XML_CONFIG_FILE_NAME); for (const char* fileName : fileNames) { for (int i = 0; i < kConfigLocationListSize; i++) { PolicySerializer serializer; snprintf(audioPolicyXmlConfigFile, sizeof(audioPolicyXmlConfigFile), "%s/%s", kConfigLocationList[i], fileName); ret = serializer.deserialize(audioPolicyXmlConfigFile, config); if (ret == NO_ERROR) { return ret; } } } return ret;}今天要说的重点就是这个for循环了，serializer.deserialize(audioPolicyXmlConfigFile, config)
先看下PolicySerializer位于/frameworks/av/services/audiopolicy/common/managerdefinitions/include/目录下
以下举例的所有标签均来自audio_policy_configuration.x下对应的第一行标签
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; } //上面都是解析校验xml的一些属性标签啥的，此处开始才是正式加载，首先是module的加载 // 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;}其中这两行代码便开始了真正的解析
deserializeCollection<ModuleTraits>(doc, cur, modules, &config); config.setHwModules(modules);deserializeCollection是个通用方法
template <class Trait>static status_t deserializeCollection(_xmlDoc *doc, const _xmlNode *cur, typename Trait::Collection &collection, typename Trait::PtrSerializingCtx serializingContext){ const xmlNode *root = cur->xmlChildrenNode; while (root != NULL) { if (xmlStrcmp(root->name, (const xmlChar *)Trait::collectionTag) && xmlStrcmp(root->name, (const xmlChar *)Trait::tag)) { root = root->next; continue; } const xmlNode *child = root; if (!xmlStrcmp(child->name, (const xmlChar *)Trait::collectionTag)) { child = child->xmlChildrenNode; } while (child != NULL) { if (!xmlStrcmp(child->name, (const xmlChar *)Trait::tag)) { typename Trait::PtrElement element; status_t status = Trait::deserialize(doc, child, element, serializingContext); if (status != NO_ERROR) { return status; } if (collection.add(element) < 0) { ALOGE("%s: could not add element to %s collection", __FUNCTION__, Trait::collectionTag); } } child = child->next; } if (!xmlStrcmp(root->name, (const xmlChar *)Trait::tag)) { return NO_ERROR; } root = root->next; } return NO_ERROR;}const char *const ModuleTraits::childAttachedDevicesTag = "attachedDevices";const char *const ModuleTraits::childAttachedDeviceTag = "item";const char *const ModuleTraits::childDefaultOutputDeviceTag = "defaultOutputDevice";const char *const ModuleTraits::tag = "module";const char *const ModuleTraits::collectionTag = "modules";const char ModuleTraits::Attributes::name[] = "name";const char ModuleTraits::Attributes::version[] = "halVersion";status_t ModuleTraits::deserialize(xmlDocPtr doc, const xmlNode *root, PtrElement &module, PtrSerializingCtx ctx){ //解析modules下的module标签，我们可以看下configuration.xml下module的name是primary，当我们如果需要修改时记得module标签里的name一定不能为空 string name = getXmlAttribute(root, Attributes::name); if (name.empty()) { ALOGE("%s: No %s found", __FUNCTION__, Attributes::name); return BAD_VALUE; } uint32_t versionMajor = 0, versionMinor = 0; string versionLiteral = getXmlAttribute(root, Attributes::version); if (!versionLiteral.empty()) { sscanf(versionLiteral.c_str(), "%u.