初窥Android AudioFlinger

Android  AudioFlinger 是Android音频系统的两大服务之一，另一个服务是AudioPolicyService，这两大服务都在系统启动时有MediaSever加载，加载的代码位于：frameworks/base/media/mediaserver/main_mediaserver.cpp。

int main(int argc, char** argv)
{
    signal(SIGPIPE, SIG_IGN);
    char value[PROPERTY_VALUE_MAX];
    ..........
        sp<ProcessState> proc(ProcessState::self());
        sp<IServiceManager> sm = defaultServiceManager();
        ALOGI("ServiceManager: %p", sm.get());
        AudioFlinger::instantiate();
        MediaPlayerService::instantiate();
        CameraService::instantiate();
        AudioPolicyService::instantiate();
        registerExtensions();
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
    }
}

这里只说下 AudioFlinger::instantiate();的实现。

instantiate()函数调用位于：framework/base/native/include/binder.h

template<typename SERVICE>
class BinderService
{
public:
    static status_t publish(bool allowIsolated = false) {
        sp<IServiceManager> sm(defaultServiceManager());
        return sm->addService(
                String16(SERVICE::getServiceName()),
                new SERVICE(), allowIsolated);
    }

    ........
    static void instantiate() { publish(); }
    ........
}

刚看到此段源码时候有点疑惑，为什么是调用BinderService类的instantiate()方法的？后来找了AudioFlinger的类定义，发现是继承关系。

class AudioFlinger :
    public BinderService<AudioFlinger>,
    public BnAudioFlinger
{
    friend class BinderService<AudioFlinger>;   // for AudioFlinger()
public:
    static const char* getServiceName() ANDROID_API { return "media.audio_flinger"; }

    virtual     status_t    dump(int fd, const Vector<String16>& args);
    ........
｝

step 1. BinderService 设计为模版类，并由AudioFlinger继承。回头看下BinderService 的static status_t publish(bool allowIsolated = false)方法。

sm->addService(
                String16(SERVICE::getServiceName()),
                new SERVICE(), allowIsolated);
BinderService<AudioFlinger> 兑换后
sm->addService(
                String16(AudioFlinger::getServiceName()),
                new AudioFlinger(), allowIsolated);

因此 AudioFlinger::instantiate(); 可以总结为：
setp 1 ： 调用父类BinderService<AudioFlinger>的instantiate（）函数。
setp 2 ： 调用父类BinderService<AudioFlinger>的publish()函数，真正的创建了AudioFlinger实例，并添加到servicemanager的服务列表中。