It is often useful to inspect the execution of a particular method. Suppose we want to monitor geometry-changes of graphical objects. The utility predicate tracepce/1 (see also section D) may be used:
1 ?- tracepce((graphical->geometry)). Trace method: graphical ->geometry
As -> is a standard Prolog operator with priority
over 1000,
tracepce/1
requires two brackets. Get-methods may be specified in a similar way. <-
is not an operator. It is suggested to define <- as an
infix operator in the XPCE initialisation file, so
this operator is available during program development:
:- op(100, xfx, <-).
Instance variables may be specified as <class>-<variable>. A trace-point on an instance variable makes both reading and writing the variable visible.
The predicate notracepce/1 disables a tracepoint.