%u", &versionMajor, &versionMinor); ALOGV("%s: mHalVersion = major %u minor %u", __FUNCTION__, versionMajor, versionMajor); } ALOGV("%s: %s %s=%s", __FUNCTION__, tag, Attributes::name, name.c_str()); //可以看下Serializer.h里关于ModuleTraits的结构体定义 typedef HwModule Element; //因此这new 了一个Hwmodule，我们先简单看一下Hwmodule的代码，位于frameworks/av/services/audiopolicy/common/managerdefinitions/src/HwModule.cpp HwModule::HwModule(const char *name, uint32_t halVersionMajor, uint32_t halVersionMinor) : mName(String8(name)), mHandle(AUDIO_MODULE_HANDLE_NONE) { setHalVersion(halVersionMajor, halVersionMinor); } //其实就是把<module name="primary" halVersion="3.0">里的name和halVersion解析并初始化给了HwModule module = new Element(name.c_str(), versionMajor, versionMinor); // Deserialize childrens: Audio Mix Port, Audio Device Ports (Source/Sink), Audio Routes MixPortTraits::Collection mixPorts; //我们可以看到module下有 <mixPorts> <devicePorts> <routes>标签，其实也是按着这个顺序解析及的。到这里多少明白了一些audio_policy_configuration.xml的解析，那么解析完的数据又 //是如何初始化的呢，我继续往下看 //开始解析 <mixPorts>标签下东西 deserializeCollection<MixPortTraits>(doc, root, mixPorts, NULL); //我们继续看下mixPoritraits const char *const MixPortTraits::collectionTag = "mixPorts"; const char *const MixPortTraits::tag = "mixPort"; const char MixPortTraits::Attributes::name[] = "name"; const char MixPortTraits::Attributes::role[] = "role"; const char MixPortTraits::Attributes::flags[] = "flags"; const char MixPortTraits::Attributes::maxOpenCount[] = "maxOpenCount"; const char MixPortTraits::Attributes::maxActiveCount[] = "maxActiveCount"; status_t MixPortTraits::deserialize(_xmlDoc *doc, const _xmlNode *child, PtrElement &mixPort, PtrSerializingCtx /*serializingContext*/) { string name = getXmlAttribute(child, Attributes::name); if (name.empty()) { ALOGE("%s: No %s found", __FUNCTION__, Attributes::name); return BAD_VALUE; } ALOGV("%s: %s %s=%s", __FUNCTION__, tag, Attributes::name, name.c_str()); string role = getXmlAttribute(child, Attributes::role); if (role.empty()) { ALOGE("%s: No %s found", __FUNCTION__, Attributes::role); return BAD_VALUE; } ALOGV("%s: Role=%s", __FUNCTION__, role.c_str()); //portRole 分为 sink和source sink可以理解为输入设备比如mic，source可以理解为输出设备比如speaker audio_port_role_t portRole = role == "source" ? AUDIO_PORT_ROLE_SOURCE : AUDIO_PORT_ROLE_SINK; //我们再去头文件里看下发现其实new是IOProfile typedef IOProfile Element;其实IOProfile继承AudioPort。 mixPort = new Element(String8(name.c_str()), portRole); //简单看下IOProfile的初始化 IOProfile(const String8 &name, audio_port_role_t role) : AudioPort(name, AUDIO_PORT_TYPE_MIX, role), maxOpenCount((role == AUDIO_PORT_ROLE_SOURCE) ? 1 : 0), curOpenCount(0), maxActiveCount(1), curActiveCount(0) {} //以上把<mixPort name="primary output" role="source"中的name和role解析完并赋值给了IOProfile。然后继续 AudioProfileTraits::Collection profiles; deserializeCollection<AudioProfileTraits>(doc, child, profiles, NULL); //我们在看下AudioProfileTraits const char *const AudioProfileTraits::collectionTag = "profiles"; const char *const AudioProfileTraits::tag = "profile"; const char AudioProfileTraits::Attributes::name[] = "name"; const char AudioProfileTraits::Attributes::samplingRates[] = "samplingRates"; const char AudioProfileTraits::Attributes::format[] = "format"; const char AudioProfileTraits::Attributes::channelMasks[] = "channelMasks"; //开始解析<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"samplingRates="48000" channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>标签下的samle