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--- tutorials:advanced:events [2015/05/11 17:31] – created gkazhoya
+++ tutorials:advanced:events [2015/05/11 19:23] – gkazhoya
@@ Line 4: / Line 4: @@
 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 [[doc/package/cram_utilities|cram_utitlities]] package from the CRAM core.
+Code-wise, the event callback mechanism is based on the notion of hooks of the [[doc/package/cram_utilities|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.
+The protocol itself is defined in the ''cram_plan_knowledge'' ROS package. That is also the package where new types of events and their handlers should be defined.
-First, load the plan knowledge system and switch to the corresponding Lisp package:
+In the nutshell, the protocol is pretty trivial: there is a generic function called ''cram-plan-knowledge: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:
 <code lisp>
 CL-USER> (asdf:load-system :cram-plan-knowledge) ; or ,r-l-s RET cram_plan_knowledge RET RET
-CL-USER> (in-package :plan-knowledge)
+CL-USER> (in-package :cram-plan-knowledge)
 </code>
@@ Line 48: / Line 50: @@
 </code>
-To trigger the event we simply call the ''cram-plan-knowledge:on-event'' function:
+To trigger the event we simply call the ''cram-plan-knowledge:on-event'' method:
 <code lisp>
 PLAN-KNOWLEDGE> (on-event (make-instance 'cat-appeared-event :cat-color "black"))
 OMG! I just saw a black cat!
+(NIL)
 </code>
+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:
+<code lisp>
+PLAN-KNOWLEDGE> (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)
+                    saw-cats))
+#<STANDARD-METHOD ON-EVENT CAT-COUNTER (CAT-APPEARED-EVENT) {100F5D3C43}>
+PLAN-KNOWLEDGE> (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)
+PLAN-KNOWLEDGE> (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)
+</code>
+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 [[http://www.gigamonkeys.com/book/variables.html#lexical-variables-and-closures|closures]]).
+  * Then we overload our ''on-event'' method while specifying ''cram-plan-knowledge::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 qualifiers we can have new handlers for a particular event (if this paragraph is hard to understand look into the definition of ''cram-utilities:define-hook'' and read up more on [[http://www.lispworks.com/documentation/HyperSpec/Body/m_defi_4.htm|method combinations]]).