Links | Search




Sections: Audio COM/ActiveX Control Panel Databases Debug Device Information .NET CF Email, POOM Emulator Enterprise eVB eVC++ Events Files and Registry GAPI Hardware Buttons HTML in Programs Images Installation IrDA, LED Misc Network, Internet Non-Programming Password Protection PIE Pocket PC 2002 Power Printing SIP Synchronization Today Tools User Interface XML

QA: Why doesn't my CButton work right?

By Michael Wolraich, September 21, 2001.
Mr. Wolraich is an independent contractor in New York City. He specializes in high-end client applications for Windows platforms.
Print version

Question

When I click I cannot press my button. If I press on the button then move stylus out of the button and then back then the button is pressed. What is wrong?

Answer

You may have noticed that when you create an MFC CButton on the Pocket PC, it doesn't highlight properly when you press it, and it doesn't always send a BN_CLICKED notification that it has been pressed. Actually, if you press and release quickly, it works fine, but if you press and hold, it un-highlights and fails to send a notification.

Such erratic behavior occurs because of a bug (feature?) in Microsoft's implementation of CWnd::OnLButtonDown(). If you look in mfc/src/wincore.cpp, you'll notice the following snippet:

void CWnd::OnLButtonDown (UINT nState, CPoint point) { if (!SHRecognizeGesture(point)) Default(); }

This method, inherited by CButton, causes two problems. First, SHRecognizeGesture() call delays the normal highlight behavior of the button. Second, when the user presses the button and holds it, SHRecognizeGesture() returns TRUE. That causes the default behavior to be skipped so that the BN_CLICKED notification is never sent when the button is released.

Happily, there is a simple workaround. Just create a new class that inherits from CButton and overwrites CButton::OnLButtonDown() as follows:

void CMyButton::OnLButtonDown (UINT nState, CPoint point) { Default(); }

or if you need to keep the SHRecognizeGesture() functionality around:

void CMyButton::OnLButtonDown (UINT nState, CPoint point) { SetState(TRUE); SHRecognizeGesture(point); Default(); }

Similar problems may occur with other MFC controls that use OnLButtonDown(). They can be worked around in a similar manner.

Sample

You can download a sample application (10 Kb) that shows working of different type of buttons (with bug, without bug, etc).

Discuss

Discuss this article. Here you can write your comments and read comments of other developers.