本文主要是从宏观上把握Android MediaPlayer的架构，尽量做到简洁，希望对感兴趣的朋友有所帮助。
（1）BBinder/BpBinder类图
说明：
a. 需要使用Android智能指针管理对象生命周期的类必须继承自Refbase；
b. BBinder用于server端，BpBinder用于client端。
注：关于Binder的知识，参见Android Binder之经典好文
（2）BnInterface/BpInterface类图
（3）MediaPlayerService类图
说明：
a. BnMediaPlayerService 为MediaPlayerService在server端的具体实现；
b. BpMediaPlayerService为MediaPlayerService在client端的具体代理；
c. 通过BpMediaPlayerService（通过ServiceManager可以获取到BpMediaPlayerService）可以享用MediaPlayerService所提供的服务（具体由BnMediaPlayerService实际完成）
d. 通过BpMediaPlayerService可以获取到IMediaRecorder、IMediaMetadataRetriever、IMediaPlayer、IOMX在client端的代理类BpXXX（IMediaRecorder、IMediaMetadataRetriever、IMediaPlayer、IOMX，这些皆是Bp和Bn的共同接口，和MediaPlayerService一样，都有Bp和Bn构成）。
（4）MediaPlayer类图
说明：
a. BpMediaPlayer为通过BpMediaPlayerService获取到的MediaPlayer在client端的代理类；
b. MediaPlayerService::Client为MediaPlayer在server的具体实现类。
（5）MediaPlayerClient类图
说明：
a. IMediaPlayerClient由BpMediaPlayerClien和BnMediaPlayerClien组成；
b. BnMediaPlayerService 和 MediaPlayerService::Client（即BnMediaPlayer）通过BpMediaPlayerClien向BnMediaPlayerClien发送消息。
// IMediaPlayerService.h
class IMediaPlayerService: public IInterface
{
public:
DECLARE_META_INTERFACE(MediaPlayerService);
virtual sp<IMediaRecorder> createMediaRecorder(pid_t pid) = 0;
virtual sp<IMediaMetadataRetriever> createMetadataRetriever(pid_t pid) = 0;
virtual sp<IMediaPlayer> create(pid_t pid, const sp<IMediaPlayerClient>& client, int audioSessionId = 0) = 0;
};
// ----------------------------------------------------------------------------
class BnMediaPlayerService: public BnInterface<IMediaPlayerService>
{
public:
virtual status_t onTransact( uint32_t code,
const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
// IMediaPlayerService.cpp
class BpMediaPlayerService: public BpInterface<IMediaPlayerService>
{
};
IMPLEMENT_META_INTERFACE(MediaPlayerService, "android.media.IMediaPlayerService");
// MediaPlayerService.h
class MediaPlayerService : public BnMediaPlayerService
{
};
// IMediaPlayer.h
class IMediaPlayer: public IInterface
{
public:
DECLARE_META_INTERFACE(MediaPlayer);
virtual status_t setDataSource(const char *url,
const KeyedVector<String8, String8>* headers) = 0;
virtual status_t prepareAsync() = 0;
virtual status_t start() = 0;
virtual status_t stop() = 0; // 后面还有一系列关于player的接口函数
};
// ----------------------------------------------------------------------------
class BnMediaPlayer: public BnInterface<IMediaPlayer>
{
public:
virtual status_t onTransact( uint32_t code,
const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
// IMediaPlayer.cpp
class BpMediaPlayer: public BpInterface<IMediaPlayer>
{
public:
BpMediaPlayer(const sp<IBinder>& impl)
: BpInterface<IMediaPlayer>(impl)
{
}
// disconnect from media player service
void disconnect()
{
Parcel data, reply;
data.writeInterfaceToken(IMediaPlayer::getInterfaceDescriptor());
remote()->transact(DISCONNECT, data, &reply);
}
status_t setDataSource(const char* url,
const KeyedVector<String8, String8>* headers)
{
Parcel data, reply;
data.writeInterfaceToken(IMediaPlayer::getInterfaceDescriptor());
data.writeCString(url);
if (headers == NULL) {
data.writeInt32(0);
} else {
// serialize the headers
data.writeInt32(headers->size());
for (size_t i = 0; i < headers->size(); ++i) {
data.writeString8(headers->keyAt(i));
data.writeString8(headers->valueAt(i));
}
}
remote()->transact(SET_DATA_SOURCE_URL, data, &reply);
return reply.readInt32();
}
// 后面还有一系列关于player的接口函数
};
IMPLEMENT_META_INTERFACE(MediaPlayer, "android.media.IMediaPlayer");
// MediaPlayerService.h
class MediaPlayerService::Client : public BnMediaPlayer
{
};
// MediaPlayer.h
class MediaPlayer : public BnMediaPlayerClient,
public virtual IMediaDeathNotifier
{
public:
MediaPlayer();
~MediaPlayer();
status_t setDataSource(
const char *url,
const KeyedVector<String8, String8> *headers);
status_t setListener(const sp<MediaPlayerListener>& listener);
status_t prepare();
status_t prepareAsync();
status_t start();
status_t stop(); // 后面还有一系列关于player的接口函数
private:
sp<IMediaPlayer> mPlayer;
};
// IMediaPlayerClient.h
class IMediaPlayerClient: public IInterface
{
public:
DECLARE_META_INTERFACE(MediaPlayerClient);
virtual void notify(int msg, int ext1, int ext2, const Parcel *obj) = 0;
};
// ----------------------------------------------------------------------------
class BnMediaPlayerClient: public BnInterface<IMediaPlayerClient>
{
public:
virtual status_t onTransact( uint32_t code,
const Parcel& data,
Parcel* reply,
uint32_t flags = 0);
};
// IMediaPlayerClient.cpp
class BpMediaPlayerClient: public BpInterface<IMediaPlayerClient>
{
public:
BpMediaPlayerClient(const sp<IBinder>& impl)
: BpInterface<IMediaPlayerClient>(impl)
{
}
virtual void notify(int msg, int ext1, int ext2, const Parcel *obj)
{
Parcel data, reply;
data.writeInterfaceToken(IMediaPlayerClient::getInterfaceDescriptor());
data.writeInt32(msg);
data.writeInt32(ext1);
data.writeInt32(ext2);
if (obj && obj->dataSize() > 0) {
data.appendFrom(const_cast<Parcel *>(obj), 0, obj->dataSize());
}
remote()->transact(NOTIFY, data, &reply, IBinder::FLAG_ONEWAY);
}
};
IMPLEMENT_META_INTERFACE(MediaPlayerClient, "android.media.IMediaPlayerClient");
// ----------------------------------------------------------------------
status_t BnMediaPlayerClient::onTransact(
uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
switch(code) {
case NOTIFY: {
CHECK_INTERFACE(IMediaPlayerClient, data, reply);
int msg = data.readInt32();
int ext1 = data.readInt32();
int ext2 = data.readInt32();
Parcel obj;
if (data.dataAvail() > 0) {
obj.appendFrom(const_cast<Parcel *>(&data), data.dataPosition(), data.dataAvail());
}
notify(msg, ext1, ext2, &obj);
return NO_ERROR;
} break;
default:
return BBinder::onTransact(code, data, reply, flags);
}
}
// MediaPlayer.h
class MediaPlayer : public BnMediaPlayerClient,
public virtual IMediaDeathNotifier
{
private:
sp<IMediaPlayer> mPlayer; // 用于保存BpMediaPlayer实例
};