1.2 The CAPI model

The CAPI provides an abstract hierarchy of classes which represent different sorts of window interface elements, along with functions for interacting with them. Instances of these classes represent window objects in an application, with their slots representing different aspects of the object, such as the text on a button, or the items on a menu. These instances are not actual window objects but provide a convenient representation of them for you. When you ask the CAPI to display your object, it creates a real window system object to represent it. This means that if you display a CAPI button, a real Windows button is created for it when running on Microsoft Windows, a real GTK+ button when running on GTK+, and a real Cocoa button when running on Cocoa.

The CAPI's approach makes the production of the screen objects the responsibility of the native window system, so it always produces the correct look and feel. Furthermore, the CAPI's use of the real interface to the window system means that it does not need to be upgraded to account for look and feel changes, and anything written with it is upwardly compatible, just like any well-written application.

1.2.1 CAPI elements

There are five types of elements in the CAPI model: interface, menu, pane, layout and pinboard-object.

Everything that the CAPI displays is contained within an interface (an instance of the class interface). When an interface is displayed a window appears containing all the menus and panes you have specified for it. Top level windows in an application are normally defined as an interface subclass, by using define-interface.

An interface can contain a number of menus collected together on a menu bar, and context menus can also appear elsewhere. Each menu can contain menu items or other menus (that is, submenus). Items can be grouped together visually and functionally inside menu components. Menus, menu items, and menu components are, respectively, instances of the classes menu, menu-item, and menu-component.

Panes are window objects such as buttons and lists. They can be positioned anywhere in an interface. The CAPI provides many different kinds of pane class, among them push-button, list-panel, text-input-pane, editor-pane, tree-view and graph-pane.

The positions of panes are controlled by a layout, which allows objects to be collected together and positioned either regularly (with instances of the classes column-layout, row-layout or grid-layout) or arbitrarily using a pinboard-layout. Layouts themselves can be laid out by other layouts — for example, a row of buttons can be laid out above a list by placing both the row-layout and the list in a column-layout.

pinboard-objects are lightweight elements that you can use to create complex display and user interaction. They must be used inside a pinboard-layout.

Note that layouts and interfaces are actually panes too (interface and layout are subclasses of simple-pane), and in most of the cases can be used where panes are used. They are listed separately because of their special role in the layout of windows.