Android WifiDisplay分析二：Wifi display连接过程

简介

这一章中我们来看Wifi Display连接过程的建立，包含P2P的部分和RTSP的部分，首先来大致看一下Wifi Display规范相关的东西。

1. HIDC: Human Interface Device Class （遵循HID标准的设备类）
2. UIBC: User Input Back Channel （UIBC分为两种，一种是Generic，包含鼠标、键盘等；另一种是HIDC，HID是一个规范，只有遵循HID的标准，都可以叫做HID设备，包含USB鼠标、键盘、蓝牙、红外等）
3. PES: Packetized Elementary Stream （数字电视基本码流）
4. HDCP: High-bandwidth Digital Content Protection （加密方式，用于加密传输的MPEG2-TS流）
5. MPEG2-TS: Moving Picture Experts Group 2 Transport Stream （Wifi display之间传输的是MPEG2-TS流）
6. RTSP: Real-Time Streaming Protocol （Wifi display通过RTSP协议来交互两边的能力）
7. RTP: Real-time Transport Protocol （Wifi display通过RTP来传输MPEG2-TS流）
8. Wi-Fi P2P: Wi-Fi Direct
9. TDLS: Tunneled Direct Link Setup （另一种方式建立两台设备之间的直连，与P2P类似，但要借助一台AP）

另一种比较重要的概念是在Wifi Display中分为Source和Sink两种角色，如下图。Source是用于encode并输出TS流；Sink用于decode并显示TS流。相当于Server/Client架构中，Source就是Server，用于提供服务；Sink就是Client。当然，我们这篇文章主要介绍在Android上Wifi display Source的流程。



从上面的架构图我们可以看到，Wifi display是建立在TCP/UDP上面的应用层协议，L2链路层是通过P2P和TDLS两种方式建立，TDLS是optional的。在L2层建立连接后，Source就会在一个特定的port上listen，等待client的TCP连接。当与Client建立了TCP连接后，就会有M1~M7七个消息的交互，用户获取对方设备的能力，包括视频编码能力、Audio输出能力、是否支持HDCP加密等等。在获取这些能力之后，Source就会选择一种视频编码格式以及Audio格式用于这次会话当中。当一个RTSP会话建立后，双方就会决定出用于传输TS流的RTP port，RTP协议是基于UDP的。当这些都准备好后，Sink设备就会发送M7消息，也就是Play给Source，双方就可以开始传输数据了。

关于M1~M7是什么，我们后面再来介绍。首先我们来介绍在Android WifiDisplay中如何建立P2P的连接。

WifiDisplay之P2P的建立

通过我们之间关于Wifi display的service启动以及enable的分析，我们知道当扫描到可用的设备后，就会显示在WifiDisplaySettings这个页面上，当我们选择其中一个后，就会开始P2P的建立了，首先到WifiDisplaySettings中的代码分析：

/packages/apps/Settings/src/com/android/settings/wfd/WifiDisplaySettings.java

private void pairWifiDisplay(WifiDisplay display) {
    if (display.canConnect()) {
        mDisplayManager.connectWifiDisplay(display.getDeviceAddress());
    }
}

WifiDisplaySettings通过AIDL调用到DisplayManagerService的connectWifiDisplay方法，关于AIDL的调用过程这里不讲了，直接到DisplayManagerService的connectWifiDisplay方法来看：

/frameworks/base/services/core/java/com/android/server/display/DisplayManagerService.java

public void connectWifiDisplay(String address) {
    if (address == null) {
        throw new IllegalArgumentException("address must not be null");
    }
    mContext.enforceCallingOrSelfPermission(Manifest.permission.CONFIGURE_WIFI_DISPLAY,
            "Permission required to connect to a wifi display");

    final long token = Binder.clearCallingIdentity();
    try {
        synchronized (mSyncRoot) {
            if (mWifiDisplayAdapter != null) {
                mWifiDisplayAdapter.requestConnectLocked(address);
            }
        }
    } finally {
        Binder.restoreCallingIdentity(token);
    }
}

首先做参数的检查，即MAC地址不能为空，然后做权限检查，调用这个方法的application必须要在manifest中声明有CONFIGURE_WIFI_DISPLAY权限，最后直接调用WifiDisplayAdapter的requestConnectLocked方法：

/frameworks/base/services/core/java/com/android/server/display/WifiDisplayAdapter.java

public void requestConnectLocked(final String address) {
    if (DEBUG) {
        Slog.d(TAG, "requestConnectLocked: address=" + address);
    }

    getHandler().post(new Runnable() {
        @Override
        public void run() {
            if (mDisplayController != null) {
                mDisplayController.requestConnect(address);
            }
        }
    });
}

这里比较简单，直接调用WifiDisplayController的requestConnect方法。前面都是直接的调用，最终做事情的还是WifiDisplayController。

/frameworks/base/services/core/java/com/android/server/display/WifiDisplayController.java

public void requestConnect(String address) {
    for (WifiP2pDevice device : mAvailableWifiDisplayPeers) {
        if (device.deviceAddress.equals(address)) {
            connect(device);
        }
    }
}

private void connect(final WifiP2pDevice device) {
    if (mDesiredDevice != null
            && !mDesiredDevice.deviceAddress.equals(device.deviceAddress)) {
        if (DEBUG) {
            Slog.d(TAG, "connect: nothing to do, already connecting to "
                    + describeWifiP2pDevice(device));
        }
        return;
    }

    if (mConnectedDevice != null
            && !mConnectedDevice.deviceAddress.equals(device.deviceAddress)
            && mDesiredDevice == null) {
        if (DEBUG) {
            Slog.d(TAG, "connect: nothing to do, already connected to "
                    + describeWifiP2pDevice(device) + " and not part way through "
                    + "connecting to a different device.");
        }
        return;
    }

    if (!mWfdEnabled) {
        Slog.i(TAG, "Ignoring request to connect to Wifi display because the "
                +" feature is currently disabled: " + device.deviceName);
        return;
    }

    mDesiredDevice = device;
    mConnectionRetriesLeft = CONNECT_MAX_RETRIES;
    updateConnection();
}

requestConnect先从mAvaiableWifiDsiplayPeers中通过Mac地址找到所有连接的WifiP2pDevice，然后调用connect方法，在connect方法中会做一系列的判断，看首先是否有正在连接中或者断开中的设备，如果有就直接返回；再看有没有已经连接上的设备，如果有，也直接返回，然后赋值mDesiredDevice为这次要连接的设备，最后调用updateConnection来更新连接状态并发起连接。updateConnection的代码比较长，我们分段来分析：

