Toolkits and Callbacks

Event-loop processing can be generalized:

• Instead of switch, use table lookup.
• Each table entry associates an event with a callback function.
• When event occurs, corresponding callback is invoked.
• Provide an API to make and delete table entries.
• Divide screen into parcels, and assign different callbacks to different parcels (X Windows does this).
• Event manager does most or all of the administration.

Modular UI functionality is provided through a set of widgets:

• Widgets are parcels of the screen that can respond to events.
• A widget has a graphical representation that suggests its function.
• Widgets may respond to events with a change in appearance, as well as issuing callbacks.
• Widgets are arranged in a parent/child hierarchy.
  • Event-process definition for parent may apply to child, and child may add additional event-process definitions
  • Event-process definition for parent may be redefined within child
• Widgets may have multiple parts, and in fact may be composed of other widgets in a heirarchy.

Some UI toolkits: Xm, Xt, SUIT, FORMS, Tk ...