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events_nodefer

Allow event handling

Description

Reenable event handling after it was previously deferred by

The purpose of this feature is to Events whose handling will be deferred are: Events that are raised while event handling is deferred will be queued and handled later. A handler for an event with the defer- property must always call events_nodefer/0 before exiting, e.g.
	deferring_event_handler :-
		writeln("This event handler"),
		writeln("will not be interrupted"),
		writeln("by other events!"),
		events_nodefer.
	

CAUTION: events_nodefer/0 is a low-level primitive that must be used with great care. Part of ECLiPSe's functionality (e.g. timeout, branch-and-bound) relies on event handling and will not work properly while event handling is deferred. Deferring and undeferring events are nonlogical operations which are not undone on backtracking. The programmer must therefore make sure that every time event handling is deferred, it is eventually reenabled by a call to events_nodefer/0, even in case of failure or abort in the deferred code sequence.

Modes and Determinism

Examples

    % Without deferring, event handlers interrupt each other, which
    % makes it seem as if events were handled in reverse order:

    simple_handler(E) :-
    	writeln(simple_handling(E)).

    ?- set_event_handler(e1, simple_handler/1),
       set_event_handler(e2, simple_handler/1),
       set_event_handler(e3, simple_handler/1).
    Yes (0.00s cpu)

    ?-  event([e1,e2,e3]).
    simple_handling(e3)
    simple_handling(e2)
    simple_handling(e1)
    Yes (0.00s cpu)


    % With deferring, event handlers are sequenced, i.e. every
    % handler is allowed to finish before the next one executes:

    deferred_handler(E) :-
    	writeln(defered_handling(E)),
	events_nodefer.

    ?- set_event_handler(e1, defers(deferred_handler/1)),
       set_event_handler(e2, defers(deferred_handler/1)),
       set_event_handler(e3, defers(deferred_handler/1)).
    Yes (0.00s cpu)

    ?- event([e1,e2,e3]).
    defered_handling(e1)
    defered_handling(e2)
    defered_handling(e3)
    Yes (0.00s cpu)
    

See Also

event_create / 3, event / 1, events_defer / 0, set_event_handler / 2