Section 7.5. Event Handling with Microsoft Windows GetMessage( ) Message Loop

7.5. Event Handling with Microsoft Windows GetMessage( ) Message Loop

There are some differences in working with the Windows event loop. In this example, you will configure and create a window that is not displayed in the application. This gives you a template for developing GUI-based Zeroconf applications for Windows. The Zeroconf events are processed as messages to this window. Here is the outline of the HandleEvents( ) function for Windows:

 void HandleEvents(DNSServiceRef inServiceRef)     {     //...   Configure and create a window that is not shown but that     // is used to process DNS-SD events as messages to the window.     wind = CreateWindow(wcex.lpszClassName, wcex.lpszClassName, 0,                          CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL,                          NULL, instance, NULL);     // ... Associate the DNS-SD browser with our window     err = WSAAsyncSelect((SOCKET) DNSServiceRefSockFD(gServiceRef), wind,                            DNSSD_EVENT, FD_READ | FD_CLOSE);     assert(err == kDNSServiceErr_NoError);     // DNS-SD events are dispatched while in this loop.     while(GetMessage(&msg, NULL, 0, 0))         {         TranslateMessage(&msg);         DispatchMessage(&msg);         }     // Clean up.     WSAAsyncSelect((SOCKET) DNSServiceRefSockFD(gServiceRef), wind,                                     DNSSD_EVENT, 0);     } 

When a window event is received, it is processed in the callback function WndProc( ). If the event is a DNS-SD event, it is passed on to DNSServiceProcessResult( ) as before. Here is the sketch of WndProc( ):

 static LRESULT CALLBACK WndProc(HWND inWindow, UINT inMsg,                                  WPARAM inWParam, LPARAM inLParam)     {     LRESULT result;     switch(inMsg)         {         case DNSSD_EVENT:             DNSServiceProcessResult(gServiceRef);             result = 0;             break; //...         }     return(result);     } 

All of the details are provided in the code listing in Example 7-9.

Example 7-9. Windows event loop example

 #include    "stdafx.h" #include    <assert.h> #include    <stdio.h> #include    <dns_sd.h> // Constants #define DNSSD_EVENT        (WM_USER + 0x100)                       // Message sent to Window when a DNS-SD event occurs. // Prototypes void HandleEvents(DNSServiceRef inServiceRef); static LRESULT CALLBACK WndProc(HWND inWindow,                                  UINT inMsg,                                  WPARAM inWParam,                                  LPARAM inLParam); static void DNSSD_API     BrowserCallBack(DNSServiceRef     inServiceRef,                      DNSServiceFlags inFlags,                      uint32_t inIFI,                      DNSServiceErrorType    inError,                      const char * inName,                      const char * inType,                      const char * inDomain,                      void * inContext); // Globals static DNSServiceRef        gServiceRef = NULL; // Main entry point for application. void HandleEvents(DNSServiceRef inServiceRef)     {     HINSTANCE        instance;     WNDCLASSEX        wcex;     HWND            wind;     MSG            msg;     int            err;     gServiceRef = inServiceRef;     // Create the window. This window won't actually be shown,     // but it demonstrates how to use DNS-SD with Windows GUI     // applications by having DNS-SD events processed as messages     // to a Window.     instance = GetModuleHandle(NULL);     assert(instance);     wcex.cbSize = sizeof(wcex);     wcex.style     = 0;     wcex.lpfnWndProc = (WNDPROC) WndProc;     wcex.cbClsExtra     = 0;     wcex.cbWndExtra     = 0;     wcex.hInstance = instance;     wcex.hIcon     = NULL;     wcex.hCursor = NULL;     wcex.hbrBackground = NULL;     wcex.lpszMenuName = NULL;     wcex.lpszClassName = TEXT("ZeroconfExample");     wcex.hIconSm = NULL;     RegisterClassEx(&wcex);     wind = CreateWindow(wcex.lpszClassName, wcex.lpszClassName, 0,                          CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL,                          NULL, instance, NULL);     assert(wind);     // Associate the DNS-SD browser with our window     // using the WSAAsyncSelect mechanism. Whenever something     // related to the DNS-SD browser occurs, our private Windows message     // will be sent to our window so we can give DNS-SD a     // chance to process it. This allows DNS-SD to avoid using a     // secondary thread (and all the issues with synchronization that     // would introduce), but still process everything asynchronously.     // This also simplifies app code because DNS-SD will only run when we     // explicitly call it.     err = WSAAsyncSelect((SOCKET) DNSServiceRefSockFD(gServiceRef), wind,                            DNSSD_EVENT, FD_READ | FD_CLOSE);     assert(err == kDNSServiceErr_NoError);     // Main event loop for the application. All DNS-SD events are     // dispatched while in this loop.     while(GetMessage(&msg, NULL, 0, 0))         {         TranslateMessage(&msg);         DispatchMessage(&msg);         }     // Clean up. This is not strictly necessary since the normal     // process cleanup will close DNS-SD socket(s) and release memory,     // but it's here to demonstrate how to do it.     WSAAsyncSelect((SOCKET) DNSServiceRefSockFD(gServiceRef), wind,                      DNSSD_EVENT, 0);     } // Callback for the Window. DNS-SD events are delivered here. static LRESULT CALLBACK WndProc(HWND inWindow, UINT inMsg,                                  WPARAM inWParam, LPARAM inLParam)     {     LRESULT result;     switch(inMsg)         {         case DNSSD_EVENT:           // Process the DNS-SD event. All DNS-SD callbacks occur from           // within this function.           if (DNSServiceProcessResult(gServiceRef) != kDNSServiceErr_NoError)             result = -1;           else             result = 0;           break;         default:           result = DefWindowProc(inWindow, inMsg, inWParam, inLParam);           break;         }     return(result);     }