format 和chanel status_t AudioProfileTraits::deserialize(_xmlDoc */*doc*/, const _xmlNode *root, PtrElement &profile, PtrSerializingCtx /*serializingContext*/){ string samplingRates = getXmlAttribute(root, Attributes::samplingRates); string format = getXmlAttribute(root, Attributes::format); string channels = getXmlAttribute(root, Attributes::channelMasks); //再看下头文件的定义typedef AudioProfile Element发现new的是AudioProfile,顺便看下初始化做了什么 //我们明白了是把<profile name="" format="AUDIO_FORMAT_PCM_16_BIT"samplingRates="48000" channelMasks="AUDIO_CHANNEL_OUT_STEREO"/>标签下的samle format 和chanel //全部赋值给AudioProfile。 AudioProfile(audio_format_t format, audio_channel_mask_t channelMasks, uint32_t samplingRate) : mName(String8("")), mFormat(format) { mChannelMasks.add(channelMasks); mSamplingRates.add(samplingRate); } profile = new Element(formatFromString(format, gDynamicFormat), channelMasksFromString(channels, ","), samplingRatesFromString(samplingRates, ",")); //以下3个函数调用我们只简单分析一个，逻辑都是一样的 //void setDynamicFormat(bool dynamic) { mIsDynamicFormat = dynamic; }实际就是把foramte赋值给我AudioProfile下的mIsDynamicFormat profile->setDynamicFormat(profile->getFormat() == gDynamicFormat); profile->setDynamicChannels(profile->getChannels().isEmpty()); profile->setDynamicRate(profile->getSampleRates().isEmpty()); return NO_ERROR;} //如果profiles是空也会初始化个默认的，也就是每个<mixPort>标签下一定要有个<profile> if (profiles.isEmpty()) { sp <AudioProfile> dynamicProfile = new AudioProfile(gDynamicFormat, ChannelsVector(), SampleRateVector()); dynamicProfile->setDynamicFormat(true); dynamicProfile->setDynamicChannels(true); dynamicProfile->setDynamicRate(true); profiles.add(dynamicProfile); } //mixport即IOProfile，profiles即AudioProfiles，把AudioProfiles赋值给了IOProfile mixPort->setAudioProfiles(profiles); string flags = getXmlAttribute(child, Attributes::flags); //如果flag标签存在，再设置下flag if (!flags.empty()) { // Source role if (portRole == AUDIO_PORT_ROLE_SOURCE) { mixPort->setFlags(OutputFlagConverter::maskFromString(flags)); } else { // Sink role mixPort->setFlags(InputFlagConverter::maskFromString(flags)); } } //下边这俩标签一般都不会使用，解析出来赋给mixport，一般在使用时如果没有特殊需求，一般使用的都是默认的 string maxOpenCount = getXmlAttribute(child, Attributes::maxOpenCount); if (!maxOpenCount.empty()) { convertTo(maxOpenCount, mixPort->maxOpenCount); } string maxActiveCount = getXmlAttribute(child, Attributes::maxActiveCount); if (!maxActiveCount.empty()) { convertTo(maxActiveCount, mixPort->maxActiveCount); } // Deserialize children //解析<profile>下的<gain>这个在mixporit下通常也是没有的 AudioGainTraits::Collection gains; deserializeCollection<AudioGainTraits>(doc, child, gains, NULL); mixPort->setGains(gains); return NO_ERROR;} //moudle即HwModule，将解析的mixPorts(IOProfiles)存储给module的setProfiles，到此<MixPort>标签里的内容就全部解析完了 module->setProfiles(mixPorts); //说下setProfiles这个函数 void HwModule::setProfiles(const IOProfileCollection &profiles){ for (size_t i = 0; i < profiles.size(); i++) { addProfile(profiles[i]); }}//调用了addprofilestatus_t HwModule::addProfile(const sp<IOProfile> &profile){ switch (profile->getRole()) { case AUDIO_PORT_ROLE_SOURCE: return addOutputProfile(profile); case AUDIO_PORT_ROLE_SINK: return addInputProfile(profile); case AUDIO_PORT_ROLE_NONE: return BAD_VALUE; } return BAD_VALUE;}//又调用了addOutputProfile和addInputProfile，其实这俩函数最终就是赋值mInputProfiles和mOutputProfiles这俩集合。