Callback Lists

In order to achieve some useful purpose, some widgets must periodically return control of the program to the application programmer's code. Such times might occur when the user clicks on a push button widget labelled ``Save'', in which case control of the application should be handed over to the relevant section of the application programmer's code dealing with saving text to a file. To achieve this, widgets that are designed to allow interaction with the application programmer's code have one or more callback lists implemented as resources. A callback list specifies which of the application programmer's routines will be invoked when a particular callback is triggered, either for a well-defined reason and sometimes as a result of a particular event.