Scribble

/* to compile: cl.exe scribble.c /link /SUBSYSTEM:WINDOWS user32.lib gdi32.lib */

#define WIN32_LEAN_AND_MEAN
#define VC_EXTRALEAN

#include <windows.h>
#include <tchar.h>

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam);

int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR szCmdLine, int iCmdShow)
{
    static TCHAR szWindowClass[] = _T("Scribble");
    static TCHAR szTitle[] = _T("Scribble");
    
    MSG msg;
    HWND hWnd;
    WNDCLASSEX wcex;

    wcex.cbSize         = sizeof(WNDCLASSEX);
    wcex.style          = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc    = WndProc;
    wcex.cbClsExtra     = 0;
    wcex.cbWndExtra     = 0;
    wcex.hInstance      = hInstance;
    wcex.hIcon          = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
    wcex.hCursor        = LoadCursor(NULL, IDC_CROSS);
    wcex.hbrBackground  = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName   = NULL;
    wcex.lpszClassName  = szWindowClass;
    wcex.hIconSm        = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_APPLICATION));
    
    RegisterClassEx(&wcex);
    
    hWnd = CreateWindow(
        szWindowClass,
        szTitle,
        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 (int)msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    static int x1, y1, x2, y2;
    HDC hdc;

    switch (message)
    {
    case WM_LBUTTONDOWN:
        InvalidateRect(hWnd, NULL, FALSE);
        x1 = LOWORD(lParam);
        y1 = HIWORD(lParam);
        return 0;
        
    case WM_RBUTTONDOWN:
        InvalidateRect(hWnd, NULL, TRUE);
        return 0;

    case WM_MOUSEMOVE:
        if (wParam & MK_LBUTTON)
        {
            hdc = GetDC(hWnd);
            x2 = LOWORD(lParam);
            y2 = HIWORD(lParam);
            MoveToEx(hdc, x1, y1, NULL);
            LineTo(hdc, x2, y2);
            ReleaseDC(hWnd, hdc);
            x1 = x2;
            y1 = y2;
        }
        return 0;

    case WM_LBUTTONUP:
        InvalidateRect(hWnd, NULL, FALSE);
        return 0;

    case WM_CLOSE:
        DestroyWindow(hWnd);
        return 0;

    case WM_DESTROY:
        PostQuitMessage(0);
        return 0;
    }
    
    return DefWindowProc(hWnd, message, wParam, lParam);
}