mixport解析结束 //解析<devicePorts>标签，解析原理都相同就不再细说了，只说下每个标签解析完都做了什么。 DevicePortTraits::Collection devicePorts; //解析的源码由于篇幅原因我就说下重要部分，这个函数会解析<devicePort tagName="Earpiece" type="AUDIO_DEVICE_OUT_EARPIECE" role="sink">标签下的各属性 deserializeCollection<DevicePortTraits>(doc, root, devicePorts, NULL); //deserializeCollection<DevicePortTraits>函数中 会 new DeviceDescriptor 并将解析的tagName和type赋值下去，这里注意role这个属性只是在解析时做的容错，真正对判断这个device是sink //还是source是通过audio_is_input_device(type)和audio_is_output_device(type)判断的 deviceDesc = new Element(type, String8(name.c_str())); //DeviceDescriptor继承自AudioPort和AudioPortConfig简单看下DeviceDescriptor 的初始化 DeviceDescriptor::DeviceDescriptor(audio_devices_t type, const String8 &tagName) : AudioPort(String8(""), AUDIO_PORT_TYPE_DEVICE, audio_is_output_device(type) ? AUDIO_PORT_ROLE_SINK : AUDIO_PORT_ROLE_SOURCE), mAddress(""), mTagName(tagName), mDeviceType(type), mId(0){ if (type == AUDIO_DEVICE_IN_REMOTE_SUBMIX || type == AUDIO_DEVICE_OUT_REMOTE_SUBMIX ) { mAddress = String8("0"); }}//最终解析完device标签，同样赋值给hwModule， module->setDeclaredDevices(devicePorts); //在这个set函数中将解析的devices分别赋值给了mDeclaredDevices和mPorts，其中mDeclaredDevices是DeviceDescriptor的集合mPorts是AudioPort的集合 void HwModule::setDeclaredDevices(const DeviceVector &devices){ mDeclaredDevices = devices; for (size_t i = 0; i < devices.size(); i++) { mPorts.add(devices[i]); }} //解析<routes>标签，要看到希望了哈，route很重要主要把source和sink连接起来 RouteTraits::Collection routes; deserializeCollection<RouteTraits>(doc, root, routes, module.get()); //我们来看下解析的源码 const char *const RouteTraits::tag = "route"; const char *const RouteTraits::collectionTag = "routes"; const char RouteTraits::Attributes::type[] = "type"; const char RouteTraits::Attributes::typeMix[] = "mix"; const char RouteTraits::Attributes::sink[] = "sink"; const char RouteTraits::Attributes::sources[] = "sources"; status_t RouteTraits::deserialize(_xmlDoc */*doc*/, const _xmlNode *root, PtrElement &element, PtrSerializingCtx ctx){ string type = getXmlAttribute(root, Attributes::type); if (type.empty()) { ALOGE("%s: No %s found", __FUNCTION__, Attributes::type); return BAD_VALUE; } //首先看<route type="mix"下的type属性，一般都是mix audio_route_type_t routeType = (type == Attributes::typeMix) ? AUDIO_ROUTE_MIX : AUDIO_ROUTE_MUX; ALOGV("%s: %s %s=%s", __FUNCTION__, tag, Attributes::type, type.c_str()); // new AudioRoute并将routeType传递下来 element = new Element(routeType); string sinkAttr = getXmlAttribute(root, Attributes::sink); if (sinkAttr.empty()) { ALOGE("%s: No %s found", __FUNCTION__, Attributes::sink); return BAD_VALUE; } // Convert Sink name to port pointer //ctx就是解析的HwModule，findPortByTagName是找到module下的mixport(IOProfile)，根据mixprot标签name找的 sp<AudioPort> sink = ctx->findPortByTagName(String8(sinkAttr.c_str())); if (sink == NULL) { ALOGE("%s: no sink found with name=%s", __FUNCTION__, sinkAttr.c_str()); return