/frameworks/base/services/core/java/com/android/server/display/WifiDisplayController.java

private void updateConnection() {
    //更新是否需要scan或者停止scan
    updateScanState();

    //如果有已经连接上的RemoteDisplay，先断开。这里先不看
    if (mRemoteDisplay != null && mConnectedDevice != mDesiredDevice) {
        ......
    }

    // 接上面的一步，段开这个group
    if (mDisconnectingDevice != null) {
        return; // wait for asynchronous callback
    }
    if (mConnectedDevice != null && mConnectedDevice != mDesiredDevice) {
        ......
    }

    // 如果有正在连接的设备，先停止连接之前的设备
    if (mCancelingDevice != null) {
        return; // wait for asynchronous callback
    }
    if (mConnectingDevice != null && mConnectingDevice != mDesiredDevice) {

    }

    // 当断开之前的连接或者启动匿名GROUP时，这里就结束了
    if (mDesiredDevice == null) {
        return; // wait for asynchronous callback
    }

    // 开始连接，这是我们要看的重点
    if (mConnectedDevice == null && mConnectingDevice == null) {
        Slog.i(TAG, "Connecting to Wifi display: " + mDesiredDevice.deviceName);

        mConnectingDevice = mDesiredDevice;
        WifiP2pConfig config = new WifiP2pConfig();
        WpsInfo wps = new WpsInfo();
        if (mWifiDisplayWpsConfig != WpsInfo.INVALID) {
            wps.setup = mWifiDisplayWpsConfig;
        } else if (mConnectingDevice.wpsPbcSupported()) {
            wps.setup = WpsInfo.PBC;
        } else if (mConnectingDevice.wpsDisplaySupported()) {
            wps.setup = WpsInfo.KEYPAD;
        } else {
            wps.setup = WpsInfo.DISPLAY;
        }
        config.wps = wps;
        config.deviceAddress = mConnectingDevice.deviceAddress;
        config.groupOwnerIntent = WifiP2pConfig.MIN_GROUP_OWNER_INTENT;

        WifiDisplay display = createWifiDisplay(mConnectingDevice);
        advertiseDisplay(display, null, 0, 0, 0);

        final WifiP2pDevice newDevice = mDesiredDevice;
        mWifiP2pManager.connect(mWifiP2pChannel, config, new ActionListener() {
            @Override
            public void onSuccess() {
                Slog.i(TAG, "Initiated connection to Wifi display: " + newDevice.deviceName);

                mHandler.postDelayed(mConnectionTimeout, CONNECTION_TIMEOUT_SECONDS * 1000);
            }

            @Override
            public void onFailure(int reason) {
                if (mConnectingDevice == newDevice) {
                    Slog.i(TAG, "Failed to initiate connection to Wifi display: "
                            + newDevice.deviceName + ", reason=" + reason);
                    mConnectingDevice = null;
                    handleConnectionFailure(false);
                }
            }
        });
        return;
    }

这段函数比较长，我们先看我们需要的，剩下的后面再来分析。首先赋值给mConnectingDevice表示当前正在连接的设备，然后构造一个WifiP2pConfig对象，这个对象包含这次连接的设备的Mac地址、wps方式以及我们自己的GROUP_OWNER intent值，然后调用advertieseDisplay方法来通知WifiDisplayAdapter相关状态的改变，WifiDisplayAdapter会发送相应的broadcast出来，这是WifiDisplaySettings可以接收这些broadcast，然后在UI上更新相应的状态。关于advertieseDisplay的实现，我们后面再来分析。

接着看updateConnection，调用WifiP2pManager的connect方法去实现两台设备的P2P连接，具体过程可以参考前面介绍的P2P连接的文章。这里的onSuccess()并不是表示P2P已经建立成功，而只是表示这个发送命令到wpa_supplicant成功，所以在这里设置了一个连接超时的timeout，为30秒。当连接成功后，会发送WIFI_P2P_CONNECTION_CHANGED_ACTION的广播出来，接着回到WifiDisplayController看如何处理连接成功的broadcast：

/frameworks/base/services/core/java/com/android/server/display/WifiDisplayController.java

private final BroadcastReceiver mWifiP2pReceiver = new BroadcastReceiver() {
    @Override
    public void onReceive(Context context, Intent intent) {
        final String action = intent.getAction();
        if (action.equals(WifiP2pManager.WIFI_P2P_STATE_CHANGED_ACTION)) {
           ......
        } else if (action.equals(WifiP2pManager.WIFI_P2P_PEERS_CHANGED_ACTION)) {
            ......
        } else if (action.equals(WifiP2pManager.WIFI_P2P_CONNECTION_CHANGED_ACTION)) {
            NetworkInfo networkInfo = (NetworkInfo)intent.getParcelableExtra(
                    WifiP2pManager.EXTRA_NETWORK_INFO);
            if (DEBUG) {
                Slog.d(TAG, "Received WIFI_P2P_CONNECTION_CHANGED_ACTION: networkInfo="
                        + networkInfo);
            }

            handleConnectionChanged(networkInfo);
        } else if (action.equals(WifiP2pManager.WIFI_P2P_THIS_DEVICE_CHANGED_ACTION)) {
           ......
        }
    }
};

private void handleConnectionChanged(NetworkInfo networkInfo) {
    mNetworkInfo = networkInfo;
    if (mWfdEnabled && networkInfo.isConnected()) {
        if (mDesiredDevice != null || mWifiDisplayCertMode) {
            mWifiP2pManager.requestGroupInfo(mWifiP2pChannel, new GroupInfoListener() {
                @Override
                public void onGroupInfoAvailable(WifiP2pGroup info) {
                    if (DEBUG) {
                        Slog.d(TAG, "Received group info: " + describeWifiP2pGroup(info));
                    }

                    if (mConnectingDevice != null && !contains(info, mConnectingDevice)) {
                        Slog.i(TAG, "Aborting connection to Wifi display because "
                                + "the current P2P group does not contain the device "
                                + "we expected to find: " + mConnectingDevice.deviceName
                                + ", group info was: " + describeWifiP2pGroup(info));
                        handleConnectionFailure(false);
                        return;
                    }

                    if (mDesiredDevice != null && !contains(info, mDesiredDevice)) {
                        disconnect();
                        return;
                    }

                    if (mWifiDisplayCertMode) {
                        boolean owner = info.getOwner().deviceAddress
                                .equals(mThisDevice.deviceAddress);
                        if (owner && info.getClientList().isEmpty()) {
                            // this is the case when we started Autonomous GO,
                            // and no client has connected, save group info
                            // and updateConnection()
                            mConnectingDevice = mDesiredDevice = null;
                            mConnectedDeviceGroupInfo = info;
                            updateConnection();
                        } else if (mConnectingDevice == null && mDesiredDevice == null) {
                            // this is the case when we received an incoming connection
                            // from the sink, update both mConnectingDevice and mDesiredDevice
                            // then proceed to updateConnection() below
                            mConnectingDevice = mDesiredDevice = owner ?
                                    info.getClientList().iterator().next() : info.getOwner();
                        }
                    }

                    if (mConnectingDevice != null && mConnectingDevice == mDesiredDevice) {
                        Slog.i(TAG, "Connected to Wifi display: "
                                + mConnectingDevice.deviceName);

