Linguagem C - SetTimer(hwnd, ID_TIMER, 50, NULL);

#include <windows.h>
#define ID_TIMER    1

LRESULT CALLBACK WndProc(HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance,
                   PSTR szCmdLine, int iCmdShow) {
    static TCHAR szAppName[] = TEXT("Bounce");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;
    wndclass.style         = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc   = WndProc;
    wndclass.cbClsExtra    = 0;
    wndclass.cbWndExtra    = 0;
    wndclass.hInstance     = hInstance;
    wndclass.hIcon         = LoadIcon(NULL, IDI_APPLICATION);
    wndclass.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = (HBRUSH) WHITE_BRUSH;
    wndclass.lpszMenuName  = NULL;
    wndclass.lpszClassName = szAppName;
    if(!RegisterClass(&wndclass)) {
        MessageBox(NULL, TEXT("This program requires Windows NT!"),
                   szAppName, MB_ICONERROR);
        return 0;
    }
    hwnd = CreateWindow(szAppName, TEXT("Bouncing Ball"),
                        WS_OVERLAPPEDWINDOW,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        CW_USEDEFAULT, CW_USEDEFAULT,
                        NULL, NULL, hInstance, NULL);
    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);
    while(GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage (&msg);
    }
    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd,UINT iMsg,WPARAM wParam,LPARAM lParam) {
    static HBITMAP hBitmap;
    static int     cxClient, cyClient, xCenter,
    yCenter, cxTotal, cyTotal, cxRadius, cyRadius,
    cxMove, cyMove, xPixel, yPixel;
    HBRUSH         hBrush;
    HDC            hdc, hdcMem;
    int            iScale;
    switch(iMsg) {
        case WM_CREATE:
            hdc = GetDC(hwnd);
            xPixel = GetDeviceCaps(hdc, ASPECTX);
            yPixel = GetDeviceCaps(hdc, ASPECTY);
            ReleaseDC(hwnd, hdc);
            SetTimer(hwnd, ID_TIMER, 50, NULL);
            return 0;
        case WM_SIZE:
            xCenter = (cxClient = LOWORD(lParam)) / 2;
            yCenter = (cyClient = HIWORD(lParam)) / 2;
            iScale = min(cxClient * xPixel, cyClient * yPixel) / 16;
            cxRadius = iScale / xPixel;
            cyRadius = iScale / yPixel;
            cxMove = max(1, cxRadius / 2);
            cyMove = max(1, cyRadius / 2);
            cxTotal = 2 * (cxRadius + cxMove);
            cyTotal = 2 * (cyRadius + cyMove);
            if(hBitmap)
                DeleteObject(hBitmap);
            hdc = GetDC(hwnd);
            hdcMem = CreateCompatibleDC(hdc);
            hBitmap=CreateCompatibleBitmap(hdc,cxTotal,cyTotal);
            ReleaseDC(hwnd, hdc);

            SelectObject(hdcMem, hBitmap);
            Rectangle(hdcMem, -1, -1, cxTotal + 1, cyTotal + 1);

            hBrush = CreateHatchBrush(HS_DIAGCROSS, 0L);
            SelectObject(hdcMem, hBrush);
            SetBkColor(hdcMem, RGB(255, 0, 255));
            Ellipse(hdcMem,cxMove,cyMove,cxTotal-cxMove,cyTotal-cyMove);
            DeleteDC(hdcMem);
            DeleteObject(hBrush);
            return 0;
        case WM_TIMER:
            if(!hBitmap)
                break;
            hdc = GetDC(hwnd);
            hdcMem = CreateCompatibleDC(hdc);
            SelectObject(hdcMem, hBitmap);
            BitBlt(hdc,xCenter-cxTotal/2,yCenter-cyTotal/2,cxTotal,cyTotal,
                   hdcMem, 0, 0, SRCCOPY);
            ReleaseDC(hwnd, hdc);
            DeleteDC(hdcMem);
            xCenter += cxMove;
            yCenter += cyMove;
            if((xCenter+cxRadius>=cxClient)||(xCenter-cxRadius<=0))
                cxMove = -cxMove;
            if((yCenter+cyRadius>=cyClient)||(yCenter-cyRadius<=0))
                cyMove =-cyMove;
            return 0;
        case WM_DESTROY:
            if(hBitmap)
                DeleteObject(hBitmap);
            KillTimer(hwnd, ID_TIMER);
            PostQuitMessage(0);
            return 0;
    }
    return DefWindowProc(hwnd, iMsg, wParam, lParam);
}