Android音频流程二（Binder部分）

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2、libmedia MediaPlayer至MediaPlayerService （binder）

client和service之间的通信通过android binder机制，只是对于用户而言是透明的。

继续按照上面play调用流程，上面已经调用到了frameworks的mediaplayer.cpp方法start

status_t MediaPlayer::start(){    LOGV("start");    Mutex::Autolock _l(mLock);    if (mCurrentState & MEDIA_PLAYER_STARTED)        return NO_ERROR;    if ( (mPlayer != 0) && ( mCurrentState & ( MEDIA_PLAYER_PREPARED |                    MEDIA_PLAYER_PLAYBACK_COMPLETE | MEDIA_PLAYER_PAUSED ) ) ) {        mPlayer->setLooping(mLoop);        mPlayer->setVolume(mLeftVolume, mRightVolume);        mPlayer->setAuxEffectSendLevel(mSendLevel);        mCurrentState = MEDIA_PLAYER_STARTED;        status_t ret = mPlayer->start();        if (ret != NO_ERROR) {            mCurrentState = MEDIA_PLAYER_STATE_ERROR;        } else {            if (mCurrentState == MEDIA_PLAYER_PLAYBACK_COMPLETE) {                LOGV("playback completed immediately following start()");            }        }        return ret;    }    LOGE("start called in state %d", mCurrentState);    return INVALID_OPERATION;}

其中mPlayer是setDataSource时候创建的播放器的实例，调用到的方法setLooping setVolume等都是在frameworks/base/include/media的IMediaPlayer.h中声明

以Ixx开头的头文件是binder机制的接口文件，每个这种接口文件同级目录下都会有对应一个.cpp文件，这里对应的是IMediaPlayer.cpp，

这个C++文件定义binder的本地和代理函数，本地的是以Bn开头，native的意思，BnMediaPlyaer；代理的是以Bp开头，proxy的意思，BpMediaPlayer

这两个类之间调用是通过在kernel驱动的、在dev/下生成的一个binder程序来通信！代理端使用remote()->transact()，本地端响应请求onTransact()

代码如下：

class BpMediaPlayer: public BpInterface<IMediaPlayer>{public://...    status_t start()    {        Parcel data, reply;        data.writeInterfaceToken(IMediaPlayer::getInterfaceDescriptor());        remote()->transact(START, data, &reply);        return reply.readInt32();}//...}status_t BnMediaPlayer::onTransact(    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags){switch(code) {//...        case START: {            CHECK_INTERFACE(IMediaPlayer, data, reply);            reply->writeInt32(start());            return NO_ERROR;        } break;//...}//...}

播放器实例获得是通过setDataSource函数创建的，而这个函数会调用MediaPlayerService

const sp<IMediaPlayerService>& service(getMediaPlayerService());

再调用MediaPlayerService的creat和setDataSource

总结下上面流程应该是：

======================================================================

|Applications Music.apk

| |

|Framework MediaPlayer API

======================================================================

--JNI----------------------------------------IMediaPlayer--------------------------------------------------

| |

|libmedia MediaPlayer

| |

--Binder---------------------------------------BpXXX-------------------------------------------------------

-------------------------------------------------BnXXX-------------------------------------------------------

| |

|MediaServer MediaPlayerService

======================================================================

Binder在直观上看到是直接函数对应调用，实际上是有透过kernel空间的