Android WifiDisplay分析三：RTSP交互以及数据传输

前面我们分析到WifiDisplaySource会调用ANetworkSession的接口去创建一个socket，并在这个socket上监听是否有客户端的连接请求。先来看看Wifi Display规范的一些流程图：

从之前的一篇文章中，当ANetworkSession创建好RTSP的listen socket后，就会把它加入到selelct中等待对方的连接，那我们首先来看ANetworkSession的threadLoop方法：

void ANetworkSession::threadLoop() {    int res = select(maxFd + 1, &rs, &ws, NULL, NULL /* tv */);    {        Mutex::Autolock autoLock(mLock);        List<sp<Session> > sessionsToAdd;        for (size_t i = mSessions.size(); res > 0 && i-- > 0;) {            const sp<Session> &session = mSessions.valueAt(i);            int s = session->socket();            if (s < 0) {                continue;            }            if (FD_ISSET(s, &rs) || FD_ISSET(s, &ws)) {                --res;            }            if (FD_ISSET(s, &rs)) {                if (session->isRTSPServer() || session->isTCPDatagramServer()) {                    struct sockaddr_in remoteAddr;                    socklen_t remoteAddrLen = sizeof(remoteAddr);                    int clientSocket = accept(                            s, (struct sockaddr *)&remoteAddr, &remoteAddrLen);                    if (clientSocket >= 0) {                        status_t err = MakeSocketNonBlocking(clientSocket);                        if (err != OK) {                        } else {                            in_addr_t addr = ntohl(remoteAddr.sin_addr.s_addr);                            ALOGI("incoming connection from %d.%d.%d.%d:%d "                                  "(socket %d)",                                  (addr >> 24),                                  (addr >> 16) & 0xff,                                  (addr >> 8) & 0xff,                                  addr & 0xff,                                  ntohs(remoteAddr.sin_port),                                  clientSocket);                            sp<Session> clientSession =                                new Session(                                        mNextSessionID++,                                        Session::CONNECTED,                                        clientSocket,                                        session->getNotificationMessage());                            clientSession->setMode(                                    session->isRTSPServer()                                        ? Session::MODE_RTSP                                        : Session::MODE_DATAGRAM);                            sessionsToAdd.push_back(clientSession);                        }                    } else {                        ALOGE("accept returned error %d (%s)",                              errno, strerror(errno));                    }                }             }        while (!sessionsToAdd.empty()) {            sp<Session> session = *sessionsToAdd.begin();            sessionsToAdd.erase(sessionsToAdd.begin());            mSessions.add(session->sessionID(), session);            ALOGI("added clientSession %d", session->sessionID());        }    }

上面在selelct循环中，首先只有刚创建的RTSP的listen socket，接着如果有客户端的连接请求，就会跳出select语句，然后调用accept去接收对方的连接。接着会去创建一个新的Session会话，我们去看它的构造函数：

ANetworkSession::Session::Session(        int32_t sessionID,        State state,        int s,        const sp<AMessage> ¬ify)    : mSessionID(sessionID),      mState(state),      mMode(MODE_DATAGRAM),      mSocket(s),      mNotify(notify),      mSawReceiveFailure(false),      mSawSendFailure(false),      mUDPRetries(kMaxUDPRetries),      mLastStallReportUs(-1ll) {    if (mState == CONNECTED) {        struct sockaddr_in localAddr;        socklen_t localAddrLen = sizeof(localAddr);        int res = getsockname(                mSocket, (struct sockaddr *)&localAddr, &localAddrLen);        CHECK_GE(res, 0);        struct sockaddr_in remoteAddr;        socklen_t remoteAddrLen = sizeof(remoteAddr);        res = getpeername(                mSocket, (struct sockaddr *)&remoteAddr, &remoteAddrLen);        CHECK_GE(res, 0);        sp<AMessage> msg = mNotify->dup();        msg->setInt32("sessionID", mSessionID);        msg->setInt32("reason", kWhatClientConnected);        msg->setString("server-ip", localAddrString.c_str());        msg->setInt32("server-port", ntohs(localAddr.sin_port));        msg->setString("client-ip", remoteAddrString.c_str());        msg->setInt32("client-port", ntohs(remoteAddr.sin_port));        msg->post();    }}

