Advanced MFC Programming

Chapter 2. Menu
ON_COMMAND(ID_EDIT_COPY, OnEditCopy)
ON_COMMAND(ID_EDIT_CUT, OnEditCut)
ON_COMMAND(ID_EDIT_PASTE, OnEditPaste)
//}}AFX_MSG_MAP
END_MESSAGE_MAP()
3) Three blank message handlers are added in file “MenuDoc.cpp”:
void CMenuDoc::OnEditCopy()
{
}
void CMenuDoc::OnEditCut()
{
}
void CMenuDoc::OnEditPaste()
{
}
When first added, these functions are empty. We have to add our own code in order to support
command execution.
By compiling and executing the sample application at this point, we will see that Edit | Copy, Edit |
Cut and Edit | Paste menu items are all enabled. This is because three blank message handlers have just
been added.
Enabling & Disabling a Command
The sample application will not actually cut, copy or paste data. The three commands will be
implemented just to simulate data copy and paste procedure. Before going on to implement it, we need to
make following assumptions.
Suppose the application supports only internal data copy, cut and paste (it does not accept data from
other applications through using system clipboard). Before command Edit | Copy or Edit | Cut is
executed, there should be no data stored in the “local clipboard”. Therefore, if we execute Edit | Paste
command at this time, there will be an error. To avoid this, we need to disable Edit | Paste command
before data has been copied to the clipboard.
The state of menu item can be set thought handling UPDATE_COMMAND_UI message. The parameter
comes along with this message is a pointer to CCmdUI type object, which can be used to enable or disable a
command, set or remove check for a menu item. Handling this message for menu items is the same with
that of tool bar controls.
So it is easy to find out a mechanism for updating command Edit | Paste: we need to declare a
Boolean type variable in class CMenuDoc and initialize it to FALSE. We can set this flag to TRUE when
Edit | Cut or Edit | Copy command is executed, and enable Edit | Paste command only if this flag is set.
In the sample application, this Boolean variable is CMenuDoc::m_bPasteAvailable. The following
code fragment shows how it is declared and initialized in the constructor of class CMenuDoc:
……
……
class CMenuDoc : public CDocument
{
protected:
BOOL m_bPasteAvailable;
}
CMenuDoc::CMenuDoc()
{
m_bPasteAvailable=FALSE;
}
The value of CmenuDoc::m_bPasteAvailable is set to TRUE when user executes either Edit | Copy or
Edit | Cut command:
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