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Lesson - 3:Messages and Commands
3.3 : Learning How ClassWizard Helps You Catch Commands and Command Updates
The ClassWizard dialog box shown in Figure 4.1 has the classname highlighted in the box labeled Object IDs. Below that are resource IDs of every resource (menu, toolbar, dialog box controls, and so on) that can generate a command or message when this object (view, dialog, and so on) is on the screen. If you highlight one of those, the list of messages associated with it is much smaller, as you see in Figure 3.4. Only two messages are associated with each resource ID: COMMAND and UPDATE_COMMAND_UI. The first enables you to add a function to handle the user selecting the menu option or clicking the button—that is, to catch the command. The second enables you to add a function to set the state of the menu item, button, or other control just as the operating system is about to display it—that is, to update the command. (The COMMAND choice is boldface in Figure 3.3 because this class already catches that command.) FIG. 3.3 ClassWizard enables you to catch or update commands. Clicking Add Function to add a function that catches or updates a command involves an extra step. ClassWizard gives you a chance to change the default function name, as shown in Figure 3.4. This is almost never appropriate. There is a regular pattern to the suggested names, and experienced MFC programmers come to count on function names that follow that pattern. Command handler functions, like message handlers, have names that start with On. Typically, the remainder of the function name is formed by removing the ID and the underscores from the resource ID and capitalizing each word. Command update handlers have names that start with OnUpdate and use the same conventions for the remainder of the function name. For example, the function that catches ID_APP_EXIT should be called OnAppExit(), and the function that updates ID_APP_EXIT should be called OnUpdateAppExit(). FIG. 3.4 It’s possible, but not wise, to change the name for your command handler or command update handler from the name suggested by ClassWizard. Not every command needs an update handler. The framework does some very nice work graying and ungraying for you automatically. Say you have a menu item—Network, Send—whose command is caught by the document. When there is no open document, this menu item is grayed by the framework, without any coding on your part. For many commands, it’s enough that an object that can handle them exists, and no special updating is necessary. For others, you may want to check that something is selected or highlighted or that no errors are present before making certain commands available. That’s when you use command updating.

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