MediaPlayer-MediaPlayerService-MediaPlayerService::Client的三角关系

• 摘要：1.MediaPlayer 是客户端2.MediaPlayerService和MediaPlayerService::Client是服务器端。 2.1MediaPlayerService实现IMediaPlayerService定义的业务逻辑,其主要功能是根据 MediaPlayer::setDataSource输入的URL调用create函数创建对应的 Player.2.2MediaPlayerService::Client实现IMediaPlayer定义的业务逻辑,其主要功能包括

• 1. MediaPlayer是客户端

   

  2. MediaPlayerService和MediaPlayerService::Client是服务器端。

   

  2.1 MediaPlayerService实现IMediaPlayerService定义的业务逻辑,其主要功能是根据MediaPlayer::setDataSource输入的URL调用create函数创建对应的Player.

   

  2.2 MediaPlayerService::Client实现IMediaPlayer定义的业务逻辑,其主要功能包括start, stop, pause, resume...,其实现方法是调用MediaPlayerService create的Player中的对应方法来实现具体功能。

   

  2.3 它们的三角恋如下图所示:

   

  

   

  3. IXX是什么东东?

   

  在一个BnXXX或BpXXX都派生于两个类,具体情况如下:

   

  class BpXXX : public IXXX, public BpRefBase

   

  class BnXXX : public IXXX, public BBinder

   

  3.1 看到了吧,BpXXX和BnXXX都派生于IXXX,哪IXXX又是做什么的呢? 明确地告诉你,定义业务逻辑,但在BpXXX与BnXXX中的实现方式不同:

   

  1) 在BpXXX中,把对应的binder_transaction_data打包之后通过BpRefBase中的mRemote(BpBinder)发送出去,并等待结果

   

  2) 在BnXXX中,实现对应的业务逻辑,通过调用BnXXX派生类中的方法来实现,如MediaPlayerService::Client

   

  懂了吧, IXXX定义业务逻辑

   

  3.2 哪另外一个做什么呢?

   

  从上图可以看出,是用来进行进程间通信用的。

   

  BpRefBase中有一个mRemote(BpBinder)用来与Binder驱动交互用的。

   

  BBinder是用来从Binder驱动中接收相关请求,并进行相关处理的。

  4. 不得不说的MediaPlayerService

   

  class MediaPlayerService : public BnMediaPlayerService

   

  它本身用来实现IMediaPlayerService中定义的业务逻辑,可以看看IMediaPlayerService到底定义了些什么东东?

  
  class IMediaPlayerService: public IInterface
  {
  public:
  DECLARE_META_INTERFACE(MediaPlayerService);
  virtual sp createMediaRecorder(pid_t pid) = 0;
  virtual sp createMetadataRetriever(pid_t pid) = 0;
  virtual sp create(pid_t pid, const sp &; client, const char* url, const KeyedVector *headers = NULL) = 0;
  virtual sp create(pid_t pid, const sp &; client, int fd, int64_t offset, int64_t length) = 0;
  virtual sp createMediaConfig(pid_t pid ) = 0;
  virtual sp decode(const char* url, uint32_t *pSampleRate, int* pNumChannels, int* pFormat) = 0;
  virtual sp decode(int fd, int64_t offset, int64_t length, uint32_t *pSampleRate, int* pNumChannels, int* pFormat) = 0;
  virtual sp getOMX() = 0;
  // Take a peek at currently playing audio, for visualization purposes.
  // This returns a buffer of 16 bit mono PCM data, or NULL if no visualization buffer is currently available.
  virtual sp snoop() = 0;
  };

  从上面可以看出,还说自己是一个MediaPlayerService,而且还向大内总管ServiceManager进行了注册,就这么个create 和decode,能做什么呢? 我们最熟悉的start ,stop, pause, resume...怎么不见呢?没有这些东东,MediaPlayer需要的功能如何实现呢?

   

  带着这么多疑问,我们先看看MediaPlayerService是如何实现上述功能的,它到底做了些什么?

