This section describes Some of the predicates available from the XPCE/Prolog library.
The predicates in this section used to be XPCE principal predicates. Changes to XPCE, the interface and our current understanding about programming the XPCE/Prolog environment have made these predicates less important.
send_list([], _) :- !.
send_list(_, []) :- !.
send_list([Object|Objects], Selectors) :- !, 
        send_list(Object, Selectors), 
        send_list(Objects, Selectors).
send_list(Object, [Selector|Selectors]) :- !, 
        send_list(Object, Selector), 
        send_list(Object, Selectors).
send_list(Object, Selector) :-
        send(Object, Selector).
Note that, since send/2 
accepts Selector(Arg...) the following is now valid code:
        ...,
        send_list(Box,
                  [ colour(red),
                    fill_pattern(colour(greed))
                  ]),
topmost_graphical(Pict, Pos, Gr) :-
        get(Pict, graphicals, Grs0),
        chain_list(Grs0, Grs1),
        topmost(Grs1, Pos, @nil, Gr),
        Gr \== @nil.
topmost([], _, Gr, Gr).
topmost([H|T], Pos, _, Gr) :-
        send(H, overlap, Pos), !,
        topmost(T, Pos, H, Gr).
topmost([_|T], Pos, Gr0, Gr) :-
        topmost(T, Pos, Gr0, Gr).
Or, using XPCE's list processing:
topmost_graphical(Dev, Pos, Gr) :-
        get(Dev, graphicals, Grs),
        get(Grs, find_all,
            message(@arg1, overlap, Pos), O),
        get(O, tail, Gr),
        send(O, done).
The second implementation is not only shorter, it also requires far less data conversions between Prolog and XPCE and is therefore much faster.
get_chain(Receiver, Selector, List) :-
        get(Receiver, Selector, Chain),
        chain_list(Chain, List).
See comments with chain_list/2.
The predicates in this section provide shorthands for common commands for debugging XPCE programs. See section 12 for more information on debugging XPCE/Prolog programs.
->), get- (<-) method or 
variable (-) and cause invocations thereof to be printed on 
the console.
Syntax note: (->) is a standard Prolog operator with
priority > 1000. Therefore many Prolog systems require 
additional brackets:
1 ?- tracepce((graphical ->selected)).
In SWI-Prolog this is not necessary. To be able to trace get-methods 
with this predicate (<-) must be declared as an infix 
operator.
->_check' 
on them. This performs various consistency checks on the objects and 
prints diagnostic messages if something is wrong. `object->_check' 
checks all objects it can (recursively) find through slot-references, 
chains, vectors and hash-tables and deals properly with loops in the 
data-structure.
1 ?- new(@move_gesture, move_gesture).
2 ?- show_slots(@move_gesture).
@move_gesture/move_gesture
        active                @on/bool
        button                middle
        modifier              @810918/modifier
        condition             @nil/constant
        status                inactive
        cursor                @default/constant
        offset                @548249/point
A graphical tool for inspecting instance variables is described in section 12.5.
1 ?- manpce(box). 2 ?- manpce((view->caret)).