Android(安卓)Audio底层原理（二）
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 create(const sp& client, audio_session_taudioSessionId = AUDIO_SESSION_ALLOCATE) = 0; virtual sp getOMX() = 0; virtual sp makeHDCP(bool createEncryptionModule) = 0; virtual sp getCodecList() const = 0; };class BnMediaPlayerService: public BnInterface { 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 class BnInterface : public INTERFACE, public BBinder { public: virtual sp 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。
templateclass BpInterface : public INTERFACE, public BpRefBase { public: explicit BpInterface(const sp& 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::sp 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(); //析构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
create(...) //纯虚函数 (2) sp
getOMX() //纯虚函数 (3) sp
makeHDCP(bool createEncryptionModule) //纯虚函数 （4) sp
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 client=interface_cast player = create(client, audioSessionId); reply->writeStrongBinder(IInterface::asBinder(player)); return NO_ERROR; } break; ... ... case GET_OMX: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp omx = getOMX(); reply->writeStrongBinder(IInterface::asBinder(omx)); return NO_ERROR; } break; case MAKE_HDCP: { CHECK_INTERFACE(IMediaPlayerService, data, reply); bool createEncryptionModule = data.readInt32(); sp 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 client(interface_cast(data.readStrongBinder())); if (client == NULL) { reply->writeStrongBinder(NULL); return NO_ERROR; } String8 iface(data.readString8()); sp display(listenForRemoteDisplay(opPackageName, client,iface)); reply->writeStrongBinder(IInterface::asBinder(display)); return NO_ERROR; } break; case GET_CODEC_LIST: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp 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 client =interface_cast player = create(client, audioSessionId); reply->writeStrongBinder(IInterface::asBinder(player)); return NO_ERROR; } break; ... ... case GET_OMX: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp omx = getOMX(); reply->writeStrongBinder(IInterface::asBinder(omx)); return NO_ERROR; } break; case MAKE_HDCP: { CHECK_INTERFACE(IMediaPlayerService, data, reply); bool createEncryptionModule = data.readInt32(); sp 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 client(interface_cast(data.readStrongBinder())); if (client == NULL) { reply->writeStrongBinder(NULL); return NO_ERROR; } String8 iface(data.readString8()); sp display(listenForRemoteDisplay(opPackageName, client, iface)); reply->writeStrongBinder(IInterface::asBinder(display)); return NO_ERROR; } break; case GET_CODEC_LIST: { CHECK_INTERFACE(IMediaPlayerService, data, reply); sp mcl = getCodecList(); reply->writeStrongBinder(IInterface::asBinder(mcl)); return NO_ERROR; } break; default: return BBinder::onTransact(code, data, reply, flags); } } 例如，当proxy发来调用服务端的getCodecList( )函数(getCodecList()函数为获取编码器列表)。则调用
remote()->transact(GET_CODEC_LIST,data,&reply);经过binder驱动，会回调至onTransact( )，处理客户端需求。如GET_CODEC_LIST满足case GET_CODEC_LIST，则执行
而IMediaPlayerService类中的create( )，makeDHCP( )，getCodecList( )具体实现不在IMediaPlayerService类中。
writeStrongBinder(IInterface::asBinder(mcl))
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