Android 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<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::sp<I##INTERFACE> 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<IMediaPlayerService> 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。

class BpMediaPlayerService: public BpInterface<IMediaPlayerService> {

public:

explicit BpMediaPlayerService(const sp<IBinder>& impl):BpInterface<IMediaPlayerService>(impl){ }

... ...

virtual sp<IMediaPlayer> create(const sp<IMediaPlayerClient>& client, audio_session_t, audioSessionId) {

Parcel data, reply;
data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());

data.writeStrongBinder(IInterface::asBinder(client));

data.writeInt32(audioSessionId); remote()->transact(CREATE, data, &reply);

return interface_cast<IMediaPlayer>(reply.readStrongBinder());
}

virtual sp<IOMX> getOMX() {

Parcel data, reply;

data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());

remote()->transact(GET_OMX, data, &reply);

return interface_cast<IOMX>(reply.readStrongBinder());

}

virtual sp<IHDCP> makeHDCP(bool createEncryptionModule) {

Parcel data, reply;

data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());

data.writeInt32(createEncryptionModule);

remote()->transact(MAKE_HDCP, data, &reply);

return interface_cast<IHDCP>(reply.readStrongBinder());

}

... ...

virtual sp<IRemoteDisplay> listenForRemoteDisplay(const String16 &opPackageName, const sp<IRemoteDisplayClient>& client, const String8& iface){

Parcel data, reply;

data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());

data.writeString16(opPackageName);

data.writeStrongBinder(IInterface::asBinder(client));

data.writeString8(iface);

remote()->transact(LISTEN_FOR_REMOTE_DISPLAY, data, &reply);

return interface_cast<IRemoteDisplay>(reply.readStrongBinder());

}

virtual sp<IMediaCodecList> getCodecList() const {

Parcel data, reply;

data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());

remote()->transact(GET_CODEC_LIST, data, &reply);

return interface_cast<IMediaCodecList>(reply.readStrongBinder());

}

};

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);

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);

}

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

writeStrongBinder(IInterface::asBinder(mcl))

而IMediaPlayerService类中的create( )，makeDHCP( )，getCodecList( )具体实现不在IMediaPlayerService类中。