Hey, I want animations too! - A quick tutorial on buffered paint animation.

#include <windowsx.h>
#include <uxtheme.h>

TCHAR g_currentState = L'0';
TCHAR g_drawnState;

BOOL OnCreate( HWND hwnd, LPCREATESTRUCT wParam );
void OnDestroy( HWND hwnd );
void OnChar( HWND hwnd, TCHAR ch, int cRepeats );
void OnPaint( HWND hwnd );
void PaintContent( HWND hwnd, HDC hdc, LPRECT rc, TCHAR state );

// Add to code
void OnChar( HWND hwnd, TCHAR ch, int cRepeats )
{
        switch ( ch )
        {
        case ' ':
                g_currentState = g_drawnState;
                g_currentState++;

                if ( g_currentState > L'~' )
                        g_currentState = L'!';

                InvalidateRect(hwnd, NULL, TRUE ); /* InvalidateRect gets our window to repaint (i.e. our OnPaint function gets called right away) */
                break;
        }
}

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
        switch (message)
        {
                HANDLE_MSG(hWnd, WM_CREATE, OnCreate);
                HANDLE_MSG(hWnd, WM_PAINT, OnPaint);
                HANDLE_MSG(hWnd, WM_CHAR, OnChar);
                HANDLE_MSG(hWnd, WM_DESTROY, OnDestroy);

        default:
                return DefWindowProc(hWnd, message, wParam, lParam);
        }

        return 0;
}

// Add to code
BOOL OnCreate( HWND hwnd, LPCREATESTRUCT wParam )
{
        BufferedPaintInit();
        return TRUE;
}

void OnDestroy( HWND hwnd )
{
        BufferedPaintUnInit();
        BufferedPaintStopAllAnimations(hwnd); /* Stops any animations that are in progress so that the buffered paint engine isn't trying to animate a window that isn't there! */
        PostQuitMessage(0);
}

void OnPaint( HWND hwnd )
{
        PAINTSTRUCT ps;
        BeginPaint(hwnd, &ps);

        /* 
         * BufferedPaintRenderAnimation will return TRUE if this paint call is a result of WM_PAINT fired by the engine. The animation works by the buffered paint engine
         * sending WM_PAINTS into your window. What we do here is detect if we are inside one of those WM_PAINTs. If we are, then we skip our code and say
         * we're done here - somebody else did the painting for me! If not, then we continue to pain our content to render the animation.
         */
        if ( !BufferedPaintRenderAnimation( hwnd, ps.hdc))
        {
                RECT rc;
                BP_ANIMATIONPARAMS params = {0};
                GetClientRect( hwnd, &rc );

                params.cbSize     = sizeof(params);
                params.style      = BPAS_LINEAR; /* Linear transition */
                params.dwDuration = g_currentState == g_drawnState ? 0 : 1000; /* If the state hasn't changed, then set duration to zero. The engine doesn't perform any animation if this is zero */

                HDC hdcFrom;
                HDC hdcTo;

                /*
                 * BeginBufferedAnimation will give us a handle to two device contexts: A DC to render the previous state and a DC to render the current state.
                 * These are essentially key frames. hdcFrom is the DC to paint the initial state of the animation. hdcTo is the DC to paint the final state of the animation.
                 *
                 * hdcFrom will be NULL on the first call, only the final state is to be painted. 
                 */
                HANIMATIONBUFFER hBuffer = BeginBufferedAnimation( hwnd, ps.hdc, &rc, BPBF_COMPATIBLEBITMAP, NULL, &params, &hdcFrom, &hdcTo );

                if ( hBuffer )
                {
                        if ( hdcFrom )
                                PaintContent( hwnd, hdcFrom, &rc, g_drawnState );

                        if ( hdcTo )
                                PaintContent( hwnd, hdcTo, &rc, g_currentState );

                        /* Here we update the drawn state to the current state*/
                        g_drawnState = g_currentState;
                        EndBufferedAnimation( hBuffer, TRUE );
                }
        }

        EndPaint(hwnd, &ps);
}

void PaintContent( HWND hwnd, HDC hdc, LPRECT rc, TCHAR state )
{
        COLORREF prevText = SetTextColor( hdc, GetSysColor( COLOR_WINDOWTEXT ) );
        COLORREF prevBack = SetBkColor( hdc, GetSysColor( COLOR_WINDOW ) );
        HFONT prevFont    = SelectFont( hdc, GetStockFont( SYSTEM_FONT ) );

        RECT rcClient;
        GetClientRect( hwnd, &rcClient );
        FillRect( hdc, &rcClient, GetSysColorBrush( COLOR_WINDOW ) ); /* Fill the background */

        /* A little bit of tweaking here */
        int cyRow = ( rcClient.bottom - rcClient.top ) / 22 * 2;
        if ( cyRow < 2 )
                cyRow = 2;

        int i;
        for ( i = 0; i < 10; ++i )
        {
                int offset = ( ( state * 2 ) % cyRow );
                ExtTextOutW(hdc, 20, cyRow * i + offset, 0, NULL, &state, 1, NULL );
        }

        SelectFont( hdc, prevFont );
        SetBkColor( hdc, prevBack );
        SetTextColor( hdc, prevText );
}

wcex.hbrBackground   = NULL;