Android学习（七）由MediaPlayer切入（3）

由MediaPlayer切入（3）

继续MediaPlayer的分析：

/*\frameworks\av\media\libmedia\mediaplayer.cpp*/

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length) {     ALOGV("setDataSource(%d, %lld, %lld)", fd, offset, length);     status_t err = UNKNOWN_ERROR;     const sp<IMediaPlayerService>& service(getMediaPlayerService());     if (service != 0) {         sp<IMediaPlayer> player(service->create(this, mAudioSessionId));         if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||             (NO_ERROR != player->setDataSource(fd, offset, length))) {             player.clear();         }         err = attachNewPlayer(player);     }     return err; }

通过getMediaPlayerService()的调用，得到了一个BpMediaPlayerService对象了。

接下来分析service->create(this, mAudioSessionId)：

virtual sp<IMediaPlayer> create(             const sp<IMediaPlayerClient>& client, int audioSessionId) {         Parcel data, reply;         data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());         data.writeStrongBinder(client->asBinder());         data.writeInt32(audioSessionId);           remote()->transact(CREATE, data, &reply);         return interface_cast<IMediaPlayer>(reply.readStrongBinder());     }

同样是调用remote()->transact(CREATE, data, &reply);只不过现在BpBinder的handle值是MediaPlayerService的handle值，发送的指令也变成了CREATE：

/*\Android4.4\frameworks\native\libs\binder\BpBinder.cpp*/

status_t BpBinder::transact(     uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags) {     // Once a binder has died, it will never come back to life.     if (mAlive) {         status_t status = IPCThreadState::self()->transact(             mHandle, code, data, reply, flags);         if (status == DEAD_OBJECT) mAlive = 0;         return status;     }       return DEAD_OBJECT; }

看看CREATE的定义：

enum {     CREATE = IBinder::FIRST_CALL_TRANSACTION,     DECODE_URL,     DECODE_FD,     CREATE_MEDIA_RECORDER,     CREATE_METADATA_RETRIEVER,     GET_OMX,     MAKE_CRYPTO,     MAKE_DRM,     MAKE_HDCP,     ADD_BATTERY_DATA,     PULL_BATTERY_DATA,     LISTEN_FOR_REMOTE_DISPLAY,     UPDATE_PROXY_CONFIG, };

可见CREATE值为1。

status_t IPCThreadState::executeCommand(int32_t cmd) {     case BR_TRANSACTION:         {             binder_transaction_data tr;             result = mIn.read(&tr, sizeof(tr));             ALOG_ASSERT(result == NO_ERROR,                 "Not enough command data for brTRANSACTION");             if (result != NO_ERROR) break;                         Parcel buffer;à定义一个buff，然后填充数据             buffer.ipcSetDataReference(                 reinterpret_cast<const uint8_t*>(tr.dataNaNr.buffer),                 tr.data_size,                 reinterpret_cast<const size_t*>(tr.dataNaNr.offsets),                 tr.offsets_size/sizeof(size_t), freeBuffer, this);                                                             Parcel reply;               if (tr.targetNaNr) {                 sp<BBinder> b((BBinder*)tr.cookie);                 const status_t error = b->transact(tr.code, buffer, &reply, tr.flags);                 if (error < NO_ERROR) reply.setError(error);               } else {                 const status_t error = the_context_object->transact(tr.code, buffer, &reply, tr.flags);                 if (error < NO_ERROR) reply.setError(error);             }                        if ((tr.flags & TF_ONE_WAY) == 0) {                 LOG_ONEWAY("Sending reply to %d!", mCallingPid);                 sendReply(reply, 0);             } else {                 LOG_ONEWAY("NOT sending reply to %d!", mCallingPid);             }                         mCallingPid = origPid;             mCallingUid = origUid;                     }         break;

 

tr.targetNaNr为NULL默认the_context_object（暂时不知道哪里来的）来处理；不为NULL，将tr.cookie强制转换为BBinder，由此可见BBinder就是Server与IPCThreadState沟通的桥梁。

先看看MediaPlayerService的注册过程：

int main(int argc, char** argv) {         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(); }

 

void MediaPlayerService::instantiate() {     defaultServiceManager()->addService(             String16("media.player"), new MediaPlayerService()); }

这个地方已经new MediaPlayerService ()一个服务对象。

而看看MediaPlayerService的定义：

\Android4.4\frameworks\av\media\libmediaplayerservice\ MediaPlayerService.cpp

class MediaPlayerService : public BnMediaPlayerService { } class BnMediaPlayerService: public BnInterface<IMediaPlayerService> { } 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(); };

可见MediaPlayerService就是一个BBinder。

class BnMediaPlayerService: public BnInterface<IMediaPlayerService> { public:     virtual status_t    onTransact( uint32_t code,                                     const Parcel& data,                                     Parcel* reply,                                     uint32_t flags = 0); };

然后用onTransact做回调。

到此为止，虽然很多地方还没有了解清楚，但基本知道了BpBinder、BnBinder、BBinder的区别和联系，了解了ServiceManager的工作流程，也就是基本了解了android中Binder的通信原理。