[移动开发]android MediaPlayer架构

MediaPlayer C++实现在libmedia里面，和MediaPlayer.java对应起来，这两者之间通过jni实现android_media_MediaPlayer.cpp联系起来，setDataSource的时候，MediaPlayer会先获取MediaplayerService，然后通过service的create创建MediaPlayerService::Client，MediaPlayer中的
mPlayer就是创建的MediaPlayerService::Client对象。

BP(binder proxy)和BN(binder native)是通过binder来通信的，Bp端可以通过BinderProxy的transact()方法与Bn端发送请求，而Bn端通过继承Binder类并重写onTransact()接收并处理来自Bp端的请求。


Bn意味着Binder Native端，Bp是Binder Proxy端，Bn端通过onTransact()方法接收来自Bp端的请求的代码，然后把结果写入reply，Bp端再从reply中读取返回值并返回给调用者。

MediaPlayer::setDataSource：

status_t MediaPlayer::setDataSource(const sp<IDataSource> &source)
{
    ALOGV("setDataSource(IDataSource)");
    status_t err = UNKNOWN_ERROR;
    const sp<IMediaPlayerService> service(getMediaPlayerService());
    if (service != 0) {
        sp<IMediaPlayer> player(service->create(this, mAudioSessionId));
        if ((NO_ERROR != doSetRetransmitEndpoint(player)) ||
            (NO_ERROR != player->setDataSource(source))) {
            player.clear();
        }
        err = attachNewPlayer(player);
    }
    return err;
}

调用BpMediaPlayerService:: create：

    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());
    }

BnMediaPlayerService::onTransact里面创建调用create：

status_t BnMediaPlayerService::onTransact(
    uint32_t code, const Parcel& data, Parcel* reply, uint32_t flags)
{
    switch (code) {
        case CREATE: {
            CHECK_INTERFACE(IMediaPlayerService, data, reply);
            sp<IMediaPlayerClient> client =
                interface_cast<IMediaPlayerClient>(data.readStrongBinder());
            audio_session_t audioSessionId = (audio_session_t) data.readInt32();
            sp<IMediaPlayer> player = create(client, audioSessionId);
            reply->writeStrongBinder(IInterface::asBinder(player));
            return NO_ERROR;
        } break;

MediaPlayerService::create：

sp<IMediaPlayer> MediaPlayerService::create(const sp<IMediaPlayerClient>& client,
        audio_session_t audioSessionId)
{
    pid_t pid = IPCThreadState::self()->getCallingPid();
    int32_t connId = android_atomic_inc(&mNextConnId);

    // MediaPlayerService::Client
    sp<Client> c = new Client(
            this, pid, connId, client, audioSessionId,
            IPCThreadState::self()->getCallingUid());


    ALOGV("Create new client(%d) from pid %d, uid %d, ", connId, pid,
         IPCThreadState::self()->getCallingUid());


    wp<Client> w = c;
    {
        Mutex::Autolock lock(mLock);
        mClients.add(w);
    }
    return c;
}

然后MediaPlayer持有了MediaPlayerService:: Client对象，player相关的所有操作都是通过这个Client对象完成的。