Other Events Types

Many X applications require input from sources other than the X event queue. The X Toolkit provides a way for registering procedures to be invoked when there is input coming from a source such as UNIX file. The call to xtAppAddInput/6 accomplishes that. The first argument in xtAppAddInput/6 is the application context, the UNIX file descriptor for the input source is second, the third is the input condition mask and the next two arguments are the predicate and client data. An Id is returned in the final argument, which can be used to unregister the procedure.

Another useful facility in the X Toolkit allows applications to register callback procedures to be invoked when a specified interval has elapsed. The call to xtAppAddTimeOut/5 accomplishes that. The time interval is measured in milliseconds and the registered procedure must be of arity 2, where the first argument is the client data and the second is the interval id. Following is an extension to the first example, which sets a timer when the push button is activated. This in turn activates a procedure some 2 seconds later.

     timer(_ClientData,_IntervalId) :-
            write('Button pressed approximately 2 seconds earlier...'),
     set_timer(Widget,Interval,_CallData) :-
            xtWidgetToApplicationContext(Widget, App),
     time :-

The X Toolkit includes another type of callback mechanism, known as a WorkProc, that provides a limited form of background processing. It allows the application to invoke a callback whenever there are no events pending. The application can register WorkProc procedures using the call to xtAppAddWorkProc/4.