                        mHandler.removeCallbacks(mConnectionTimeout);
                        mConnectedDeviceGroupInfo = info;
                        mConnectedDevice = mConnectingDevice;
                        mConnectingDevice = null;
                        updateConnection();
                    }
                }
            });
        }
    } else {
        ......
    }
}

当WifiDisplayController收到WIFI_P2P_CONNECTION_CHANGED_ACTION广播后，会调用handleConnectionChanged来获取当前P2P Group相关的信息，如果获取到的P2P Group信息里面没有mConnectingDevice或者mDesiredDevice的信息，则表示连接出错了，直接退出。如果当前连接信息与前面设置的mConnectingDevice一直，则表示连接P2P成功，这里首先会移除前面设置的连接timeout的callback，然后设置mConnectedDevice为当前连接的设备，并设置mConnectingDevice为空，最后调用updateConnection来更新连接状态信息。我们又回到updateConnection这个函数了，但这次进入的分支与之前连接请求的分支又不同了，我们来看代码：

/frameworks/base/services/java/com/android/server/display/WifiDisplayController.java

private void updateConnection() {
    // 更新是否需要scan或者停止scan
    updateScanState();

    // 如果有连接上的RemoteDisplay，这里先断开
    if (mRemoteDisplay != null && mConnectedDevice != mDesiredDevice) {

    }

    // 接着上面的一步，先断开之前连接的设备
    if (mDisconnectingDevice != null) {
        return; // wait for asynchronous callback
    }
    if (mConnectedDevice != null && mConnectedDevice != mDesiredDevice) {

    }

    // 如果有正在连接的设备，先断开之前连接的设备
    if (mCancelingDevice != null) {
        return; // wait for asynchronous callback
    }
    if (mConnectingDevice != null && mConnectingDevice != mDesiredDevice) {

    }

    // 当断开之前的连接或者匿名GO时，这里就结束了
    if (mDesiredDevice == null) {

    }

    // 如果有连接请求，则进入此
    if (mConnectedDevice == null && mConnectingDevice == null) {

    }

    // 当连接上P2P后，就进入到此
    if (mConnectedDevice != null && mRemoteDisplay == null) {
        Inet4Address addr = getInterfaceAddress(mConnectedDeviceGroupInfo);
        if (addr == null) {
            Slog.i(TAG, "Failed to get local interface address for communicating "
                    + "with Wifi display: " + mConnectedDevice.deviceName);
            handleConnectionFailure(false);
            return; // done
        }

        mWifiP2pManager.setMiracastMode(WifiP2pManager.MIRACAST_SOURCE);

        final WifiP2pDevice oldDevice = mConnectedDevice;
        final int port = getPortNumber(mConnectedDevice);
        final String iface = addr.getHostAddress() + ":" + port;
        mRemoteDisplayInterface = iface;

        Slog.i(TAG, "Listening for RTSP connection on " + iface
                + " from Wifi display: " + mConnectedDevice.deviceName);

        mRemoteDisplay = RemoteDisplay.listen(iface, new RemoteDisplay.Listener() {
            @Override
            public void onDisplayConnected(Surface surface,
                    int width, int height, int flags, int session) {
                if (mConnectedDevice == oldDevice && !mRemoteDisplayConnected) {
                    Slog.i(TAG, "Opened RTSP connection with Wifi display: "
                            + mConnectedDevice.deviceName);
                    mRemoteDisplayConnected = true;
                    mHandler.removeCallbacks(mRtspTimeout);

                    if (mWifiDisplayCertMode) {
                        mListener.onDisplaySessionInfo(
                                getSessionInfo(mConnectedDeviceGroupInfo, session));
                    }

                    final WifiDisplay display = createWifiDisplay(mConnectedDevice);
                    advertiseDisplay(display, surface, width, height, flags);
                }
            }

            @Override
            public void onDisplayDisconnected() {
                if (mConnectedDevice == oldDevice) {
                    Slog.i(TAG, "Closed RTSP connection with Wifi display: "
                            + mConnectedDevice.deviceName);
                    mHandler.removeCallbacks(mRtspTimeout);
                    disconnect();
                }
            }

            @Override
            public void onDisplayError(int error) {
                if (mConnectedDevice == oldDevice) {
                    Slog.i(TAG, "Lost RTSP connection with Wifi display due to error "
                            + error + ": " + mConnectedDevice.deviceName);
                    mHandler.removeCallbacks(mRtspTimeout);
                    handleConnectionFailure(false);
                }
            }
        }, mHandler,mContext.getOpPackageName());

        // Use extended timeout value for certification, as some tests require user inputs
        int rtspTimeout = mWifiDisplayCertMode ?
                RTSP_TIMEOUT_SECONDS_CERT_MODE : RTSP_TIMEOUT_SECONDS;

        mHandler.postDelayed(mRtspTimeout, rtspTimeout * 1000);
    }
}

到这里P2P的连接就算建立成功了，接下来就是RTSP的部分了

WifiDisplay之RTSP server的创建

这里首先设置MiracastMode，我认为这部分应该放在enable WifiDisplay时，不知道Google为什么放在这里？ 然后从GroupInfo中取出对方设备的IP地址，利用默认的CONTROL PORT构建mRemoteDisplayInterface，接着调用RemoteDisplay的listen方法去listen指定的IP和端口上面的TCP连接请求。最后会设置Rtsp的连接请求的timeout，当用于Miracast认证时是120秒，正常的使用中是30秒，如果在这么长的时间内没有收到Sink的TCP请求，则表示失败了。下面来看RemoteDisplay的listen的实现：

