The fundamental graphical element in MrEd's windowing toolbox is an area . It has no graphical representation or event-handling capabilities. As suggested by the above listing, certain areas, called containers , manage certain other areas, called containees . Some areas, such as panels, are both containers and containees.
Most areas are windows , but some are non-windows .
A window, such as a panel, has a graphical representation, receives keyboard and mouse events, and can be disabled or hidden. The menu can appear in a menu bar, in a popup menu, or as a submenu in another menu. When the item is selected, its callback procedure is invoked. The complete type hierarchy for the menu system is shown in Figure 2.3 .