Created
April 28, 2011 21:15
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Event dispatching and the expression problem
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(ns event-expr) | |
(declare send-event) | |
(defprotocol PrintEventHandler | |
(handle-print [this])) | |
(defprotocol BarEventHandler | |
(handle-bar [this])) | |
(defprotocol SomeAspectEventHandler | |
(handle-some [this])) | |
;; no in-place event processing | |
(defrecord FooEvent [data]) | |
;; in-place event processing for BarEventHandler | |
(defrecord BarEvent [a b c] | |
BarEventHandler (handle-bar [bar] (send-event (FooEvent. "from bar"))) | |
SomeAspectEventHandler (handle-some [bar] (println "SomeAspect get an BarEvent with a: " a))) | |
(defn print-event [e] | |
(println "Event: " e)) | |
(extend-protocol PrintEventHandler | |
FooEvent (handle-print [this] (print-event this)) | |
BarEvent (handle-print [this] (print-event this))) | |
(def handlers {PrintEventHandler handle-print | |
BarEventHandler handle-bar | |
SomeAspectEventHandler handle-some}) | |
(defn send-event [e] | |
(doseq [[p m] handlers] | |
(if (satisfies? p e) (m e)))) | |
(defn run [] | |
(send-event (FooEvent. "foo1")) | |
(send-event (FooEvent. "foo2")) | |
(send-event (BarEvent. :bar1-a :bar1-b :bar1-c))) | |
(defprotocol OddEvenEventHandler | |
(handle-odd-even [this])) | |
(defrecord OddEvenEvent [i]) | |
(defrecord EvenEvent [i]) | |
(defrecord OddEvent [i]) | |
(extend-protocol OddEvenEventHandler | |
OddEvenEvent | |
(handle-odd-even [{i :i}] | |
(send-event (EvenEvent. i))) | |
OddEvent | |
(handle-odd-even [{i :i}] | |
(if (zero? i) | |
(println " ...odd.") | |
(send-event (EvenEvent. (dec i))))) | |
EvenEvent | |
(handle-odd-even [{i :i}] | |
(if (zero? i) | |
(println " ...even.") | |
(send-event (OddEvent. (dec i)))))) | |
(def handlers (merge handlers | |
{ OddEvenEventHandler handle-odd-even})) | |
(defn run-odd-even [] | |
(println "7 is... ") | |
(send-event (OddEvenEvent. 7)) | |
(println "14 is... ") | |
(send-event (OddEvenEvent. 14))) |
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