/frameworks/base/media/java/android/media/RemoteDisplay.java

public static RemoteDisplay listen(String iface, Listener listener, Handler handler) {
    if (iface == null) {
        throw new IllegalArgumentException("iface must not be null");
    }
    if (listener == null) {
        throw new IllegalArgumentException("listener must not be null");
    }
    if (handler == null) {
        throw new IllegalArgumentException("handler must not be null");
    }

    RemoteDisplay display = new RemoteDisplay(listener, handler);
    display.startListening(iface);
    return display;
}

这里首先进行参数的检查，然后创建一个RemoteDisplay对象（这里不能直接创建RemoteDisplay对象，因为它的构造函数是private的），接着调用RemoteDisplay的startListening方法：

/frameworks/base/media/java/android/media/RemoteDisplay.java

private void startListening(String iface) {
    mPtr = nativeListen(iface);
    if (mPtr == 0) {
        throw new IllegalStateException("Could not start listening for "
                + "remote display connection on \"" + iface + "\"");
    }
    mGuard.open("dispose");
}

nativeListen会调用JNI中的实现，相关代码在android_media_RemoteDisplay.cpp中。注意上面的mGuard是CloseGuard对象，是一种用于显示释放一些资源的机制。

/frameworks/base/core/jni/android_media_RemoteDisplay.cpp

static jint nativeListen(JNIEnv* env, jobject remoteDisplayObj, jstring ifaceStr) {
    ScopedUtfChars iface(env, ifaceStr);

    sp<IServiceManager> sm = defaultServiceManager();
    sp<IMediaPlayerService> service = interface_cast<IMediaPlayerService>(
            sm->getService(String16("media.player")));
    if (service == NULL) {
        ALOGE("Could not obtain IMediaPlayerService from service manager");
        return 0;
    }

    sp<NativeRemoteDisplayClient> client(new NativeRemoteDisplayClient(env, remoteDisplayObj));
    sp<IRemoteDisplay> display = service->listenForRemoteDisplay(
            client, String8(iface.c_str()));
    if (display == NULL) {
        ALOGE("Media player service rejected request to listen for remote display '%s'.",
                iface.c_str());
        return 0;
    }

    NativeRemoteDisplay* wrapper = new NativeRemoteDisplay(display, client);
    return reinterpret_cast<jint>(wrapper);
}

上面的代码中先从ServiceManager中获取MediaPlayerService的Bpbinder引用，然后由传入的第二个参数remoteDisplayObj，也就是RemoteDisplay对象构造一个NativeRemoteDisplayClient，在framework中，我们经常看到像这样的用法，类似于设计模式中的包装模式，例如在framework中对Java层的BnBinder也是做了一层封装JavaBBinder。在NativeRemoteDisplayClient中通过JNI的反向调用，就可以直接回调RemoteDisplay中的一些函数，实现回调方法了，下面来看它的实现：

/frameworks/base/core/jni/android_media_RemoteDisplay.cpp

class NativeRemoteDisplayClient : public BnRemoteDisplayClient {
public:
    NativeRemoteDisplayClient(JNIEnv* env, jobject remoteDisplayObj) :
            mRemoteDisplayObjGlobal(env->NewGlobalRef(remoteDisplayObj)) {
    }

protected:
    ~NativeRemoteDisplayClient() {
        JNIEnv* env = AndroidRuntime::getJNIEnv();
        env->DeleteGlobalRef(mRemoteDisplayObjGlobal);
    }

public:
    virtual void onDisplayConnected(const sp<IGraphicBufferProducer>& bufferProducer,
            uint32_t width, uint32_t height, uint32_t flags, uint32_t session) {
        env->CallVoidMethod(mRemoteDisplayObjGlobal,
                gRemoteDisplayClassInfo.notifyDisplayConnected,
                surfaceObj, width, height, flags, session);
    }

    virtual void onDisplayDisconnected() {

    }

    virtual void onDisplayError(int32_t error) {

    }

private:
    jobject mRemoteDisplayObjGlobal;

    static void checkAndClearExceptionFromCallback(JNIEnv* env, const char* methodName) {

        }
    }
};

在NativeRemoteDisplayClient的构造函数中，把RemoteDisplay对象先保存到mRemoteDisplayObjGlobal中，可以看到上面主要实现了三个回调函数，onDisplayConnected、onDisplayDisconnected、onDisplayError，这三个回调函数对应到RemoteDisplay类的notifyDisplayConnected、notifyDisplayDisconnected和notifyDisplayError三个方法。接着回到nativeListen中，接着会调用MediaPlayerService的listenForRemoteDisplay方法去监听socket连接，这个方法是返回一个RemoteDisplay对象，当然经过binder的调用，最终返回到nativeListen的是BpRemoteDisplay对象，然后会由这个BpRemoteDisplay对象构造一个NativeRemoteDisplay对象并把它的指针地址返回给上层RemoteDisplay使用。

/frameworks/base/core/jni/android_media_RemoteDisplay.cpp

class NativeRemoteDisplay {
public:
    NativeRemoteDisplay(const sp<IRemoteDisplay>& display,
            const sp<NativeRemoteDisplayClient>& client) :
            mDisplay(display), mClient(client) {
    }

    ~NativeRemoteDisplay() {
        mDisplay->dispose();
    }

    void pause() {
        mDisplay->pause();
    }

    void resume() {
        mDisplay->resume();
    }

private:
    sp<IRemoteDisplay> mDisplay;
    sp<NativeRemoteDisplayClient> mClient;
};

来看一下这时Java层的RemoteDisplay和Native层RemoteDisplay之间的关系：



WifiDisplayController通过左边的一条线路关系去控制WifiDisplaySource，而WifiDisplaySource又通过右边一条线路关系去回调WifiDisplayController的一些方法。

接着来看MediaPlayerService的listenForRemoteDisplay方法：

/frameworks/av/media/libmediaplayerservice/MediaPlayerService.cpp

sp<IRemoteDisplay> MediaPlayerService::listenForRemoteDisplay(
        const sp<IRemoteDisplayClient>& client, const String8& iface) {
    if (!checkPermission("android.permission.CONTROL_WIFI_DISPLAY")) {
        return NULL;
    }

    return new RemoteDisplay(client, iface.string());
}

首先进行权限的检查，然后创建一个RemoteDisplay对象（注意现在已经在C++层了），这里看RemoteDisplay.cpp文件。RemoteDisplay继承于BnRemoteDisplay，并实现BnRemoteDisplay中的一些方法，有兴趣的可以去看一下IRemoteDisplay的实现。接下来来看RemoteDisplay的构造函数：

