Question

Problem is like this:
I have two winapi application’s. There is only one way communication. App1 sends message to App2. App2 should receive WM_COPYDATA with structure which has coordinates of two shapes and then it should paint those shapes.
Message is received. Data from structure is ok. Code in WM_COPYDATA case is executed which should paint window but nothing happens ( window has just solid white background)

Sending message looks like this:

HWND secondApp = FindWindowEx(NULL, NULL, _T("lab2app2"), NULL ); 
int error = GetLastError();
if(secondApp != 0) // window found, send message
{
    // prepare data
    FiguresData figuresData;
    // Set Data About Location and dimensions of figures
    // TODO
    figuresData.square_origin_x = square_origin_x;
    figuresData.square_origin_y = square_origin_y;
    figuresData.circle_origin_x = circle_origin_x;
    figuresData.circle_origin_y = circle_origin_y;

    COPYDATASTRUCT data;

    data.dwData = FIGURESDATA;
    data.cbData = sizeof( figuresData );
    data.lpData = &figuresData;
    bool value =  SendMessage(secondApp, WM_COPYDATA, (WPARAM)(HWND)mainWindow, (LPARAM)(LPVOID)&data);
}

FiguresData structure is straightforward and has only 4 ints in it.

The App2 receives messages and code for that is:

LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
PAINTSTRUCT ps;
HDC hdc;

switch (message)
{
case WM_PAINT:

    RECT clientRectangle;
GetClientRect(hWnd, &clientRectangle);

hdc = BeginPaint(hWnd, &ps);

    // nothing really here

    EndPaint(hWnd, &ps);
    break;

case WM_COPYDATA:
    {
    PCOPYDATASTRUCT pData;
    pData = (PCOPYDATASTRUCT) lParam;

    int square_origin_x = ((FiguresData *) ( pData->lpData ))->square_origin_x;
    int square_origin_y = ((FiguresData *) ( pData->lpData ))->square_origin_y;
    int circle_origin_x = ((FiguresData *) ( pData->lpData ))->circle_origin_x;
    int circle_origin_y = ((FiguresData *) ( pData->lpData ))->circle_origin_y;
    hdc = BeginPaint(hWnd, &ps);
    HPEN oldPen = (HPEN)SelectObject(hdc, pen);
    HBRUSH oldBrush = (HBRUSH)SelectObject(hdc, backgroundBrush);
    COLORREF oldBackgroundColor = SetBkColor(hdc, backgroundColor);

    Rectangle(hdc, square_origin_x, square_origin_y, square_origin_x+ SQUARE_SIDE, square_origin_y + SQUARE_SIDE);
    Ellipse(hdc, circle_origin_x, circle_origin_y, circle_origin_x+ SQUARE_SIDE, circle_origin_y + SQUARE_SIDE);


    // Put everything back the way we found it
    SelectObject(hdc, oldPen);
    SetBkColor(hdc, oldBackgroundColor);

    EndPaint(hWnd, &ps);
    break;
    }
case WM_DESTROY:
    PostQuitMessage(0);
    break;
default:
    return DefWindowProc(hWnd, message, wParam, lParam);
    break;
}

return 0;
}

On debugger I see that window handle is ok. Message is sent as well as received by App2 and code in the WM_COPYDATA case is executed but it has no effect.
No shapes are drawn on the App2’s window and I have no idea why. Any clue?

Answer 1

You are calling BeginPaint outside of a WM_PAINT handler. BeginPaint returns a device context that allows drawing only on the invalid region of the window. Since you have not called InvalidateRect, the window is not yet invalid, so BeginPaint does not paint anything.

As a general rule, you should restrict your painting to your WM_PAINT handler. Have your WM_COPYDATA handler remember what needs to be painted, then invalidate the window and have the WM_PAINT handler actually paint it.