8 DUIM-Gadgets Library
8.2 Callbacks and keys
When an event occurs in a user interface (for example, a button is pressed, a menu command is chosen, or an item in a list is double-clicked), you usually want some operation to be performed. If the user of your application chooses the File > Open command, a File Open dialog should be displayed. If the user clicks on an OK button in a dialog, the dialog should be dismissed and the appropriate changes to the application state to be performed. In DUIM, you can provide this functionality by specifying a function known as a callback.
Generally speaking, a callback gets passed a single argument, which is the gadget that is affected. Thus, the argument passed to the callback for a button is the button itself. Callbacks do not need to have a return value, although they are not forbidden either. If a value is returned by a callback function, then it is just ignored.
Callbacks are used in preference to event handlers because Dylan does not let you write methods that specialize on individual instances. In languages such as C, you uniquely name each element in an interface, and then provide behavior for each element by writing event handlers that contain case statements that let you discriminate on individual elements. This is a somewhat inelegant solution. Instead, in Dylan you specify the names of the callbacks for each element in an interface when you create the elements. It is then a simple matter for the system to know what behavior goes with what elements, and is much less tedious than having to write many cumbersome methods for handle-event.
In Dylan, you use events in order to create new kinds of class. If you were creating a new kind of button, you would need to define a new method for handle-event in order to describe what happens when you click on an instance of that button. You would then write callbacks to deal with particular instance of the new class of button.
By contrast with callbacks, you can also provide functions in DUIM known as keys, which are specific to collection gadgets. A key is used to set the value of some aspect of the collection gadget for which the key is defined. With keys, therefore, the values returned by the function are fundamental to the operation of the gadget. There are two keys that are generally used by gadgets, known as the value key and the label key. The value key is a function that is used to calculate the value of the gadget for which the key is defined. The label key is used to calculate the printed representation, or label, of all the items in a collection gadget.