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

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<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, const char* url, const KeyedVector<String8, String8> *headers = NULL) = 0;
    virtual sp<IMediaPlayer>    create(pid_t pid, const sp<IMediaPlayerClient>& client, int fd, int64_t offset, int64_t length) = 0;
    virtual sp<IMediaConfig>    createMediaConfig(pid_t pid ) = 0;
    virtual sp<IMemory>         decode(const char* url, uint32_t *pSampleRate, int* pNumChannels, int* pFormat) = 0;
    virtual sp<IMemory>         decode(int fd, int64_t offset, int64_t length, uint32_t *pSampleRate, int* pNumChannels, int* pFormat) = 0;
    virtual sp<IOMX>            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<IMemory>         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<IMediaPlayer> MediaPlayerService::create(
        pid_t pid, const sp<IMediaPlayerClient>& client, const char* url,
        const KeyedVector<String8, String8> *headers)
{
    int32_t connId = android_atomic_inc(&mNextConnId);
    sp<Client> 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<Client> 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<ISurface>& 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 h00186041  add 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。