/frameworks/av/media/libmediaplayerservice/RemoteDisplay.cpp

RemoteDisplay::RemoteDisplay(
        const sp<IRemoteDisplayClient> &client,
        const char *iface)
    : mLooper(new ALooper),
      mNetSession(new ANetworkSession) {
    mLooper->setName("wfd_looper");

    mSource = new WifiDisplaySource(mNetSession, client);
    mLooper->registerHandler(mSource);

    mNetSession->start();
    mLooper->start();

    mSource->start(iface);
}

RemoteDisplay类包含三个比较重要的元素：ALooper、ANetworkSession、WifiDisplaySource。首先来看一下在Native层的类图：



ALooper中会创建一个Thread，并且不断的进行Looper循环去收消息，并dispatch给WifiDisplaySource去处理消息。首先来看它的构造函数和setName以及registerHandler这三个方法：

/frameworks/av/media/libstagefright/foundation/ALooper.cpp

ALooper::ALooper()
    : mRunningLocally(false) {
}

void ALooper::setName(const char *name) {
    mName = name;
}

ALooper::handler_id ALooper::registerHandler(const sp<AHandler> &handler) {
    return gLooperRoster.registerHandler(this, handler);
}

这三个方法都比较简单，我们看LooperRoster的registerHandler方法：

/frameworks/av/media/libstagefright/foundation/ALooperRoster.cpp

ALooper::handler_id ALooperRoster::registerHandler(
        const sp<ALooper> looper, const sp<AHandler> &handler) {
    Mutex::Autolock autoLock(mLock);

    if (handler->id() != 0) {
        CHECK(!"A handler must only be registered once.");
        return INVALID_OPERATION;
    }

    HandlerInfo info;
    info.mLooper = looper;
    info.mHandler = handler;
    ALooper::handler_id handlerID = mNextHandlerID++;
    mHandlers.add(handlerID, info);

    handler->setID(handlerID);

    return handlerID;
}

这里为每一个注册的AHandler分配一个handlerID，并且把注册的AHandler保存在mHandlers列表中，后面使用时，就可以快速的通过HandlerID找到对应的AHandler以及ALooper了。注意这里HandlerInfo结构中的mLooper和mHander都是wp，是一个弱引用，在使用中必须调用其promote方法获取sp指针才能使用。再回到RemoteDisplay的构造函数中看ALooper的start方法：

/frameworks/av/media/libstagefright/foundation/ALooper.cpp

status_t ALooper::start(
        bool runOnCallingThread, bool canCallJava, int32_t priority) {
    if (runOnCallingThread) {
        {
            Mutex::Autolock autoLock(mLock);
            if (mThread != NULL || mRunningLocally) {
                return INVALID_OPERATION;
            }
            mRunningLocally = true;
        }
        do {
        } while (loop());
        return OK;
    }

    Mutex::Autolock autoLock(mLock);

    mThread = new LooperThread(this, canCallJava);

    status_t err = mThread->run(
            mName.empty() ? "ALooper" : mName.c_str(), priority);
    if (err != OK) {
        mThread.clear();
    }

    return err;
}

这里的runOnCallingThread会根据默认形参为false，所以会新建一个LooperThread来不断的做循环，LooperThread是继承于Thread，并实现它的readyToRun和threadLoop方法，在threadLoop方法中去调用ALooper的loop方法，代码如下：

/frameworks/av/media/libstagefright/foundation/ALooper.cpp

    virtual bool threadLoop() {
        return mLooper->loop();
    }

bool ALooper::loop() {
    Event event;

    {
        Mutex::Autolock autoLock(mLock);

        if (mEventQueue.empty()) {
            mQueueChangedCondition.wait(mLock);
            return true;
        }
        int64_t whenUs = (*mEventQueue.begin()).mWhenUs;
        int64_t nowUs = GetNowUs();

        if (whenUs > nowUs) {
            int64_t delayUs = whenUs - nowUs;
            mQueueChangedCondition.waitRelative(mLock, delayUs * 1000ll);

            return true;
        }

        event = *mEventQueue.begin();
        mEventQueue.erase(mEventQueue.begin());
    }

    gLooperRoster.deliverMessage(event.mMessage);

    return true;
}

在loop方法中，不断的从mEventQueue取出消息，并dispatch给LooperRoster处理，mEventQueue是一个list链表，其元素都是Event结构，Event结构又包含消息处理的时间以及消息本身AMessage。再来看ALooperRoster的deliverMessage方法：

/frameworks/av/media/libstagefright/foundation/ALooperRoster.cpp

void ALooperRoster::deliverMessage(const sp<AMessage> &msg) {
    sp<AHandler> handler;

    {
        Mutex::Autolock autoLock(mLock);

        ssize_t index = mHandlers.indexOfKey(msg->target());

        if (index < 0) {
            ALOGW("failed to deliver message. Target handler not registered.");
            return;
        }

        const HandlerInfo &info = mHandlers.valueAt(index);
        handler = info.mHandler.promote();

        if (handler == NULL) {
            ALOGW("failed to deliver message. "
                 "Target handler %d registered, but object gone.",
                 msg->target());

            mHandlers.removeItemsAt(index);
            return;
        }
    }

    handler->onMessageReceived(msg);
}

这里首先通过AMessage的target找到需要哪个AHandler处理，然后调用这个AHandler的onMessageReceived去处理这个消息。注意前面的info.mHandler.promote()用于当前AHandler的强引用指针，也可以用来判断当前AHandler是否还存活在。由前面的知识我们知道，这里会调用到WifiDisplaySource的onMessageReceived方法，至于这些消息如何被处理，我们后面再来分析。再回到RemoteDisplay的构造函数中，ANetworkSession用于处理与网络请求相关的工作，比如创建socket，从socket中收发数据，当然这些工作都是由WifiDisplaySource控制的，我们先来看ANetworkSession的构造方法和start方法：

/frameworks/av/media/libstagefright/foundation/ANetworkSession.cpp

ANetworkSession::ANetworkSession()
    : mNextSessionID(1) {
    mPipeFd[0] = mPipeFd[1] = -1;
}

status_t ANetworkSession::start() {
    if (mThread != NULL) {
        return INVALID_OPERATION;
    }

    int res = pipe(mPipeFd);
    if (res != 0) {
        mPipeFd[0] = mPipeFd[1] = -1;
        return -errno;
    }

    mThread = new NetworkThread(this);

    status_t err = mThread->run("ANetworkSession", ANDROID_PRIORITY_AUDIO);

    if (err != OK) {
        mThread.clear();

        close(mPipeFd[0]);
        close(mPipeFd[1]);
        mPipeFd[0] = mPipeFd[1] = -1;

        return err;
    }

    return OK;
}

在start方法中，首先创建一个管道，这里创建的管理主要用于让ANetworkSession不断的做select循环，当有事务要处理时，就从select中跳出来处理，我们后面会分析到具体的代码。接着创建一个NetworkThread，NetworkThread也是继承于Thread，并实现threadLoop方法，在threadLoop方法中只是简单的调用ANetworkSession的threadLoop方法，我们来分析threadLoop方法：

