This under-the-hood tour of how MFC connects user actions such as window resizing or menu choices to your code is almost finished. All that’s left is to handle the graying of menus and buttons, a process called command updating.

Imagine you are designing an operating system, and you know it’s a good idea to have some menu items grayed to show they can’t be used right now. There are two ways you can go about implementing this.

One is to have a huge table with one entry for every menu item and a flag to indicate whether it’s available. Whenever you have to display the menu, you can quickly check the table. Whenever the program does anything that makes the item available or unavailable, it updates the table. This is called the continuous-update approach.

The other way is not to have a table but to check all the conditions just before your program displays the menu. This is called the update-on-demand approach and is the approach taken in Windows. In the old C way of doing things—to check whether each menu option should be grayed—the system sent a WM_INITMENUPOPUP message, which means "I’m about to display a menu." The giant switch in the WindProc caught that message and quickly enabled or disabled each menu item. This wasn’t very object-oriented though. In an object-oriented program, different pieces of information are stored in different objects and aren’t generally made available to the entire program.

When it comes to updating menus, different objects know whether each item should be grayed. For example, the document knows whether it has been modified since it was last saved, so it can decide whether File, Save should be grayed. However, only the view knows whether some text is currently highlighted; therefore, it can decide if Edit, Cut and Edit, Copy should be grayed. This means that the job of updating these menus should be parcelled out to various objects within the application rather than handled within the WindProc.

The MFC approach is to use a little object called a CCmdUI, a command user interface, and give this object to whoever catches a CN_UPDATE_COMMAND_UI message. You catch those messages by adding (or getting ClassWizard to add) an ON_UPDATE_COMMAND_UI macro in your message map. If you want to know what’s going on behind the scenes, it’s this: The operating system still sends WM_INITMENUPOPUP; then the MFC base classes such as CFrameWnd take over. They make a CCmdUI, set its member variables to correspond to the first menu item, and call one of that object’s own member functions, DoUpdate(). Then, DoUpdate() sends out the CN_COMMAND_UPDATE_UI message with a pointer to itself as the CCmdUI object the handlers use. The same CCmdUI object is then reset to correspond to the second menu item, and so on, until the entire menu is ready to be displayed. The CCmdUI object is also used to gray and ungray buttons and other controls in a slightly different context.

CCmdUI has the following member functions:

Determining which member function you want to use is usually clear-cut. Here is a shortened version of the message map from an object called CWhoisView, a class derived from CFormView that is showing information to a user. This form view contains several edit boxes, and the user may want to paste text into one of them. The message map contains an entry to catch the update for the ID_EDIT_PASTE command, like this:

The function that catches the update, OnUpdateEditPaste(), looks like this:

This calls the API function ::IsClipboardFormatAvailable() to see whether there is text in the Clipboard. Other applications may be able to paste in images or other nontext Clipboard contents, but this application cannot and grays the menu item if there is no text available to paste. Most command update functions look just like this: They call Enable() with a parameter that is a call to a function that returns TRUE or FALSE, or perhaps a simple logical expression. Command update handlers must be fast because five to ten of them must run between the moment the user clicks to display the menu and the moment before the menu is actually displayed.