Understanding GUI-Based Programming: Event Loops and Windows in Software Engineering

1. GUI-Based Programming ECE 417/617:Elements of Software Engineering Stan Birchfield Clemson University

2. Paradigms Compared Application callbacks draw output Widgets Application output input input The User Traditional command-line GUI-based

3. Event/Message loop

4. Event loop – pseudocode int main() { return WinMain(); } WinMain() { while (1) { // loop forever, waiting for an event if (event_exists) { //there is an event, figure out what to do if (event == keydown_a) display(‘user pressed the A key’); else if (event == window_resize) display(‘window resized’); else if (event == repaint) display(‘need to repaint window’); else if (event == keydown_escape) exit_program(); } } }

5. Event loop – WinMain int WINAPI WinMain (HINSTANCE hInst, HINSTANCE hPrevInst, char * cmdParam, int cmdShow) { char className [] = "Winnie"; WinClass winClass (WindowProcedure, className, hInst); winClass.Register (); WinMaker win ("Hello Windows!", className, hInst); win.Show (cmdShow); MSG msg; int status; while ((status = ::GetMessage (& msg, 0, 0, 0)) != 0) { if (status == -1) return -1; ::DispatchMessage (& msg); } return msg.wParam; }

6. Event loop – WindowProc LRESULT CALLBACK WindowProcedure (HWND hwnd, unsigned int message, WPARAM wParam, LPARAM lParam) { switch (message) { case WM_DESTROY: ::PostQuitMessage (0); return 0; } return ::DefWindowProc (hwnd, message, wParam, lParam ); }

7. Event loop (cont.) int WINAPI WinMain(HINSTANCE hinstance, HINSTANCE hprev, PSTR cmdline, int ishow) { HWND hwnd; MSG msg; //initialization code goes here while(1) { // Get message(s) if there is one if(PeekMessage(&msg,hwnd,0,0,PM_REMOVE)) { if(msg.message == WM_QUIT) break; TranslateMessage(&msg); DispatchMessage(&msg); //this calls the CALLBACK function WinProc() } else { DrawScene(); //display the OpenGL/DirectX scene } } }

8. Event loop (cont.) LRESULT CALLBACK WinProc(HWND hwnd, UINT message, WPARAM wparam, LPARAM lparam) { PAINTSTRUCT ps; // Depending on the message -- we'll do different stuff switch(message) { case WM_PAINT: Draw(); return 0; // ESC will quit program case WM_KEYDOWN: //user pressed a key if(GetAsyncKeyState(VK_ESCAPE)) //it was the escape key PostQuitMessage(0); //quit program return 0; case WM_DESTROY: //windows wants the program to die case WM_CLOSE: //or the user closed the window PostQuitMessage(0); //quit program return 0; } return DefWindowProc(hwnd, message, wparam, lparam); }

9. GUI Concepts • Widget – graphic object with functionality; e.g., button, toolbar, ... • Window – holds widgets • Child/parent – relationships between windows • Event / message – how windows communicate

10. Anatomy of a Window title bar menu toolbar client area status bar

11. Microsoft Windows Programming • History: • Win32 API: core library written in C • MFC: C++ wrappers around most common Win32 API functions • Lots of macros • Lots of legacy code • Not free, not portable • But it works (generally speaking)