/frameworks/av/media/libstagefright/foundation/ANetworkSession.cpp

void ANetworkSession::threadLoop() {
    fd_set rs, ws;
    FD_ZERO(&rs);
    FD_ZERO(&ws);

    FD_SET(mPipeFd[0], &rs);
    int maxFd = mPipeFd[0];

    {
        Mutex::Autolock autoLock(mLock);

        for (size_t i = 0; i < mSessions.size(); ++i) {
            const sp<Session> &session = mSessions.valueAt(i);

            int s = session->socket();

            if (s < 0) {
                continue;
            }

            if (session->wantsToRead()) {
                FD_SET(s, &rs);
                if (s > maxFd) {
                    maxFd = s;
                }
            }

            if (session->wantsToWrite()) {
                FD_SET(s, &ws);
                if (s > maxFd) {
                    maxFd = s;
                }
            }
        }
    }

    int res = select(maxFd + 1, &rs, &ws, NULL, NULL /* tv */);

    if (res == 0) {
        return;
    }

    if (res < 0) {
        if (errno == EINTR) {
            return;
        }

        ALOGE("select failed w/ error %d (%s)", errno, strerror(errno));
        return;
    }

}

这个函数比较长，我们分段来看，首先看select前半段部分，首先将mPipeFd[0]作为select监听的一个fd。然后循环的从mSessions中取出各个子Session（Session即为一个回话，在RTSP中当双方连接好TCP连接，并交互完Setup以后，就表示一个回话建立成功了，在RTSP中，可以在一对Server & Client之间建立多个回话，用于传输不同的数据），并通过socket类型添加到ReadFd和WirteFd中，最后调用select去等待是否有可读或者可写的事件发生。mSessions是一个KeyedVector，保存所有的Session及其SessionID，方便查找。关于Session何时创建，如何创建，我们后面再来分析。

接着回到RemoteDisplay的构造函数，再来分析WifiDisplaySource，WifiDisplaySource继承于AHandler，并实现其中的onMessageReceived方法用于处理消息。先来看WifiDisplaySource的构造函数：

/frameworks/av/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp

WifiDisplaySource::WifiDisplaySource(
        const sp<ANetworkSession> &netSession,
        const sp<IRemoteDisplayClient> &client,
        const char *path)
    : mState(INITIALIZED),
      mNetSession(netSession),
      mClient(client),
      mSessionID(0),
      mStopReplyID(0),
      mChosenRTPPort(-1),
      mUsingPCMAudio(false),
      mClientSessionID(0),
      mReaperPending(false),
      mNextCSeq(1),
      mUsingHDCP(false),
      mIsHDCP2_0(false),
      mHDCPPort(0),
      mHDCPInitializationComplete(false),
      mSetupTriggerDeferred(false),
      mPlaybackSessionEstablished(false) {
    if (path != NULL) {
        mMediaPath.setTo(path);
    }

    mSupportedSourceVideoFormats.disableAll();

    mSupportedSourceVideoFormats.setNativeResolution(
            VideoFormats::RESOLUTION_CEA, 5);  // 1280x720 p30

    // Enable all resolutions up to 1280x720p30
    mSupportedSourceVideoFormats.enableResolutionUpto(
            VideoFormats::RESOLUTION_CEA, 5,
            VideoFormats::PROFILE_CHP,  // Constrained High Profile
            VideoFormats::LEVEL_32);    // Level 3.2
}

首先给一些变量出初始化处理，由默认形参我们知道path为空。接着去清空VideoFormats中所有的设置，并把1280*720p以上的所有分辨率打开。VideoFormats是用于与Sink回复的M3作比对用的，可以快速找出我们和Sink支持的分辨率以及帧率，作为回复M4消息用，也用作后续传输TS数据的格式。首先来看VideoFormats的构造函数：

/frameworks/av/media/libstagefright/wifi-display/VideoFormats.cpp

VideoFormats::VideoFormats() {
    memcpy(mConfigs, mResolutionTable, sizeof(mConfigs));

    for (size_t i = 0; i < kNumResolutionTypes; ++i) {
        mResolutionEnabled[i] = 0;
    }

    setNativeResolution(RESOLUTION_CEA, 0);  // default to 640x480 p60
}

mResolutionTable是按照Wifi Display 规范定义好的一个3*32数组，里面的元素是config_t类型：

/frameworks/av/media/libstagefright/wifi-display/VideoFormats.h

struct config_t {
    size_t width, height, framesPerSecond;
    bool interlaced;
    unsigned char profile, level;
};

config_t包含了长、宽、帧率、隔行视频、profile和H.264 level。然后在构造函数中，对mResolutionEnabled[]数组全部置为0，mResolutionEnabled数组有三个元素，分别对应CEA、VESA、HH被选取的位，如果在mConfigs数组中相应的格式被选取，就会置mResolutionEnabled对应的位为1；相反取消支持一种格式时，相应的位就被置为0。在来看setNativeResolution：

/frameworks/av/media/libstagefright/wifi-display/VideoFormats.cpp

void VideoFormats::setNativeResolution(ResolutionType type, size_t index) {
    CHECK_LT(type, kNumResolutionTypes);
    CHECK(GetConfiguration(type, index, NULL, NULL, NULL, NULL));

    mNativeType = type;
    mNativeIndex = index;

    setResolutionEnabled(type, index);
}

首先做参数检查，检查输入的type和index是否合法，然后调用setResolutionEnabled去设置mResolutionEnabled和mConfigs中的相应的值：

/frameworks/av/media/libstagefright/wifi-display/VideoFormats.cpp

void VideoFormats::setResolutionEnabled(
        ResolutionType type, size_t index, bool enabled) {
    CHECK_LT(type, kNumResolutionTypes);
    CHECK(GetConfiguration(type, index, NULL, NULL, NULL, NULL));

    if (enabled) {
        mResolutionEnabled[type] |= (1ul << index);
        mConfigs[type][index].profile = (1ul << PROFILE_CBP);
        mConfigs[type][index].level = (1ul << LEVEL_31);
    } else {
        mResolutionEnabled[type] &= ~(1ul << index);
        mConfigs[type][index].profile = 0;
        mConfigs[type][index].level = 0;
    }
}