BAD_VALUE; } //找到sink属性，将sink值即Earpiece赋值给AudioRoute的setSink 标签<route type="mix" sink="Earpiece" element->setSink(sink); //解析sources属性<route type="mix" sink="Earpiece"sources="primary output,deep_buffer,BT SCO Headset Mic"/> 我们发现sources下有好多因此我们用循环来处理 string sourcesAttr = getXmlAttribute(root, Attributes::sources); if (sourcesAttr.empty()) { ALOGE("%s: No %s found", __FUNCTION__, Attributes::sources); return BAD_VALUE; } // Tokenize and Convert Sources name to port pointer AudioPortVector sources; char *sourcesLiteral = strndup(sourcesAttr.c_str(), strlen(sourcesAttr.c_str())); char *devTag = strtok(sourcesLiteral, ","); while (devTag != NULL) { if (strlen(devTag) != 0) { //还记得之前解析的mixport实际是IOProfile，而IOProfile继承自AudioPort，因此这里找的便是之前的mixport。 sp<AudioPort> source = ctx->findPortByTagName(String8(devTag)); if (source == NULL) { ALOGE("%s: no source found with name=%s", __FUNCTION__, devTag); free(sourcesLiteral); return BAD_VALUE; } sources.add(source); } devTag = strtok(NULL, ","); } free(sourcesLiteral); //将audioroute赋值到audioport中 sink->addRoute(element); for (size_t i = 0; i < sources.size(); i++) { sp<AudioPort> source = sources.itemAt(i); source->addRoute(element); } //audioroute的setSources element->setSources(sources); return NO_ERROR;} //说下setRoutes这个函数 module->setRoutes(routes); //我们看下HwModule.cpp中的实现 void HwModule::setRoutes(const AudioRouteVector &routes){ mRoutes = routes; // Now updating the streams (aka IOProfile until now) supported devices refreshSupportedDevices();}//继续看refreshSupportedDevices这个函数void HwModule::refreshSupportedDevices(){ // Now updating the streams (aka IOProfile until now) supported devices //mInputProfiles就是我们解析mixport时setProfiles时赋值的，因此先遍历所有的mInputProfiles for (const auto& stream : mInputProfiles) { DeviceVector sourceDevices; //解析route标签时sink->addRoute(element);已添加过，这里开始遍历这个mInputProfile下的所有route for (const auto& route : stream->getRoutes()) { //route->getSink()也是解析route标签时element->setSink(sink)下来的，判断这个sinkmInputProfiles中是否同一个，如果相等继续 sp<AudioPort> sink = route->getSink(); if (sink == 0 || stream != sink) { ALOGE("%s: Invalid route attached to input stream", __FUNCTION__); continue; } //先说下getRouteSourceDevices函数，找route下的source标签下的device，如果是source即输出设备，就存入sourceDevices集合 DeviceVector HwModule::getRouteSourceDevices(const sp<AudioRoute> &route) const { //DeviceVector : public SortedVector<sp<DeviceDescriptor> > DeviceVector sourceDevices; for (const auto& source : route->getSources()) { if (source->getType() == AUDIO_PORT_TYPE_DEVICE) { sourceDevices.add(mDeclaredDevices.getDeviceFromTagName(source->getTagName())); } } return sourceDevices; } //继续看sourceDevicesForRoute 我们知道是route标签source属性里所有输出device DeviceVector sourceDevicesForRoute = getRouteSourceDevices(route); if (sourceDevicesForRoute.isEmpty()) { ALOGE("%s: invalid source devices for %s", __FUNCTION__, stream->getName().string()); continue; } sourceDevices.add(sourceDevicesForRoute); } if (sourceDevices.isEmpty()) { ALOGE("%s: invalid source devices for %s", __FUNCTION__, stream->getName().string()); continue; } //将这些输出devices关联到inputProfile上，作为inputProfile的支持devices