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.

Code-wise, 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_occasions_events ROS package. The actual event types are defined in cram_plan_occasions_events. Event handlers are mostly defined in cram_bullet_reasoning_belief_state package or you can define your own handlers.

In the nutshell, the protocol is pretty trivial: there is a generic function called cram-occasions-events:on-event which is supposed to be overloaded with new handlers and called to emit new events.

So, let's first load the plan knowledge system and switch to its corresponding Lisp package:

CL-USER> (ros-load:load-system "cram_plan_occasions_events" :cram-plan-occasions-events)
;; or ,r-l-s RET cram_plan_occasions_events RET RET
CL-USER> (in-package :cram-plan-occasions-events)

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

CPOE> (defclass cat-appeared-event (event)
          :initarg :cat-color :reader cat-color
          :initform (error
                     :format-control "CAT-APPEARED-EVENT requires CAT-COLOR"))))
CPOE> (describe 'cat-appeared-event)
CAT-APPEARED-EVENT names the standard-class #<STANDARD-CLASS
  Direct superclasses: EVENT
  No subclasses.
  Not yet finalized.
  Direct slots:
      Initargs: :CAT-COLOR
                       "CAT-APPEARED-EVENT requires CAT-COLOR")
      Readers: CAT-COLOR

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

CPOE> (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-occasions-events:on-event method:

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

Now, imagine that we would like to have multiple handlers for the same event. E.g., in addition to screaming excitedly each time we see a cat, we would also like to count the number of cats seen so far:

CPOE> (let ((saw-cats 0))
        (defmethod on-event cat-counter ((event cat-appeared-event))
          (incf saw-cats)
          (format t "number of cats seen so far: ~a~%" saw-cats)
CPOE> (on-event (make-instance 'cat-appeared-event :cat-color "black"))
number of cats seen so far: 1
OMG! I just saw a black cat!
(1 NIL)
CPOE> (on-event (make-instance 'cat-appeared-event :cat-color "black"))
number of cats seen so far: 2
OMG! I just saw a black cat!
(2 NIL)

Let's break this down.

  • We define a lexical variable saw-cats which will be our counter (if saw-cats confuses you read up on closures).
  • Then we overload our on-event method while specifying cram-plan-occasions-events::cat-counter as a method combination qualifier. The default method combination of generic function on-event is cram-utilities:hooks which simply combines the results of all the suitable methods into a list. That is clearly illustrated in the result of the last on-event call in the code snippet above: (2 NIL), where 2 is the result of the method qualified with cat-counter and NIL results from the format statement of the other method. Combining the results of all the suitable methods means that the qualifier itself (in our case cat-counter) is of no importance. So, as long as we can come up with new unique qualifiers we can have new handlers for our event (if this paragraph is hard to understand look into the definition of cram-utilities:define-hook and read up more on method combinations).