初窺Android AudioFlinger

         Android  AudioFlinger 是Android音頻系統的兩大服務之一，另外一個服務是AudioPolicyService，這兩大服務都在系統啓動時有MediaSever加載，加載的代碼位於：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();
    }
}

這裏只說下 AudioFlinger::instantiate();的實現。

instantiate()函數調用位於：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(); }
    ........
}

剛看到此段源碼時候有點疑惑，爲何是調用BinderService類的instantiate()方法的？後來找了AudioFlinger的類定義，發現是繼承關係。

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 設計爲模版類，並由AudioFlinger繼承。回頭看下BinderService 的static status_t publish(bool allowIsolated = false)方法。

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

所以 AudioFlinger::instantiate(); 能夠總結爲：
setp 1 ： 調用父類BinderService<AudioFlinger>的instantiate（）函數。
setp 2 ： 調用父類BinderService<AudioFlinger>的publish()函數，真正的建立了AudioFlinger實例，並添加到servicemanager的服務列表中。