Android WifiDisplay分析二：Wifi display连接过程

目录(?)[-]

WifiDisplay之P2P的建立
WifiDisplay之RTSP server的创建

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。

HIDC: Human Interface Device Class （遵循HID标准的设备类）
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）
PES: Packetized Elementary Stream （数字电视基本码流）
HDCP: High-bandwidth Digital Content Protection （加密方式，用于加密传输的MPEG2-TS流）
MPEG2-TS: Moving Picture Experts Group 2 Transport Stream （Wifi display之间传输的是MPEG2-TS流）
RTSP: Real-Time Streaming Protocol （Wifi display通过RTSP协议来交互两边的能力）
RTP: Real-time Transport Protocol （Wifi display通过RTP来传输MPEG2-TS流）
Wi-Fi P2P: Wi-Fi Direct
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：

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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：

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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：

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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。

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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：

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private void updateConnection() {
n style="white-space:pre"> </span>//更新是否需要scan或者停止scan
updateScanState();
n style="white-space:pre"> </span>//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) {
}
// 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;
}<span style="font-family: Arial, Helvetica, sans-serif;"> </span>

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：

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} 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);
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 && !info.contains(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 && !info.contains(mDesiredDevice)) {
disconnect();
return;
}
if (mConnectingDevice != null && mConnectingDevice == mDesiredDevice) {
Slog.i(TAG, "Connected to Wifi display: "
+ mConnectingDevice.deviceName);
mHandler.removeCallbacks(mConnectionTimeout);
mConnectedDeviceGroupInfo = info;
mConnectedDevice = mConnectingDevice;
mConnectingDevice = null;
updateConnection();
}
}
});
}
}

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：

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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);
// 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，博主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：

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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：

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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。

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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：

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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。

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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：

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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：

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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：

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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：

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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：

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status_t ALooper::start(
bool runOnCallingThread, bool canCallJava, int32_t priority) {
if (runOnCallingThread) {
}
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：

[java] view plain copy

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

[java] view plain copy

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：

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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：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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640*480 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1280*720 p30，并调用enableResolutionUpto去将1280*720 p30以上的格式都设置为支持：

[java] view plain copy

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"：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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：

[java] view plain copy

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。

[java] view plain copy

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：

[java] view plain copy

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。

[java] view plain copy

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：