A customer was using an MFC CHtmlDialog as a child
dialog and found that they needed to add the
WS_CLIPCHILDREN style to ensure
that the contents appeared on the screen; otherwise,
the parent dialog would paint its background over the child dialog,
and the child dialog would consequently appear to vanish.
On the other hand, they also had some transparent image controls
on the parent dialog,
and adding the
WS_CLIPCHILDREN style prevented
those controls from drawing transparently,
meaning that instead of having the parent dialog's background show
through, it just used a black background.
How can they get the best of both worlds,
with
WS_CLIPCHILDREN style applying
to some child controls but not others?
The controls that need the background to show through
can ask the parent dialog to draw its background.
They can do this the same way standard controls do it,
by sending a WM_CTLCOLORxxx
message to the parent to request the background brush
and text colors.
Of course, since it's an image control, the text
colors aren't interesting.
case WM_PAINT:
{
PAINTSTRUCT ps;
HDC hdc = BeginPaint(hwnd, &ps);
HBRUSH hbrBackground = (HBRUSH)
SendMessage(GetParent(hwnd), WM_CTLCOLORSTATIC,
(WPARAM)hdc, (LPARAM)hwnd);
FillRect(hdc, &ps.rcPaint, hbrBackground);
DrawTheImage(...);
EndPaint(hwnd, &ps);
return 0;
}
This technique assumes that the parent dialog responds
to
WM_CTLCOLORxxx messages.
If the parent dialog has standard colors, then the
DefWindowProc function will return
standard colors, and everything will work out.
But if the parent dialog uses custom colors, it will have
to add additional message handlers for these messages.
Alternatively, the image controls could render its background
by forwarding the WM_ERASEBKGND message
to its parent.
Since the child and parent have different coordinates, you'll have
to do some coordinate manipulation to get the parent to receive
a device context whose origin is what it expects.
case WM_ERASEBKGND:
{
HDC hdc = (HDC)wParam;
HWND hwndParent = GetParent(hwnd);
POINT ptOffset{};
MapWindowPoints(hwnd, hwndParent, &ptOffset, 1);
OffsetWindowOrgEx(hdc, ptOffset.x, ptOffset.y, &ptOrig);
LRESULT lres = SendMessage(hwndParent, WM_ERASEBKGND, wParam, lParam);
SetWindowOrgEx(hdc, ptOrig.x, ptOrig.y, nullptr);
return lres;
}
Note that neither of these techniques actually draw the
control transparently.
Rather, they attempt to simulate the effect by asking the parent
to draw its background into the control.
It also means that if the parent draws something interesting in its
WM_PAINT handler,
it won't show up in the "background" of the control.
Nevertheless, this usually works well enough for most purposes.