这里首先还是做参数的检查，由默认形参我们知道，enable是true，则设置mResolutionEnabled相应type中的对应格式为1，并设置mConfigs中的profile和level值为CBP和Level 3.1。这里设置640480 p60是因为在Wifi Display规范中，这个格式是必须要强制支持的，在Miracast认证中，这种格式也会被测试到。然后回到WifiDisplaySource的构造函数中，接下来会调用setNativeResolution去设置当前系统支持的默认格式为1280720 p30，并调用enableResolutionUpto去将1280*720 p30以上的格式都设置为支持：

/frameworks/av/media/libstagefright/wifi-display/VideoFormats.cpp

void VideoFormats::enableResolutionUpto(
        ResolutionType type, size_t index,
        ProfileType profile, LevelType level) {
    size_t width, height, fps, score;
    bool interlaced;
    if (!GetConfiguration(type, index, &width, &height,
            &fps, &interlaced)) {
        ALOGE("Maximum resolution not found!");
        return;
    }
    score = width * height * fps * (!interlaced + 1);
    for (size_t i = 0; i < kNumResolutionTypes; ++i) {
        for (size_t j = 0; j < 32; j++) {
            if (GetConfiguration((ResolutionType)i, j,
                    &width, &height, &fps, &interlaced)
                    && score >= width * height * fps * (!interlaced + 1)) {
                setResolutionEnabled((ResolutionType)i, j);
                setProfileLevel((ResolutionType)i, j, profile, level);
            }
        }
    }
}

这里采用width * height * fps * (!interlaced + 1)的方式去计算一个score值，然后遍历所有的mResolutionTable中的值去检查是否计算到的值比当前score要高，如果大于当前score值，就将这种分辨率enable，并设置mConfigs中对应分辨率的profile和H.264 level为CHP和Level 3.2。到这里WifiDisplaySource的构造函数分析完了，接着回到RemoteDisplay构造函数中，它会调用WifiDisplaySource的start方法，参数是的"ip:rtspPort"：

/frameworks/av/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp

status_t WifiDisplaySource::start(const char *iface) {
    CHECK_EQ(mState, INITIALIZED);

    sp<AMessage> msg = new AMessage(kWhatStart, id());
    msg->setString("iface", iface);

    sp<AMessage> response;
    return PostAndAwaitResponse(msg, &response);
}

static status_t PostAndAwaitResponse(
        const sp<AMessage> &msg, sp<AMessage> *response) {
    status_t err = msg->postAndAwaitResponse(response);

    if (err != OK) {
        return err;
    }

    if (response == NULL || !(*response)->findInt32("err", &err)) {
        err = OK;
    }

    return err;
}

在start函数中，构造一个AMessage，消息种类是kWhatStart，id()返回在ALooperRoster注册的handlerID值，ALooperRoster通过handlerID值可以快速找到对应的AHandler，我们知道，这里的id()返回WifiDisplaySource这个AHander的id值，这个消息最终也会被WifiDisplaySource的onMessageReceived方法处理。首先来看AMessage的postAndAwaitResponse方法：

/frameworks/av/media/libstagefright/foundation/AMessage.cpp

status_t AMessage::postAndAwaitResponse(sp<AMessage> *response) {
    return gLooperRoster.postAndAwaitResponse(this, response);
}

这里直接调用LooperRoster的postAndAwaitResponse方法，这里比较重要的是gLooperRoster在这里只是被extern引用：extern ALooperRoster gLooperRoster，其最终的声明和定义是在我们前面讲到的ALooper中。接着去看LooperRoster的postAndAwaitResponse方法：

/frameworks/av/media/libstagefright/foundation/ALooperRoster.cpp

status_t ALooperRoster::postAndAwaitResponse(
        const sp<AMessage> &msg, sp<AMessage> *response) {
    Mutex::Autolock autoLock(mLock);

    uint32_t replyID = mNextReplyID++;

    msg->setInt32("replyID", replyID);

    status_t err = postMessage_l(msg, 0 /* delayUs */);

    if (err != OK) {
        response->clear();
        return err;
    }

    ssize_t index;
    while ((index = mReplies.indexOfKey(replyID)) < 0) {
        mRepliesCondition.wait(mLock);
    }

    *response = mReplies.valueAt(index);
    mReplies.removeItemsAt(index);

    return OK;
}

首先会为每个需要reply的消息赋予一个replyID，后面会根据这个replyID去mReplies找到对应的response。再来看postMessage_l的实现：

/frameworks/av/media/libstagefright/foundation/ALooperRoster.cpp

status_t ALooperRoster::postMessage_l(
        const sp<AMessage> &msg, int64_t delayUs) {
    ssize_t index = mHandlers.indexOfKey(msg->target());

    if (index < 0) {
        ALOGW("failed to post message '%s'. Target handler not registered.",
              msg->debugString().c_str());
        return -ENOENT;
    }

    const HandlerInfo &info = mHandlers.valueAt(index);

    sp<ALooper> looper = info.mLooper.promote();

    if (looper == NULL) {
        ALOGW("failed to post message. "
             "Target handler %d still registered, but object gone.",
             msg->target());

        mHandlers.removeItemsAt(index);
        return -ENOENT;
    }

    looper->post(msg, delayUs);

    return OK;
}

首先从mHandler数组中找到当前AMessage对应的ALooper，然后调用ALooper的post方法，来看一下实现：

/frameworks/av/media/libstagefright/foundation/ALooper.cpp

void ALooper::post(const sp<AMessage> &msg, int64_t delayUs) {
    Mutex::Autolock autoLock(mLock);

    int64_t whenUs;
    if (delayUs > 0) {
        whenUs = GetNowUs() + delayUs;
    } else {
        whenUs = GetNowUs();
    }

    List<Event>::iterator it = mEventQueue.begin();
    while (it != mEventQueue.end() && (*it).mWhenUs <= whenUs) {
        ++it;
    }