这里的状态mState为CONNECTED，所以会构建一个AMessage并post出去，由前一章的知识，我们知道这里的mNotify是一个kWhatRTSPNotify消息，会在WifiDisplaySource调用createRTSPServer时传进来的一个参数，所以这里创建的AMessage最终还是会被WifiDisplaySource去处理，跳过中间AMessage、ALooperRoster、ALooper的调用关系，我们直接到WifiDisplaySource的onMessageReceived去看如何处理：

        case kWhatRTSPNotify:        {            int32_t reason;            CHECK(msg->findInt32("reason", &reason));            switch (reason) {                case ANetworkSession::kWhatError:                {                    break;                }                case ANetworkSession::kWhatClientConnected:                {                    int32_t sessionID;                    CHECK(msg->findInt32("sessionID", &sessionID));                    if (mClientSessionID > 0) {                        ALOGW("A client tried to connect, but we already "                              "have one.");                        mNetSession->destroySession(sessionID);                        break;                    }                    CHECK_EQ(mState, AWAITING_CLIENT_CONNECTION);                    CHECK(msg->findString("client-ip", &mClientInfo.mRemoteIP));                    CHECK(msg->findString("server-ip", &mClientInfo.mLocalIP));                    if (mClientInfo.mRemoteIP == mClientInfo.mLocalIP) {                        // Disallow connections from the local interface                        // for security reasons.                        mNetSession->destroySession(sessionID);                        break;                    }                    CHECK(msg->findInt32(                                "server-port", &mClientInfo.mLocalPort));                    mClientInfo.mPlaybackSessionID = -1;                    mClientSessionID = sessionID;                    ALOGI("We now have a client (%d) connected.", sessionID);                    mState = AWAITING_CLIENT_SETUP;                    status_t err = sendM1(sessionID);                    CHECK_EQ(err, (status_t)OK);                    break;                }                case ANetworkSession::kWhatData:                {                    break;                }                case ANetworkSession::kWhatNetworkStall:                {                    break;                }                default:                    TRESPASS();            }            break;        }

这里的reason是kWhatClientConnected，跳过前面不必要的case语句。如果先前已经连上其它的Sink device，这里就先断开之前的连接；如果没有，将新的SessionID赋予给mClientSessionID，并更改状态为AWAITING_CLIENT_SETUP，接着去看sendM1消息，这时候就要开始WifiDisplay M1~M7消息的发送了。

下面列举M1~M7消息的格式，有兴趣的可以去对照代码分析，我们后面着重分析M6(SetUp)和M7(Play)两个消息。

M1 reqeust:

OPTIONS * RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:24 +0000
Server: stagefright/1.2 (Linux;Android 4.4)
CSeq: 1
Require: org.wfa.wfd1.0

M1 respose:

RTSP/1.0 200 OK
CSeq: 1
Date: Fri, Jan 01 2014 09:02:37 GMT
Public: org.wfa.wfd1.0, GET_PARAMETER, SET_PARAMETER

M2 request:

OPTIONS * RTSP/1.0
CSeq: 2
Require: org.wfa.wfd1.0

M2 response:

RTSP/1.0 200 OK
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 2
Public: org.wfa.wfd1.0, SETUP, TEARDOWN, PLAY, PAUSE, GET_PARAMETER, SET_PARAMETER

M3 request:

GET_PARAMETER rtsp://localhost/wfd1.0 RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 2
Content-Type: text/parameters
Content-Length: 83

wfd_content_protection
wfd_video_formats
wfd_audio_codecs
wfd_client_rtp_ports

M3 response:

RTSP/1.0 200 OK
CSeq: 2
Content-Length: 124
Content-Type: text/parameters

wfd_audio_codecs: LPCM 00000003 00, AAC 00000007 00
wfd_video_formats: 00 00 02 02 0000FFFF 0FFFFFFF 00000FFF 00 0000 0000 01 none none
wfd_content_protection: none
wfd_client_rtp_ports: RTP/AVP/UDP;unicast 19990 0 mode=play

M4 request:

SET_PARAMETER rtsp://localhost/wfd1.0 RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 3
Content-Type: text/parameters
Content-Length: 247
wfd_video_formats: 00 00 02 02 00000020 00000000 00000000 00 0000 0000 00 none none
wfd_audio_codecs: AAC 00000001 00
wfd_presentation_URL: rtsp://192.168.5.200/wfd1.0/streamid=0 none
wfd_client_rtp_ports: RTP/AVP/UDP;unicast 19990 0 mode=play

M4 response:

RTSP/1.0 200 OK
CSeq: 3

M5 request:

SET_PARAMETER rtsp://localhost/wfd1.0 RTSP/1.0
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 4
Content-Type: text/parameters
Content-Length: 27
wfd_trigger_method: SETUP

M5 response:

