Events in CRAM

This is a short tutorial to showcase how to trigger and handle custom-defined events. CRAM has an event protocol that makes the above-mentioned very simple.

The event callback mechanism is based on the notion of hooks of the cram_utitlities package from the CRAM core. The protocol itself is defined in the ``cram_plan_knowledge`` package. That is also the package where new types of events and their handlers should be defined. That's why we will work in that package.

First, load the plan knowledge system and switch to the corresponding Lisp package:

CL-USER> (asdf:load-system :cram-plan-knowledge) ; or ,r-l-s RET cram_plan_knowledge RET RET
CL-USER> (in-package :plan-knowledge)

Next, we define a custom event class that inherits from cram-plan-knowledge:event and has one data slot:

PLAN-KNOWLEDGE> (defclass cat-appeared-event (event)
                  ((color-of-cat
                    :initarg :cat-color :reader cat-color
                    :initform (error
                               'simple-error
                               :format-control "CAT-APPEARED-EVENT requires CAT-COLOR"))))
#<STANDARD-CLASS CAT-APPEARED-EVENT>
 
PLAN-KNOWLEDGE> (describe 'cat-appeared-event)
CRAM-PLAN-KNOWLEDGE::CAT-APPEARED-EVENT
  [symbol]
CAT-APPEARED-EVENT names the standard-class #<STANDARD-CLASS
                                              CAT-APPEARED-EVENT>:
  Direct superclasses: EVENT
  No subclasses.
  Not yet finalized.
  Direct slots:
    COLOR-OF-CAT
      Initargs: :CAT-COLOR
      Initform: (ERROR 'SIMPLE-ERROR :FORMAT-CONTROL
                       "CAT-APPEARED-EVENT requires CAT-COLOR")
      Readers: CAT-COLOR

Now that we have our custom event type we define an event handler for it:

PLAN-KNOWLEDGE> (defmethod on-event ((event cat-appeared-event))
                  (format t "OMG! I just saw a ~a cat!~%" (cat-color event)))

To trigger the event we simply call the cram-plan-knowledge:on-event function:

PLAN-KNOWLEDGE> (on-event (make-instance 'cat-appeared-event :cat-color "black"))
OMG! I just saw a black cat!