Common controls are child windows that send notification messages to the parent window when events, such as input from the user, occur in the control.
The application relies on these notification messages to determine what action the user wants it to take. Most common controls send notification messages as WM_NOTIFY messages. Windows 3.x controls send most notification messages as WM_COMMAND messages. CWnd::OnNotify is the handler for the WM_NOTIFY message. As with CWnd::OnCommand, the implementation of OnNotify dispatches the notification message to OnCmdMsg for handling in message maps. The message-map entry for handling notifications is ON_NOTIFY. For more information, see Technical Note 61: ON_NOTIFY and WM_NOTIFY Messages.
Alternately, a derived class can handle its own notification messages using "message reflection." For more information, see Technical Note 62: Message Reflection for Windows Controls.