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Lesson - 3:Messages and Commands
3.2 : Understanding Message Loops
The heart of any Windows program is the message loop, typically contained in a WinMain() routine. The WinMain() routine is, like the main() in DOS or UNIX, the function called by the operating system when you run the program. You won’t write any WinMain() routines because it is now hidden away in the code that AppWizard generates for you. Still, there is a WinMain(), just as there is in Windows C programs. Listing 3.2 shows a typical WinMain(). Listing 3.2 Typical WinMain() Routine int APIENTRY WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nCmdShow) { MSG msg; if (! InitApplication (hInstance)) return (FALSE); if (! InitInstance (hInstance, nCmdShow)) return (FALSE); while (GetMessage (&msg, NULL, 0, 0)){ TranslateMessage (&msg); DispatchMessage (&msg); } return (msg.wParam); } In a Windows C program like this, InitApplication() typically calls RegisterWindow(), and InitInstance() typically calls CreateWindow(). Then comes the message loop, the while loop that calls GetMessage(). The API function GetMessage() fills msg with a message destined for this application and almost always returns TRUE, so this loop runs over and over until the program is finished. The only thing that makes GetMessage() return FALSE is if the message it receives is WM_QUIT. TranslateMessage() is an API function that streamlines dealing with keyboard messages. Most of the time, you don’t need to know that "the A key just went down" or "the A key just went up," and so on. It’s enough to know that "the user pressed A." TranslateMessage() deals with that. It catches the WM_KEYDOWN and WM_KEYUP messages and usually sends a WM_CHAR message in their place. Of course, with MFC, most of the time you don’t care that the user pressed A. The user types into an edit box or similar control, and you can retrieve the entire string out of it later, when the user has clicked OK. Don’t worry too much about TranslateMessage(). The API function DispatchMessage() calls the WndProc for the window that the message is headed for. The WndProc() function for a Windows C program is a huge switch statement with one case for each message the programmer planned to catch, such as the one in Listing 3.3. Listing 3.3 Typical WndProc() Routine LONG APIENTRY MainWndProc (HWND hWnd, // window handle UINT message, // type of message UINT wParam, // additional information LONG lParam) // additional information { switch (message) { case WM_MOUSEMOVE: //handle mouse movement break; case WM_LBUTTONDOWN: //handle left click break; case WM_RBUTTONDOWN: //handle right click break; case WM_PAINT: //repaint the window break; case WM_DESTROY: // message: window being destroyed PostQuitMessage (0); break; default: return (DefWindowProc (hWnd, message, wParam, lParam)); } return (0); } As you can imagine, these WndProcs become very long in a hurry. Program maintenance can be a nightmare. MFC solves this problem by keeping information about message processing close to the functions that handle the messages, freeing you from maintaining a giant switch statement that is all in one place. Read on to see how it’s done.

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