src/swipl-devel/packages/xpce/man/refmanual/md/timer.md

In rswipl: Embed 'SWI'-'Prolog'

class timer {#class-timer}

A timer is an object that -when the system is dispatching messages- generates an event at regular intervals. Timers are used to implement simulation, blinking (caret) or to wake-up the user under some (time-based) conditions.

The interval of a timer is specified as a real-number. The actual accuracy depends on the timing capabilities offered by the X-implementation on which PCE has been build.

When executing the message, no arguments are forwarded. @receiver is bound to the timer itself. See ->execute.

NOTE: When multiple objects are animated at a similar speed performance will be much better if a single timer is used. The code below provides a skeleton:

:- pce_global(@stepping, chain).

start_stepping :-
    send(new(@stepper,
             timer(1, message(@stepper, for_all,
                              step))),
         start).

step(Obj) :-
    send(@stepping, append, Obj).

@see class date

Instance variables {#class-timer-instvars}

• timer<-interval: real Interval in seconds. Note that

  • The timer is only executing if PCE is in its idle loop (see display ->dispatch).

  • Granularity depends on the underlying implementation. For the X11 version, consult the manual page for select().

  The interval may be changed when the timer is running. In this case it will first fire <-interval seconds from the time is was changed.

• timer<->message: code* Code executed each time. See ->execute for details.

• timer<->service: bool If @on, execution cannot be debugged. See also application ->kind.

• timer<-status: {idle,repeat,once} Status of the timer. Its values:

  | idle   | Timer is not running                  |
  | once   | After ->execute'ing once it will stop |
  | repeat | Timer will repeat ->execute'ing       |
  

Send methods {#class-timer-send}

• timer->delay Delay for <-interval. XPCE processes events while execution is suspended.

• timer->execute Execute the timers <-message with the following arguments:

  @receiver       The timer itself.
  

• timer->initialise: interval=real, message=[code]* Create a timer object from its interval and the message to execute. After creation, the timer is idle. Use ->start or ->status to start the timer.

• timer->start: how=[{repeat,once}] Equivalent to ->status: [repeat].

• timer->stop Equivalent to ->status: idle.

• timer->running: running=bool Start/stop the timer in repeat mode.

  | @on, ->start | is equivalent to ->status: repeat |
  | @off, ->stop | is equivalent to ->status: idle.  |
  

rswipl documentation built on June 16, 2026, 5:07 p.m.