stream->setSupportedDevices(sourceDevices); } //同理遍历mOutputProfiles，找到mOutputProfiles里和routes里匹配的mOutputProfile对应的route，将route里sink标签里是输入的devices，作为mOutputProfile支持的输入device for (const auto& stream : mOutputProfiles) { DeviceVector sinkDevices; for (const auto& route : stream->getRoutes()) { sp<AudioPort> source = route->getSources().findByTagName(stream->getTagName()); if (source == 0 || stream != source) { ALOGE("%s: Invalid route attached to output stream", __FUNCTION__); continue; } sp<DeviceDescriptor> sinkDevice = getRouteSinkDevice(route); if (sinkDevice == 0) { ALOGE("%s: invalid sink device for %s", __FUNCTION__, stream->getName().string()); continue; } sinkDevices.add(sinkDevice); } stream->setSupportedDevices(sinkDevices); }}//到此还未结束，回到module标签的开始会发现<attachedDevices>和<defaultOutputDevice>标签还未解析，继续 const xmlNode *children = root->xmlChildrenNode; while (children != NULL) { if (!xmlStrcmp(children->name, (const xmlChar *)childAttachedDevicesTag)) { ALOGV("%s: %s %s found", __FUNCTION__, tag, childAttachedDevicesTag); const xmlNode *child = children->xmlChildrenNode; while (child != NULL) { if (!xmlStrcmp(child->name, (const xmlChar *)childAttachedDeviceTag)) { xmlChar *attachedDevice = xmlNodeListGetString(doc, child->xmlChildrenNode, 1); if (attachedDevice != NULL) { ALOGV("%s: %s %s=%s", __FUNCTION__, tag, childAttachedDeviceTag, (const char*)attachedDevice); //解析<attachedDevices>标签找到和device标签下name相同的DeviceDescriptor sp<DeviceDescriptor> device = module->getDeclaredDevices().getDeviceFromTagName(String8((const char*)attachedDevice)); //ctx即audioPolicyConfig ctx->addAvailableDevice(device); //看下addAvailableDevice这个函数，将<attachedDevices>标签里的device分到mAvailableOutputDevices和mAvailableInputDevices中 void addAvailableDevice(const sp<DeviceDescriptor> &availableDevice) { if (audio_is_output_device(availableDevice->type())) { mAvailableOutputDevices.add(availableDevice); } else if (audio_is_input_device(availableDevice->type())) { mAvailableInputDevices.add(availableDevice); } } xmlFree(attachedDevice); } } child = child->next; } } //同理解析<defaultOutputDevice>后通过AudioPolicyConfig设置下默认的输出设备即mDefaultOutputDevices if (!xmlStrcmp(children->name, (const xmlChar *)childDefaultOutputDeviceTag)) { xmlChar *defaultOutputDevice = xmlNodeListGetString(doc, children->xmlChildrenNode, 1);; if (defaultOutputDevice != NULL) { ALOGV("%s: %s %s=%s", __FUNCTION__, tag, childDefaultOutputDeviceTag, (const char*)defaultOutputDevice); sp<DeviceDescriptor> device = module->getDeclaredDevices().getDeviceFromTagName(String8((const char*)defaultOutputDevice)); if (device != 0 && ctx->getDefaultOutputDevice() == 0) { ctx->setDefaultOutputDevice(device); ALOGV("%s: default is %08x", __FUNCTION__, ctx->getDefaultOutputDevice()->type()); } xmlFree(defaultOutputDevice); } } children = children->next; } return NO_ERROR;}最终解析完的所有module， config.setHwModules(modules)设置下去。到此基本就差不多了，剩下以下的的原理一样就不说了。
// deserialize volume section deserializeCollection<VolumeTraits>(doc, cur, volumes, &config);//// Global ConfigurationGlobalConfigTraits::deserialize(cur, config);总结
整个xml文件就解析完成了，下一章结合具体的audio_policy_configuration.xml在说下解析过程，如果有任何问题欢迎沟通指正。