WinApi capturing messages in default proc is not "definitive"? - winapi

Question

I can disable window movement by doing this in the message loop:
case WM_MOVING:
GetWindowRect(hWnd, (RECT*)lParam);
break;
This, however does not work if you remove the call to GetWindowRect(). Why ?
This looks like the message loop is only modifying the message in passing (WM_MOVING.lParam points to the destination RECT of the movement). The message still gets processed (by the system ?). Just doing a break; does not throw away the message, the window is movable.
The message is processed no matter what (even if you don't call DefWindowProc() on it), you can just set its lParam so that the window goes back to its initial position. Is there no way to discard the message completely ? And who's doing the processing ? (obviously, the system is, is there a ubiquitous DefWindowProc() up there in the sky - that you cannot turn off- ?)
What is exactly going on ?

Answer 1

WM_MOVING is one of calbacks done inside callig SetWindowPos. Window is moved after returnig from callback, and after finally moving window next is send WM_MOVE. There are also WM_NCCALCSIZE, WM_WINDOWPOSCHANGING, WM_WINDOWPOSCHANGE, WM_GETMINMAXINFO, WM_SIZING, WM_SIZE.
Some messages when routed to DefWindowProc do real work (for example WM_NCPAINT, WM_ERASEBKGND), some are used for gathering information for further processing (WM_NCHITTEST, WM_GETMINMAXINFO), some are used for notifying you about changes (WM_MOVING, WM_MOVE, WM_SETTINGCHANGE) or actions (WM_COMMAND, WM_LBUTTONDOWN). It is recommended that when you don't process message, you should route it through DefWindowProc. If you fail to do this for some messages that wouldn't have any effect, but for others you will miss essential functionality.
For modifying interactive moving or sizing of a window, WM_NCHITTEST is a good choice. You can disable default action activated by part of window, or implement selected action on any part of the window.
case WM_NCHITTEST:
{
LRESULT r = DefWindowProc( hwnd, msg, wparam, lparam );
if ( r == HTCAPTION )
r = HTNOWHERE;
return r;
}
Or try bellow code and see what happens when left or top border is dragged.
case WM_NCHITTEST:
{
LRESULT r = DefWindowProc( hwnd, msg, wparam, lparam );
if ( r == HTLEFT )
r = HTTOP;
else if ( r == HTTOP )
r = HTLEFT;
return r;
}

Answer 2

The "Window Procedures" doc says:
For example, the system defines a window procedure for the combo box
class (COMBOBOX); all combo boxes then use that window procedure
I imagine that procedure belongs with the system (the appropriate system DLL, comctl32.dll I guess, unless you subclass it), but it executes in the app thread of course (after it loads that DLL). What that excerpt, and the apparent behavior suggest is the following:
If the style you select requires a title bar, a title bar window (a
system facility) is instantiated for your app, with its own
windowProc, let's say "sysTtlBarProc".
When you try to move this window,
messages (system-created, starting from the input driver) are sent to
both your app's windowProc() and to sysTtlBarProc()
sysTtlBarProc() waits on your windowProc() (which can call DefWindowProc() or not) to return before acting on the message
This would explain the behavior: this protocol gives your windowProc() a chance to act (on its own) and/or modify the message, thus potentially modifying sysTtlBarProc's behavior. This is just a general concept, some messages I guess are purely informative (you can't change sysTtlBarProc's behavior for these), you can only monitor what's happening, and do something of your own next to what sysTtlBarProc() does.
This can easily be demonstrated by doing this :
case WM_SIZING:
case WM_MOVING:
Sleep(1000);
break;
You can then try and move/resize the window and watch the new "sluggish" behavior. Whoever is processing those two WM_SIZING/MOVING messages (you're not calling DefWinProc(), and you're not doing anything with the messages) is definitely delaying his work by a second now.