WM_NCLBUTTONUP message patch

 Environment: MFC, Windows 95, 98, Me, NT 3.51, 4.0, 5.0

If you spend time investigating what happens when you click and release the left
button over the title bar, you will find out that instead of getting “non-client
left button up” message, you just get “left button up”. One could actually work
with it, if the message found it’s way into the application. It does not.

I was in a middle of writing an application when I discovered this. I looked
all over the internet for articles regarding WM_NCLBUTTONUP problem, but the only
thing I could find were questions about the problem. After some more
investigating I have come up with a patch that could be adopted by each
application requiring such notification.

The patch consists of installing a “windows hook” that will
intercept all mouse messages for this application before they enter into the message pump.
To do that you need to call SetWindowsHookEx(…) function, soon after
the main window is created. Here is the call:


hMHook = SetWindowsHookEx(
// hook type:
WH_MOUSE,
// hook procedure:
(HOOKPROC) MouseHookProc,
// handle to application instance:
AfxGetInstanceHandle(),
// thread identifier:
AfxGetThread()->m_nThreadID
);

It is very important that you supply handle to application instance and
thread identifier, otherwise every application running on your computer will
attempt to hook it’s mouse messages through your program and it could be
disastrous. By supplying these two parameters you will insure that only messages
from your application will end up in your callback function.

Equally important is a call to remove the hook before your application
terminates. The UnhookWindowsHookEx(…) function removes a hook procedure
installed in a hook chain. Most likely you will call it somewhere in OnDestroy(),
like this:

   if(hMHook != NULL)
UnhookWindowsHookEx(hMHook);

The callback function is where you will receive WM_NCLBUTTONDOWN message and
the next time you receive WM_LBUTTONUP message you will post WM_NCLBUTTONUP directly
into the application message pump. Therefore, no special handling will be required
to service these messages. You will simply write your code inside of
OnNcLButtonUp(…), just like you would for any other message.

Here is the callback code:


// ////////////////////////////////////////////////////
// handle to the mouse hook

HHOOK hMHook = NULL;

// status of non-client left button down
BOOL bNcLButtonDown = FALSE;

// /////////////////////////////////////////////////////
// Mouse hook process

LRESULT CALLBACK MouseHookProc( int nCode,
WPARAM wParam,
LPARAM lParam)
{
if(nCode == HC_ACTION)
{
// get a pointer to the mouse hook struct.
PMOUSEHOOKSTRUCT mhs = (PMOUSEHOOKSTRUCT) lParam;

// intercept messages for left button down and up
switch(wParam)
{
case WM_NCLBUTTONDOWN:
{
// get the pointer to the main window
CWnd *pWnd = AfxGetMainWnd();

// if the message is from your window and
// the hit test indicates title bar

if((mhs->hwnd == pWnd->GetSafeHwnd())
&& (mhs->wHitTestCode == HTCAPTION))
{
// then indicate non-client left button down
bNcLButtonDown = TRUE;

// there is no problem with this message
// so you don’t have to do anything else

}
}
break;

case WM_NCLBUTTONUP:
// you will get this message if you double-click
// on the title bar
// reset the status

bNcLButtonDown = FALSE;
break;

case WM_LBUTTONUP:
{
// get the pointer to the main window
CWnd *pWnd = AfxGetMainWnd();

// if the message is from your window and
// non-client left button is down

if((mhs->hwnd == pWnd->GetSafeHwnd())
&& (bNcLButtonDown == TRUE))
{
// then post WM_NCLBUTTONUP message directly
// into your window message pump
// Note: I’m hardcoding HTCAPTION because the
// left button was down, and while it is down,
// the mouse does not move in respect to the
// window, but it does in respect to the screen,
// so the mouse should still be over the caption
// bar of your window when you release the button.

pWnd->PostMessage(WM_NCLBUTTONUP, HTCAPTION,
MAKELONG(mhs->pt.x,mhs->pt.y));

// reset non-client left button down
bNcLButtonDown = FALSE;
}
}
break;

default:
break;
}
}
// let the messages through to the next hook
return CallNextHookEx(hMHook, nCode, wParam, lParam);
}

I am including two sample projects. The “nclbxExample” is technical, and
the “AlphaDialogExample” is more practical. The “nclbxExample” is better documented
so you can see how and were I have implemented the code.

NOTE: If you are going to use mousepatch.cpp the way I’m using it, DO NOT
add it to your project.

Downloads

Download nclbxExample project – 38 Kb

Download AlphaDialogExample project – 118 Kb (will only work in Win2K)

Download source – 2 Kb

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