MediaPlayer(三)--C++ binder框架
MediaPlayer(三)--C++ binder框架
结构框架
上一篇文章MediaPlayer--MediaPlayer基本框架没有将C++的mediaplayer binder通讯展开，这篇文章主要展开这一部分。
大概画了下C++ MediaPlayer的uml图，不是很满意，请高手指教
这里面主要的client和service实现类是以下两个文件(Android8.1 路径)
frameworks/av/media/libmedia/mediaplayer.cpp
frameworks/av/media/libmediaplayerservice/MediaPlayerService.cpp
这里面涉及了三对IPC的通讯，即
IMediaPlayerClient
IMediaPlayerService
IMediaPlayer
其中 IMediaPlayerService ， IMediaPlayer 的服务端在MediaPlayerService.cpp， 客户端在mediaplayer.cpp。IMediaPlayerClient 的服务端在mediaplayer.cpp， 客户端在MediaPlayerService.cpp， 这是MediaPlayerService回调mediaplayer使用的。 IMediaPlayerService会创建IMediaPlayer的实例
流程
MediaPlayer 同MediaPlayerService通讯 是通过变量 sp<IMediaPlayer> mPlayer, mPlayer赋值是在setDataSource
status_t MediaPlayer::setDataSource(
const sp<IMediaHTTPService> &httpService,
const char *url, const KeyedVector<String8, String8> *headers)
{
ALOGV("setDataSource(%s)", url);
status_t err = BAD_VALUE;
if (url != NULL) {
const sp<IMediaPlayerService> service(getMediaPlayerService());
if (service != 0) {
sp<IMediaPlayer> player(service->create(this, mAudioSessionId));
if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
(NO_ERROR != player->setDataSource(httpService, url, headers))) {
player.clear();
}
err = attachNewPlayer(player);
}
}
return err;
}关于创建mPlayer这块主要分3步：
1 const sp<IMediaPlayerService> service(getMediaPlayerService()); 这一步其实是获取了BpMediaPlayerService
2 sp<IMediaPlayer> player(service->create(this, mAudioSessionId)); 这一步是通过BpMediaPlayerService 的create方法返回了BpMediaPlayer
3 attachNewPlayer 将player赋值给mPlayer
看一下 1 和 2 的两个函数的实现
/*static*/const sp<IMediaPlayerService>
IMediaDeathNotifier::getMediaPlayerService()
{
ALOGV("getMediaPlayerService");
Mutex::Autolock _l(sServiceLock);
if (sMediaPlayerService == 0) {
sp<IServiceManager> sm = defaultServiceManager();
sp<IBinder> binder;
do {
binder = sm->getService(String16("media.player"));
if (binder != 0) {
break;
}
ALOGW("Media player service not published, waiting...");
usleep(500000); // 0.5 s
} while (true);
if (sDeathNotifier == NULL) {
sDeathNotifier = new DeathNotifier();
}
binder->linkToDeath(sDeathNotifier);
sMediaPlayerService = interface_cast<IMediaPlayerService>(binder);
}
ALOGE_IF(sMediaPlayerService == 0, "no media player service!?");
return sMediaPlayerService;
}这里简单讲一下
1 sp<IServiceManager> sm = defaultServiceManager(); 获取到ServiceManager
2 sm->getService(String16("media.player")); 通过ServiceManager获取到 MediaPlayerService，MediaPlayerService在android初始化时就向ServiceManager注册了
3 interface_cast<IMediaPlayerService>(binder); 这里实际是new BpMediaPlayerService(binder)
class BpMediaPlayerService: public BpInterface<IMediaPlayerService>
{
public:
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());
}create方法做了两件事
1 remote()->transact(CREATE, data, &reply) 通过 IMediaPlayerService binder通讯，将IMediaPlayerClient的IBinder传给了MediaPlayerService， MediaPlayerServcie后面可以通过IMediaPlayerClient 回调MediaPlayer
2 interface_cast<IMediaPlayer>(reply.readStrongBinder()); 上一步binder通讯MeidaPlayer的binder对象， 同过binder是创建了 BpMediaPlayer 实例
Binder可分为实名binder和匿名binder， 实名binder是在ServiceManager注册的servce，可以通过ServiceManager的getService获取，而匿名binder没在ServiceManager中注册，没法通过ServiceManager获取，需要想办法获取service的binder实例才能通讯。对于MediaPlayer的结构，MediaPlayerServcie 属于实名binder， IMediaPlayer 和 IMediaPlayerClient 属于匿名binder。MediaPlayer 通过 MediaPlayerServcie实名binder， 将IMediaPlayerClient binder 传递给MediaPlayerServcie，同时MediaPlayerServcie将IMediaPlayer binder返回MediaPlayer。 这样MediaPlayer 就能通过 IMediaPlayer 调用 MediaPlayerServcie::client接口， MediaPlayerServcie可以通过IMediaPlayerClient调用MediaPlayer接口
