8 DUIM-Gadgets Library

8.3 Gadget protocols

Gadgets are objects that make up an interface: the menus, buttons, sliders, check lists, tool bars, menu bars, and so on. Gadget classes may support three protocols, value, items, and activate.

Gadgets that support the value protocol respond to the gadget-value message, a value-changed callback, and have a setter function associated with them.
Gadgets that support the items protocol respond to gadget-items and have a gadget setter function associated with them.
Gadgets that support the activate protocol have an activation callback associated with them.

Gadgets have a set of slots, or properties, associated with them: gadget-label, gadget-value, gadget-items, and gadget-enabled? Every gadget has some or all of these properties.

gadget-label: This slot holds the label that appears on the gadget on the screen. If a gadget does not have a label, the gadget-label function returns #f.
gadget-value: This slot holds the value(s) of the gadget. If a gadget does not have any values, the gadget-value function returns #f.
gadget-items: This slot is a list of the contents of the gadget. If the gadget does not have items, for example a button, gadget-items returns nothing.
gadget-enabled?
: This slot tests whether or not the gadget is active. All gadgets have a gadget-enabled? slot.

An introduction to the protocols supported by different sorts of gadget can also be found in the Building Applications Using DUIM book.