Advanced MFC Programming

Chapter 5. Common Controls
compare the handle obtained from function CWnd::GetFocus() with the handles of combo boxes. If there is
a hit, we could update the content of that combo box.
Unfortunately, since a combo box is made up of two controls: an edit box and a list box, if we are
trying to input characters into the combo box, it is the edit box that has the current focus. Thus if we call
CWnd::GetFocus() to obtain handle of the window that has the current focus, we will actually get the
handle of the edit box. The edit box is the child window of the combo box window, and it has a different
handle with its parent. So comparing the handle of the edit box with the handles of the combo boxes will
never result in any hit. The correct step would be: for each combo box, obtaining the handle of its edit box,
then comparing it with the handle of the focused window. This will eventually result in a hit.
Class CWnd has a member function that can be used to find a window’s child windows:
CWnd *CWnd::GetWindow(UINT nCmd);
Here nCmd specifies what kind of window is being looked for. To enumerate all the child windows, we
need to call this function using GW_CHILD flag to find the first child window, then, use GW_HWNDNEXT to call
the same function repeatedly until it returns a NULL value. This will enumerate all the sibling windows of
the first child window.
There are still problems here: function CWnd::GetWindow(…) returns a CWnd type pointer, we can not
obtain further information about that window (i.e. is it an edit box or a list box?). Since a combo box has
two child windows, although we can access both of them with the above-mentioned method, we do not
know which one is the edit box.
In Windows, before a new type of window is created, it must register a special class name to the
system. Every window has its own class name, which could be used to tell the window’s type. In the case
of combo box, its edit box’s class name is “Edit” and its list box’s class name is “ComboLBox”. Please
note that this class name has nothing to do with MFC classes. It is used by the operating system to identify
the window types rather than a programming implementation.
In MFC, the procedure of creating windows is handled automatically, so we never bother to register
class names for the windows being created, therefore, we seldom need to know the class names of our
windows.
A window’s class name can be retrieved from its handle by calling an API function:
int ::GetClassName(HWND hWnd, LPTSTR lpClassName, int nMaxCount);
The first parameter hWnd is the handle of window whose class name is being retrieved; the second
parameter lpClassName is the pointer to a buffer where the class name string can be put; the third
parameter nMaxCount specifies the length of this buffer.
We can access the first child window of the combo box, see if its class name is “Edit”. If not, the other
child window must be the edit box. This is because a combo box has only two child windows.
A window’s handle can be obtained by calling function CWnd::GetSafeHwnd(). If the window that has
the current focus is the edit box of a combo box when RETURN is pressed, we need to notify the parent
window about this event. In the sample, a user defined message is used to implement this notification:
#define WM_COMBO_RETURN
WM_USER+1000
The following portion of function CCCtlDlg::PreTranslateMessage(…) shows how to retrieve the
handles of the edit boxes and compare them with the handle of the focused window:
……
if(nVirtKey == VK_RETURN)
{
hwndFocus=GetFocus()->GetSafeHwnd();
if(hwndFocus == NULL)return CDialog::PreTranslateMessage(pMsg);
ptrEdit=(CEdit *)m_cbDropDown.GetWindow(GW_CHILD);
hwndEdit=ptrEdit->GetSafeHwnd();
::GetClassName(hwndEdit, szClassName, 256);
if(memcmp(szClassName, "Edit", sizeof("Edit")))
{
ptrEdit=(CEdit *)ptrEdit->GetWindow(GW_HWNDNEXT);
hwndEdit=ptrEdit->GetSafeHwnd();
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