Tuesday, January 31, 2006

IsWindowVisible can be deceiving!

Recently I had to debug a bit of code that was causing an Active-x control to be hidden. I found that if i commented out the two ShowWindow lines below, the problem went away.

bool bVisibile= mChild.wnd.IsWindowVisible();
mChild.wnd.MoveWindow(x, y, w, h);
mChild.wnd.ShowWindow(IsWindowVisible ? SW_SHOW : SW_HIDE);

That snippet of code basically saves the current state of the window, then hides it, then moves it, then sets the state back to what it was before. So it should be a completely safe statement right? WRONG! The concept seems to make sense but a careful look at the Microsoft documentation will reveal the problem. Look particularly at this sentance.

If the specified window, its parent window, its parent’s parent window, and so forth, have the WS_VISIBLE style, the return value is nonzero. Otherwise, the return value is zero.

So let’s assume that the child window (the one we’re trying to move) had its WS_VISIBLE bit set so that it can be visible. Let’s also assume that either its parent or its parent’s parent or both were NOT visible. Well then IsWindowVisible would indicate that the child window was in fact NOT visible. Hmm this makes sense because the user clearly cannot see the window if a parent is hidden, BUT the child window’s bit is still set to visible. Well this code snippet doesn’t realize that and sets the child window to hidden. That’s fine for now. We perform our window move but here’s where it dies. We go to set our child window back to its original state…which was what? It was set to visible but that’s not what that function told us. So we leave the child window as hidden. Now when it comes time for the parent that was hidden to be displayed and show the Active-X control, the child window was explicitly set to hidden and cannot be seen.

Bug in the code? Yeap! Bug in the documentation? Possibly. I think the biggest problem is that the naming of this function is a bit misleading. Developers who use this function should ask themselves this question from a users standpoint. “Is this window visible to the user?” That’s how IsWindowVisible will answer the question. Most developers just have the tendancy to become focused on the window they’re trying to manipulate and forget that it’s really a piece of a bigger picture.

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