What Windows message is sent to repaint a partially occluded window?

Question

I know that WM_PAINT tells a window that it needs to repaint itself entirely, but apparently that's not the message that gets sent when it's been covered partially and then the window that was in front of it is no longer in the way and it needs to repaint the dirty portion. Does anyone know what message is sent in this case?

EDIT: Found the problem:

The issue involved a Delphi control I wrote to embed a SDL rendering surface on a Delphi form. SDL has an API to build its renderer on another window's HWND, and it marks it as a "foreign window".

SDL usually handles WM_PAINT internally, so I ended up having to add some code to SDL's WindowProc to forward the message on to the external WindowProc if it's a foreign window. That was working sometimes, but it turns out there was a glitch that was stripping the foreign window flag from the window's data structure, so it was swallowing the message instead of passing it on to my app. Once I fixed that, I started getting the WM_PAINT messages all the time.

Answer 1

I'm pretty certain the Win32 API uses WM_PAINT even for partial repaints. From MSDN:

The WM_PAINT message is sent when the system or another application makes a request to paint a portion of an application's window. [My italics].

That link has the full detail on WM_PAINT but if, as you say, the WM_PAINT message is not being sent for partial redraws, the Spy++ is the tool you need to find out for sure.

Answer 2

The BeginPaint() function returns the rect that requires validation in its 2nd parameter: http://msdn.microsoft.com/en-us/library/dd183362%28VS.85%29.aspx

case WM_PAINT:
{
PAINTSTRUCT psPaint;
HDC hdc = BeginPaint( hwnd, &psPaint );
// psPaint.rcPaint contains invalidated area
EndPaint (hwnd, &psPaint);
}
return 0;

Look at psPaint.rcPaint : http://msdn.microsoft.com/en-us/library/dd162768%28VS.85%29.aspx

Answer 3

WM_PAINT is sent to indicate that some portion (including the entirity) of the window needs to be repainted.

Call GetUpdateRect() to get a rectangle that bounds the region to be updated. This information is also included in the PAINTSTRUCT (as the rcPaint field) passed to BeginPaint().

Answer 4

Why do you say it's apparently not? WM_PAINT should be called for partial redraws (the updated rect is returned by BeginPaint or GetUpdateRect). If it doesn't appear to be getting called, there may be a bug elsewhere in your app that's preventing it. What are you seeing that leads you to believe that it's not working?

Answer 5

Take a look at WM_PRINTCLIENT. There are some circumstances when WM_PAINT is not sent and a WM_PRINTCLIENT message is sent instead. (AnimateWindow for example.)