RTSP/1.0 200 OK
CSeq: 4

M6 request:

SETUP rtsp://192.168.5.200/wfd1.0/streamid=0 RTSP/1.0
CSeq: 3
Transport: RTP/AVP/UDP;unicast;client_port=19990

M6 response:

RTSP/1.0 200 OK
Date: Tue, 29 Fri 2014 02:41:25 +0000
Server: stagefright/1.2 (Linux;Android 4.3)
CSeq: 3
Session: 988982966;timeout=30
Transport: RTP/AVP/UDP;unicast;client_port=19990;server_port=22220

我们先来看处理M6 request的方法，代码在onSetupRequest中：

status_t WifiDisplaySource::onSetupRequest(        int32_t sessionID,        int32_t cseq,        const sp<ParsedMessage> &data) {    CHECK_EQ(sessionID, mClientSessionID);    if (mClientInfo.mPlaybackSessionID != -1) {        sendErrorResponse(sessionID, "400 Bad Request", cseq);        return ERROR_MALFORMED;    }    int32_t playbackSessionID = makeUniquePlaybackSessionID();    sp<AMessage> notify = new AMessage(kWhatPlaybackSessionNotify, id());    notify->setInt32("playbackSessionID", playbackSessionID);    notify->setInt32("sessionID", sessionID);    sp<PlaybackSession> playbackSession =        new PlaybackSession(                mNetSession, notify, mInterfaceAddr, mHDCP, mMediaPath.c_str());    looper()->registerHandler(playbackSession);    AString uri;    data->getRequestField(1, &uri);    if (strncasecmp("rtsp://", uri.c_str(), 7)) {        sendErrorResponse(sessionID, "400 Bad Request", cseq);        return ERROR_MALFORMED;    }    if (!(uri.startsWith("rtsp://") && uri.endsWith("/wfd1.0/streamid=0"))) {        sendErrorResponse(sessionID, "404 Not found", cseq);        return ERROR_MALFORMED;    }    RTPSender::TransportMode rtcpMode = RTPSender::TRANSPORT_UDP;    if (clientRtcp < 0) {        rtcpMode = RTPSender::TRANSPORT_NONE;    }    status_t err = playbackSession->init(            mClientInfo.mRemoteIP.c_str(),            clientRtp,            rtpMode,            clientRtcp,            rtcpMode,            mSinkSupportsAudio,            mUsingPCMAudio,            mSinkSupportsVideo,            mChosenVideoResolutionType,            mChosenVideoResolutionIndex,            mChosenVideoProfile,            mChosenVideoLevel);    if (err != OK) {        looper()->unregisterHandler(playbackSession->id());        playbackSession.clear();    }    mClientInfo.mPlaybackSessionID = playbackSessionID;    mClientInfo.mPlaybackSession = playbackSession;    mState = AWAITING_CLIENT_PLAY;    scheduleReaper();    scheduleKeepAlive(sessionID);    return OK;}

跳过前面的关于RTSP回复消息的组织，这里还会创建一个PlaybackSession对象，并调用它的init方法做初始化。

根据前面的背景知识介绍，设备之间的交互将由Session来管理。在代码中，Session的概念由WifiDisplaySource的内部类PlaybackSession来表示。先来看和其相关的类图结构，如下图所示：

由上图可知：

•  PlaybackSession及其内部类Track都从AHandler派生。故它们的工作也依赖于消息循环和处理。Track代表视频流或音频流。
• Track内部通过mMediaPull变量指向一个MediaPull对象。而MediaPull对象则保存了一个MediaSource对象。在PlaybackSession中，此MediaSource的真正类型为SurfaceMediaSource。它表明该Media的源来自Surface。
•  BufferQueue从ISurfaceTexure中派生，根据前面对SurfaceFlinger的介绍，它就是SurfaceFlinger代码示例中代表虚拟设备的State的surface变量。
• 左图中，MediaPull通过kWhatPull消息不断调用MediaSource的read函数。
• 右图中，SurfaceMediaSource的read函数由通过mBufferQueue来读取数据。

• 那么mBufferQueue的数据来自什么地方呢？对，正是来自SurfaceFlinger。

  当然，PlaybackSession拿到这些数据后还需要做编码，然后才能发送给远端设备。由于篇幅关系，本文就不再讨论这些问题了。

  
  

当双方设备准备就绪后，MediaPull会通过kWhatPull消息处理不断调用MediaSource的read函数。在SurfaceMediaSource实现的read函数中，来自SurfaceFlinger的混屏后的数据经由BufferQueue传递到MediaPull中。