Advanced MFC Programming

Chapter 2. Menu
void CMenuDoc::OnEditCopy()
{
m_bPasteAvailable=TRUE;
}
void CMenuDoc::OnEditCut()
{
m_bPasteAvailable=TRUE;
}
Now we will use CMenuDoc::m_bPasteAvailable to enable Edit | Paste menu item when it becomes
TRUE. In MFC, menu items are updated through handling UPDATE_COMMAND_UI messages. When the state
of a menu command needs to be updated, UPDATE_COMMAND_UI message will be automatically sent to the
application. If there exists a corresponding message handler, it will be called for updating the
corresponding menu item. Otherwise, the menu item will remain unchanged.
Adding an UPDATE_COMMAND_UI message handler is the same with adding a WM_COMMAND message
handler. After invoking the Class Wizard, we need to select the class name, the command ID, and highlight
“UPDATE_COMMAND_UI” instead of “WM_COMMAND” in “Messages” window. Finally, we need to
click button “Add Function”.
After adding the message handler for ID_EDIT_PASTE command, we will have a new member function
declared in class CMenuDoc, and a new message mapping macro added to the class implementation file. In
addition, we will have an empty function that could be modified to implement message handling:
void CMenuDoc::OnUpdateEditPaste(CCmdUI *pCmdUI)
{
}
The only parameter to this function is a pointer to CCmdUI type object. Here, class CCmdUI has several
member functions, which can be used to set the state of the menu item. To enable or disable the menu item,
we can call function CCmdUI::Enable(…). The function has only one Boolean type parameter, we can pass
TRUE to enable the menu item and pass FALSE to disable it. Because the state of Edit | Paste command
depends upon variable CMenuDoc::m_bPasteAvailable, we can use it to set the state of menu item:
void CMenuDoc::OnUpdateEditPaste(CCmdUI* pCmdUI)
{
pCmdUI->Enable(m_bPasteAvailable);
}
By compiling and executing the sample application at this point, we will see that Edit | Paste
command is disabled at the beginning, and after we execute either Edit | Cut or Edit | Copy command, it
will be enabled.
Changing Menu Text
We will go on to add more features to the mainframe menu. Class CCmdUI has another useful member
function CCmdUI::SetText(…), which allows us to change the text of a menu item dynamically. By using
this function, we can change the text of Edit | Paste menu item so that it can convey more information to
the user. For example, we could set text to “Do not paste” when data is not available, and to “Please paste”
when data is available. To add this feature, all we need is to call CCmdUI::SetText(…) in the above
message handler as follows:
void CMenuDoc::OnUpdateEditPaste(CCmdUI* pCmdUI)
{
pCmdUI->Enable(m_bPasteAvailable);
pCmdUI->SetText(m_bPasteAvailable ? "Please &paste":"Do not &paste");
}
Checking a Menu Item
Let’s further add some more interesting features to the menu commands. With the current
implementation we do not know if the data has been “Pasted” after it is “cut” or “copied” to the
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