android binder 机制三(匿名Service)

什么是匿名Service？凡是没有到ServiceManager上注册的Service，都是匿名Service。

还是拿上一篇的例子来举例，看代码：

status_t MediaPlayer::setDataSource(int fd, int64_t offset, int64_t length){    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(fd, offset, length))) {            player.clear();        }        err = attachNewPlayer(player);    }    return err;}

在BpMediaPlayerService中，create的实现如下：

virtual sp<IMediaPlayer> create(        const sp<IMediaPlayerClient>& client, int audioSessionId) {    Parcel data, reply;    data.writeInterfaceToken(IMediaPlayerService::getInterfaceDescriptor());    data.writeStrongBinder(client->asBinder());    data.writeInt32(audioSessionId);    remote()->transact(CREATE, data, &reply);    return interface_cast<IMediaPlayer>(reply.readStrongBinder());}

直接跳到服务端MediaPlayerService，看create的真正实现：

sp<IMediaPlayer> MediaPlayerService::create(const sp<IMediaPlayerClient>& client,        int audioSessionId){    pid_t pid = IPCThreadState::self()->getCallingPid();    int32_t connId = android_atomic_inc(&mNextConnId);    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());    /* add by Gary. start {{----------------------------------- */    c->setScreen(mScreen);    /* add by Gary. end   -----------------------------------}} */    c->setSubGate(mGlobalSubGate);  // 2012-03-12, add the global interfaces to control the subtitle gate    wp<Client> w = c;    {        Mutex::Autolock lock(mLock);        mClients.add(w);    }    return c;}

从代码中，我们可以看出，service->create(this, mAudioSessionId)是返回了一个参数为Client类型的BpMediaPlayer对象，其中Client为MediaPlayerService的私有内部类，其声明为：class Client : publicBnMediaPlayer。这样，Binder通信的服务端和客户端就建立起来了。Client端BpMediaPlayer由MediaPlayer使用，Server端BnMediaPlayer由MediaPlayerService使用。

BpMediaPlayer是如何获得BnMediaPlayer的handle值的呢？答案在MediaPlayerService返回这个binder的引用的时候，binder驱动保存了这个binder实体的各种数据，创建了节点，看下面的代码：

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());            int audioSessionId = data.readInt32();            sp<IMediaPlayer> player = create(client, audioSessionId);            reply->writeStrongBinder(player->asBinder());            return NO_ERROR;        } break;……}}

答案就在这句话中：reply->writeStrongBinder(player->asBinder());

当这个reply写到Binder驱动时，驱动会特殊处理这种IBinder类型的数据，为Bbinder建立一个handle。

通信的通路建立后，就可以进行通信了：player->setDataSource(fd, offset, length)

之后的实现，和上一篇讲的普通Service就一样了。

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