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根据前面的分析可知,MediaPlayerService驻留在MediaServer进程中,这个进程有两个线程在talkWithDriver。假设其中有一个线程收到了请求,它最终会通过executeCommand调用来处理这个请求,实现代码如下所示: **IPCThreadState.cpp** ~~~ status_t IPCThreadState::executeCommand(int32_tcmd) { BBinder*obj; RefBase::weakref_type* refs; status_tresult = NO_ERROR; switch(cmd) { case BR_ERROR: result = mIn.readInt32(); break; ...... caseBR_TRANSACTION: { binder_transaction_data tr; result = mIn.read(&tr, sizeof(tr)); if (result != NO_ERROR) break; Parcel buffer; Parcel reply; if (tr.target.ptr) { /* 看到BBinder,想起图6-3了吗? BnServiceXXX从BBinder派生, 这里的b实际就是实现BnServiceXXX的那个对象,这样就直接定位到了业务层的对象。 */ sp<BBinder> b((BBinder*)tr.cookie); const status_t error = b->transact(tr.code, buffer, &reply, 0); if (error < NO_ERROR) reply.setError(error); } else { /* the_context_object是IPCThreadState.cpp中定义的一个全局变量。可通过 setTheContextObject函数设置。 */ const status_t error = the_context_object->transact(tr.code,buffer, &reply, 0); if (error < NO_ERROR) reply.setError(error); } break; ...... ~~~ BBinder和业务层有什么关系?还记得图6-3吗?我们以MediaPlayerService为例,来梳理一下其派生关系,如图6-5所示: :-: ![](http://img.blog.csdn.net/20150802155954109?watermark/2/text/aHR0cDovL2Jsb2cuY3Nkbi5uZXQv/font/5a6L5L2T/fontsize/400/fill/I0JBQkFCMA==/dissolve/70/gravity/Center) 图6-5 MediaPlayerService家谱 BnMediaPlayerService实现了onTransact函数,它将根据消息码调用对应的业务逻辑函数,这些业务逻辑函数由MediaPlayerService来实现。这一路的历程,如下面的代码所示: **Binder.cpp** ~~~ status_t BBinder::transact( uint32_tcode, const Parcel& data, Parcel* reply, uint32_t flags) { data.setDataPosition(0); status_terr = NO_ERROR; switch(code) { casePING_TRANSACTION: reply->writeInt32(pingBinder()); break; default: //调用子类的onTransact,这是一个虚函数。 err = onTransact(code, data, reply, flags); break; } if (reply!= NULL) { reply->setDataPosition(0); } returnerr; } ~~~ **IMediaPlayerService.cpp** ~~~ status_t BnMediaPlayerService::onTransact(uint32_tcode, const Parcel& data, Parcel* reply, uint32_t flags) { switch(code) { ...... caseCREATE_MEDIA_RECORDER: { CHECK_INTERFACE(IMediaPlayerService, data, reply); //从请求数据中解析对应的参数 pid_t pid = data.readInt32(); //子类要实现createMediaRecorder函数。 sp<IMediaRecorder> recorder = createMediaRecorder(pid); reply->writeStrongBinder(recorder->asBinder()); return NO_ERROR; }break; caseCREATE_METADATA_RETRIEVER: { CHECK_INTERFACE(IMediaPlayerService, data, reply); pid_t pid = data.readInt32(); //子类要实现createMetadataRetriever函数 sp<IMediaMetadataRetriever> retriever =createMetadataRetriever(pid); reply->writeStrongBinder(retriever->asBinder()); return NO_ERROR; }break; default: return BBinder::onTransact(code, data, reply, flags); } } ~~~ 通过上面的分析,我们是否能更清晰地理解了图6-3、图6-4和图6-5所表达的意义了呢?