Advanced MFC Programming

Chapter 7. Common Dialog Boxes
7.9 Modeless Common Dialog Boxes
Tricks
It is difficult to implement modeless common dialog boxes. This is because all the common dialog
boxes are designed to work in the modal style. Therefore, if we call function Create(…) instead of
DoModal(), although the dialog box will pop up, it will not behave like a common dialog box. This is
because function Create(…) is not overridden in a class such as CColorDialog, CFontDialog.
We need to play some tricks in order to implement modeless common dialog boxes. By looking at the
source code of MFC, we can find that within function CColorDialog::DoModal() or CFontDialog::
DoModal(), the base class version of DoModal() is not called. Instead, API functions ::ChooseColor(…)and
::ChooseFont(…) are used to implement common dialog boxes.
There is no difference between the common dialog boxes implemented by API functions and MFC
classes. Actually, using API function is fairly simple. For example, if we want to implement a color dialog
box, we can first prepare a CHOOSECOLOR object, then use it to call function ::ChooseColor(…). But here we
must initialize every member of CHOOSECOLOR in order to let the dialog box have appropriate styles.
By using this method we can still create only modal common dialog boxes. A “modeless” common
dialog box can be implemented by using the following method:
1) Before creating the common dialog box, first implement a non-visible modeless window.
1) Create the modal common dialog box, use the modeless window as its parent.
Although the common dialog box is modal, its parent window is modeless. So actually we can switch
away from the common dialog box (and its parent window) without closing it. Because the common dialog
box’s parent is invisible, this gives the user an impression that the common dialog box is modeless.
But if we call function DoModal() to implement the common dialog box, we are not allowed to switch
away even it has a modeless parent window. We must call API functions to create this type of common
dialog boxes.
Hook Function
We can provide hook function when implementing common dialog boxes using API functions. In
Windows programming, a hook function is used to intercept messages sent to a window, thus by handling
these messages we can customize a window’s behavior. Both structure CHOOSEFONT and CHOOSECOLOR have
a member lpfnHook that can be used to store a hook function’s address when the common dialog box is
being implemented. If a valid hook function is provided, when a message is sent to the dialog box, the hook
function will be called first to process the message. To enable hook function, we also need to set
CF_ENABLEHOOK or CC_ENABLEHOOK bit for member Flags of structure CHOOSEFONT or CHOOSECOLOR. If the
message is not processed in the hook function, the dialog’s default behavior will not change.
A hook procedure usually looks like the following:
……
……
UINT APIENTRY HookProc(HWND hdlg, UINT uiMsg, WPARAM wParam, LPARAM lParam)
{
switch(uiMsg)
{
case WM_INITDIALOG:
{
}
}
}
break;
The first parameter is the handle of the destination window where the message is being sent. The
second parameter specifies the type of message. The third and fourth parameters are WPARAM and LPARAM
parameters of the message respectively. For different events, the messages sent to this procedure are
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