New module: message-loop

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A simple message-handling loop. Listeners can watch for/react to multiple types of messages and multiple listeners can react to the same message, although order of processing is not guaranteed.

I'm currently using this in a GUI system to simplify interdependencies and multi-step data gathering -- instead of getting lost in callback hell where various controls and/or processing can block the GUI thread, I simply post a message and move on. It makes things a lot simpler.

#lang racket

(require message-loop)

(start-message-loop)

(add-listeners
 (listener++ #:listen-for '(matriculate)
             #:action (lambda (lstnr msg)
                        (match-define (cons name school) (message-data msg))
                        (displayln (format "~a got into ~a" name school))))
 (listener++ #:listen-for '(birthday wedding)
             #:id 'life-event-listener-1
             #:action (lambda (lstnr msg)
			(displayln (format "listener id: '~a' heard that there was a ~a"
                                           (listener-id lstnr)
                                           (message-type msg)))))
 (listener++ #:listen-for '(birthday wedding)
             #:id 'life-event-listener-2
             #:action (lambda (lstnr msg)
			(displayln (format "listener id: '~a' heard that there was a ~a"
                                           (listener-id lstnr)
                                           (message-type msg))))))

(post-message (message++ #:type 'matriculate #:data (cons 'alice "Northwestern"))) 
(post-message (message++ #:type 'graduate #:data (cons 'bob "Northwestern")))
(post-message 'birthday)
(post-message 'wedding)

; output is:  
;
; alice got into Northwestern
; listener id: 'life-event-listener-1' heard that there was a birthday
; listener id: 'life-event-listener-2' heard that there was a birthday
; listener id: 'life-event-listener-1' heard that there was a wedding
; listener id: 'life-event-listener-2' heard that there was a wedding

; NB:  Order of processing is not guaranteed for the 'life-event-listener-N' messages
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