   

  还是让代码自己说话吧:

  
  sp MediaPlayerService::create(
  pid_t pid, const sp &; client, const char* url,
  const KeyedVector *headers)
  {
  int32_t connId = android_atomic_inc(&;mNextConnId);
  sp c = new Client(this, pid, connId, client);
  LOGV("Create new client(%d) from pid %d, url=%s, connId=%d", connId, pid, url, connId);
  if (NO_ERROR != c->setDataSource(url, headers))
  {
  c.clear();
  return c;
  }
  wp w = c;
  Mutex::Autolock lock(mLock);
  mClients.add(w);
  return c;
  }

  原来如此,它负责创建MediaPlayer需要的对应功能,创建了Client(MediaPlayerService),MediaPlayerService::Client的定义如下:

   

  class Client : public BnMediaPlayer, 它实现了IMediaPlayer定义的业务逻辑,哪就看看IMediaPlayer定义了些什么:

  
  class IMediaPlayer: public IInterface
  {
  public:
  DECLARE_META_INTERFACE(MediaPlayer);
  virtual void disconnect() = 0;
  virtual status_t setVideoSurface(const sp &; surface) = 0;
  virtual status_t setSurfaceXY(int x,int y) = 0;/*add by h00186041 add interface for surface left-up position*/
  virtual status_t setSurfaceWH(int w,int h) = 0;/*add by h00186041add interface for surface size*/
  virtual status_t prepareAsync() = 0;
  virtual status_t start() = 0;
  virtual status_t stop() = 0;
  virtual status_t pause() = 0;
  virtual status_t isPlaying(bool* state) = 0;
  virtual status_t seekTo(int msec) = 0;
  virtual status_t getCurrentPosition(int* msec) = 0;
  virtual status_t getDuration(int* msec) = 0;
  virtual status_t reset() = 0;
  virtual status_t setAudioStreamType(int type) = 0;
  virtual status_t setLooping(int loop) = 0;
  virtual status_t setVolume(float leftVolume, float rightVolume) = 0;
  virtual status_t suspend() = 0;
  virtual status_t resume() = 0;
  // Invoke a generic method on the player by using opaque parcels
  // for the request and reply.
  // @param request Parcel that must start with the media player
  // interface token.
  // @param[out] reply Parcel to hold the reply data. Cannot be null.
  // @return OK if the invocation was made successfully.
  virtual status_t invoke(const Parcel&; request, Parcel *reply) = 0;
  // Set a new metadata filter.
  // @param filter A set of allow and drop rules serialized in a Parcel.
  // @return OK if the invocation was made successfully.
  virtual status_t setMetadataFilter(const Parcel&; filter) = 0;
  // Retrieve a set of metadata.
  // @param update_only Include only the metadata that have changed
  //since the last invocation of getMetadata.
  //The set is built using the unfiltered
  //notifications the native player sent to the
  //MediaPlayerService during that period of
  //time. If false, all the metadatas are considered.
  // @param apply_filter If true, once the metadata set has been built based
  // on the value update_only, the current filter is
  // applied.
  // @param[out] metadata On exit contains a set (possibly empty) of metadata.
  //Valid only if the call returned OK.
  // @return OK if the invocation was made successfully.
  virtual status_t getMetadata(bool update_only,
  bool apply_filter,
  Parcel *metadata) = 0;
  }

  终于找到了思念已久的start, stop, pause, resume...

   

  所以MediaPlayer::setDataSource返回时,会创建一个与MediaPlayerService::Client对应的BpMediaPlayer,用于获取MediaPlayerService::Client的各项功能。

   

  4. 1 MediaPlayer又是如何找到MediaPlayerService::Client的呢? 只有MediaPlayerService才向ServiceManager进行了注册,所以MediaPlayer必须先获取 BpMediaPlayerService,然后通过BpMediaService的管理功能create,来创建一个 MediaPlayerService::Client.

   

  4.2 为什么不直接定义一个MediaPlayer向ServiceManager注册呢?

   

  也 许是为了系统简化吧,MediaPlayerService包含的功能不只是Client, 还有AudioOutput,AudioCache,MediaConfigClient功能。现在明白了吧,MediaPlayerService就是 一个媒体服务的接口,它先把客人接回来,再根据客人的需求,安排不同的员工去做,就这么简单。