Android Audio底层原理（二）

DSP

Android Audio底层原理（二）

2019-07-13 15:51发布生成海报

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1. Hal层类关系图

以MediaPlayer功能为例，我们可以根据Android 8.1 (Oreo)源代码绘制出其类图

2. IMediaPlayerService.h

定义相关API，方便binder进程调用。IMediaPlayerService理论上是MediaPlayerService的接口，故只定义API而把API具体实现都放在class MediaPlayerService中。

class IMediaPlayerService: public IInterface {    public:
      DECLARE_META_INTERFACE(MediaPlayerService);
      ... ...       virtual sp<IMediaPlayer> create(const sp<IMediaPlayerClient>& client, audio_session_t audioSessionId = AUDIO_SESSION_ALLOCATE) = 0; virtual sp<IOMX> getOMX() = 0;        virtual sp<IHDCP> makeHDCP(bool createEncryptionModule) = 0;        virtual sp<IMediaCodecList> getCodecList() const = 0; };
class BnMediaPlayerService: public BnInterface<IMediaPlayerService> {    public:        virtual status_t onTransact( uint32_t code,                                                    const Parcel& data,                                                    Parcel* reply,                                                    uint32_t flags = 0); };

2.1 BnMediaPlayerService IMediaPlayerService主要有两个子类，一个是BpMediaPlayerServcie类，另一个是BnMediaPlayerService类。 IMediaPlayerService.h中，可以看到BnMediaPlayerService继承模板类BnInterface。在IInterface.h中可看到BnInterface的定义如下：

template<typename INTERFACE>
class BnInterface : public INTERFACE, public BBinder
{
    public:
       virtual sp<IInterface>      queryLocalInterface(const String16& _descriptor);
         virtual const String16&     getInterfaceDescriptor() const;       protected:
         virtual IBinder*            onAsBinder();
   };

根据模板类的概念，BnMediaPlayerService相当于继承IMediaPlayerService和BBinder。同理，BpMediaPlayerService类相当于继承了IMediaPlayerService和BpRefBase类。

template<typename INTERFACE> class BpInterface : public INTERFACE, public BpRefBase { public: explicit BpInterface(const sp<IBinder>& remote); protected: virtual IBinder* onAsBinder(); };

2.2 DECLARE_META_INTERFACE( )
DECLARE_META_INTERFACE( )同样位于IInterface.h，同样是一个模板函数。

#define DECLARE_META_INTERFACE(INTERFACE)
      static const ::android::String16 descriptor;
    static ::android::spINTERFACE> asInterface( const ::android::sp<::android::IBinder>& obj);       virtual const ::android::String16& getInterfaceDescriptor() const;
      I##INTERFACE();
      virtual ~I##INTERFACE();

则DECLARE_META_INTERFACE( MediaPlayerService)可以翻译为：

      static const ::android::String16 descriptor;
    static ::android::sp asInterface( const ::android::sp<::android::IBinder>& obj);       virtual const ::android::String16& getInterfaceDescriptor() const;
      IMediaPlayerService();
      virtual ~IMediaPlayerService();                                  //释放IMediaPlayerService对象

可以总结为IMediaPlayerService主要包含了以下几个虚函数，待子类 class MediaPlayerService实现。 (1) sp<IMediaPlayer> create(...) //纯虚函数 (2) sp<IOMX> getOMX() //纯虚函数 (3) sp<IHDCP> makeHDCP(bool createEncryptionModule) //纯虚函数（4) sp<IMediaCodecList> getCodecList() const //纯虚函数

3. IMediaPlayerService.cpp

IMediaPlayer.cpp包含了class BpMediaPlayerService和class BnMediaPlayerService。 BpMediaPlayerService的主要设计满足代理模式(proxy)，可以视为代理类，隔断客户端(访问对象)和服务端(被访问对象)。客户端对象必须通过代理类才能访问到BnMediaPlayerService中对象。

BpMediaPlayerService代理类的主要代码如下，主要作用是remote->transact( )调用。通知服务端具体调用。

status_t BnMediaPlayerService::onTransact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) { switch (code) { case CREATE: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp<IMediaPlayerClient> client =interface_cast<IMediaPlayerClient(data.readStrongBinder()); audio_session_t audioSessionId = (audio_session_t) data.readInt32(); sp<IMediaPlayer> player = create(client, audioSessionId); reply->writeStrongBinder(IInterface::asBinder(player)); return NO_ERROR; } break; ... ... case GET_OMX: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp<IOMX> omx = getOMX(); reply->writeStrongBinder(IInterface::asBinder(omx)); return NO_ERROR; } break; case MAKE_HDCP: { CHECK_INTERFACE(IMediaPlayerService, data, reply); bool createEncryptionModule = data.readInt32(); sp<IHDCP> hdcp = makeHDCP(createEncryptionModule); reply->writeStrongBinder(IInterface::asBinder(hdcp)); return NO_ERROR; } break; ... ...
case LISTEN_FOR_REMOTE_DISPLAY: { CHECK_INTERFACE(IMediaPlayerService, data, reply); const String16 opPackageName = data.readString16(); sp<IRemoteDisplayClient> client(interface_cast<IRemoteDisplayClient>(data.readStrongBinder())); if (client == NULL) { reply->writeStrongBinder(NULL); return NO_ERROR; } String8 iface(data.readString8()); sp<IRemoteDisplay> display(listenForRemoteDisplay(opPackageName, client, iface)); reply->writeStrongBinder(IInterface::asBinder(display)); return NO_ERROR; } break; case GET_CODEC_LIST: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp<IMediaCodecList> mcl = getCodecList(); reply->writeStrongBinder(IInterface::asBinder(mcl)); return NO_ERROR; } break; default: return BBinder::onTransact(code, data, reply, flags); } }

BnMediaPlayerService作为实现类，重写onTransact( )函数，处理proxy发来的函数调用。

status_t BnMediaPlayerService::onTransact( uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) { switch (code) { case CREATE: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp<IMediaPlayerClient> client =interface_cast<IMediaPlayerClient(data.readStrongBinder()); audio_session_t audioSessionId = (audio_session_t) data.readInt32(); sp<IMediaPlayer> player = create(client, audioSessionId); reply->writeStrongBinder(IInterface::asBinder(player)); return NO_ERROR; } break; ... ... case GET_OMX: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp<IOMX> omx = getOMX(); reply->writeStrongBinder(IInterface::asBinder(omx)); return NO_ERROR; } break; case MAKE_HDCP: { CHECK_INTERFACE(IMediaPlayerService, data, reply); bool createEncryptionModule = data.readInt32(); sp<IHDCP> hdcp = makeHDCP(createEncryptionModule); reply->writeStrongBinder(IInterface::asBinder(hdcp)); return NO_ERROR; } break; ... ...
case LISTEN_FOR_REMOTE_DISPLAY: { CHECK_INTERFACE(IMediaPlayerService, data, reply);