    Event event;
    event.mWhenUs = whenUs;
    event.mMessage = msg;

    if (it == mEventQueue.begin()) {
        mQueueChangedCondition.signal();
    }

    mEventQueue.insert(it, event);
}

delayUs用于做延时消息使用，会加上当前时间作为消息应该被处理的时间。然后依次比较mEventQueue链表中的所有消息，并把当前消息插入到比whenUs大的前面一个位置。如果这是mEventQueue中的第一个消息，则发出一个signal通知等待的线程。前面我们知道在ALooper的loop方法中会循环的从mEventQueue获取消息并dispatch出去给WifiDisplaySource的onMessageReceived去处理，我们接着来看这部分的实现。这里绕这么大一圈，最后WifiDisplaySource发送的消息还是给自己处理，主要是为了避开主线程处理的事务太多，通过消息机制，让更多的繁杂的活都在Thread中去完成。

/frameworks/av/media/libstagefright/wifi-display/source/WifiDisplaySource.cpp

void WifiDisplaySource::onMessageReceived(const sp<AMessage> &msg) {
    switch (msg->what()) {
        case kWhatStart:
        {
            uint32_t replyID;
            CHECK(msg->senderAwaitsResponse(&replyID));

            AString iface;
            CHECK(msg->findString("iface", &iface));

            status_t err = OK;

            ssize_t colonPos = iface.find(":");

            unsigned long port;

            if (colonPos >= 0) {
                const char *s = iface.c_str() + colonPos + 1;

                char *end;
                port = strtoul(s, &end, 10);

                if (end == s || *end != '\0' || port > 65535) {
                    err = -EINVAL;
                } else {
                    iface.erase(colonPos, iface.size() - colonPos);
                }
            } else {
                port = kWifiDisplayDefaultPort;
            }

            if (err == OK) {
                if (inet_aton(iface.c_str(), &mInterfaceAddr) != 0) {
                    sp<AMessage> notify = new AMessage(kWhatRTSPNotify, id());

                    err = mNetSession->createRTSPServer(
                            mInterfaceAddr, port, notify, &mSessionID);
                } else {
                    err = -EINVAL;
                }
            }

            mState = AWAITING_CLIENT_CONNECTION;

            sp<AMessage> response = new AMessage;
            response->setInt32("err", err);
            response->postReply(replyID);
            break;
        }

首先通过AMessage获取到replayID和iface，然后把iface分割成ip和port，分别保存在mInterfaceAddr和port中。在调用ANetSession的createRTSPServer去创建一个RTSP server，最后构造一个response对象并返回。我们先来看createRTSPServer方法：

/frameworks/av/media/libstagefright/foundation/ANetworkSession.cpp

status_t ANetworkSession::createRTSPServer(
        const struct in_addr &addr, unsigned port,
        const sp<AMessage> ¬ify, int32_t *sessionID) {
    return createClientOrServer(
            kModeCreateRTSPServer,
            &addr,
            port,
            NULL /* remoteHost */,
            0 /* remotePort */,
            notify,
            sessionID);
}

status_t ANetworkSession::createClientOrServer(
        Mode mode,
        const struct in_addr *localAddr,
        unsigned port,
        const char *remoteHost,
        unsigned remotePort,
        const sp<AMessage> ¬ify,
        int32_t *sessionID) {
    Mutex::Autolock autoLock(mLock);

    *sessionID = 0;
    status_t err = OK;
    int s, res;
    sp<Session> session;

    s = socket(
            AF_INET,
            (mode == kModeCreateUDPSession) ? SOCK_DGRAM : SOCK_STREAM,
            0);

    if (s < 0) {
        err = -errno;
        goto bail;
    }

    if (mode == kModeCreateRTSPServer
            || mode == kModeCreateTCPDatagramSessionPassive) {
        const int yes = 1;
        res = setsockopt(s, SOL_SOCKET, SO_REUSEADDR, &yes, sizeof(yes));

        if (res < 0) {
            err = -errno;
            goto bail2;
        }
    }

    err = MakeSocketNonBlocking(s);

    if (err != OK) {
        goto bail2;
    }

    struct sockaddr_in addr;
    memset(addr.sin_zero, 0, sizeof(addr.sin_zero));
    addr.sin_family = AF_INET;
    } else if (localAddr != NULL) {
        addr.sin_addr = *localAddr;
        addr.sin_port = htons(port);

        res = bind(s, (const struct sockaddr *)&addr, sizeof(addr));

        if (res == 0) {
            if (mode == kModeCreateRTSPServer
                    || mode == kModeCreateTCPDatagramSessionPassive) {
                res = listen(s, 4);
            } else {

    if (res < 0) {
        err = -errno;
        goto bail2;
    }

    Session::State state;
    switch (mode) {

        case kModeCreateRTSPServer:
            state = Session::LISTENING_RTSP;
            break;

        default:
            CHECK_EQ(mode, kModeCreateUDPSession);
            state = Session::DATAGRAM;
            break;
    }

    session = new Session(
            mNextSessionID++,
            state,
            s,
            notify);

    mSessions.add(session->sessionID(), session);

    interrupt();

    *sessionID = session->sessionID();

    goto bail;

bail2:
    close(s);
    s = -1;

bail:
    return err;
}

createRTSPServer直接调用createClientOrServer，第一个参数是kModeCreateRTSPServer表示要创建一个RTSP server。createClientOrServer的代码比较长，上面是精简后的代码，其它没看到的代码我们以后遇到的过程中再来分析。上面的代码中首先创建一个socket，然后设置一下socket的reuse和no-block属性，接着bind到指定的IP和port上，然后再此socket上开始listen。接下来置当前ANetworkSession的状态是LISTENING_RTSP。然后创建一个Session会话对象，在构造函数中会传入notify作为参数，notify是一个kWhatRTSPNotify的AMessag，后面会看到如何使用它。然后添加到mSessions数组当中。接着调用interrupt方法，让ANetworkSession的NetworkThread线程跳出select语句，并重新计算readFd和writeFd用于select监听的文件句柄。

/frameworks/av/media/libstagefright/foundation/ANetworkSession.cpp

void ANetworkSession::interrupt() {
    static const char dummy = 0;

    ssize_t n;
    do {
        n = write(mPipeFd[1], &dummy, 1);
    } while (n < 0 && errno == EINTR);

    if (n < 0) {
        ALOGW("Error writing to pipe (%s)", strerror(errno));
    }
}

interrupt方法向pipe中写入一个空消息，前面我们已经介绍过threadLoop了，这里就会把刚刚创建的socket加入到监听的readFd中。到这里，关于WifiDisplay连接的建立就讲完了，后面会再从收到Sink的TCP连接请求开始讲起。最后贴一份从WifiDisplaySettings到ANetworkSession如何创建socket的时序图：

转载自：https://blog.csdn.net/lilian0118/article/details/23168531?spm=1001.2014.3001.5502