Question

When I want to redraw a window, is there any preferred function to call between InvalidateRect and RedrawWindow?

For instance, are these two calls equal: (win would be a HWND)
RedrawWindow(win, NULL, NULL, RDW_INVALIDATE);
InvalidateRect(win, NULL, NULL);

The main question(s): When should I use one or the other? Are there any differences that happen in the background? (different WM_messages / focus / order / priorities..)

The reason that I want to redraw the window is because I send a new image to it that I want it to display, meaning the content of the window is no longer valid.

Answer 1

InvalidateRect does not immediately redraw the window. It simply “schedules” a future redraw for a specific rectangular area of the window. Using InvalidateRect you may schedule as many areas as you want, making them accumulate in some internal buffer. The actual redrawing for all accumulated scheduled areas will take place later, when the window has nothing else to do. (Of course, if the window is idle at the moment when you issue the InvalidateRect call, the redrawing will take place immediately).

You can also force an immediate redraw for all currently accumulated invalidated areas by calling UpdateWindow. But, again, if you are not in a hurry, explicitly calling UpdateWindow is not necessary, since once the window is idle it will perform a redraw for all currently invalidated areas automatically.

RedrawWindow, on the other hand, is a function with a much wider and flexible set of capabilities. It can be used to perform invalidation scheduling (i.e. the same thing InvalidateRect does) or it can be used to forcefully perform immediate redrawing of the specified area, without doing any “scheduling”. In the latter case calling RedrawWindow is virtually equivalent to calling InvalidateRect and then immediately calling UpdateWindow.