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+;;; guile-2d
+;;; Copyright (C) 2013 David Thompson <dthompson2@worcester.edu>
+;;;
+;;; Guile-2d is free software: you can redistribute it and/or modify it
+;;; under the terms of the GNU Lesser General Public License as
+;;; published by the Free Software Foundation, either version 3 of the
+;;; License, or (at your option) any later version.
+;;;
+;;; Guile-2d is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+;;; Lesser General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU Lesser General Public
+;;; License along with this program.  If not, see
+;;; <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; Event listener.
+;;
+;;; Code:
+
+(define-module (2d observer)
+  #:use-module (srfi srfi-1)
+  #:use-module (srfi srfi-9)
+  #:use-module (srfi srfi-26)
+  #:export (<observer>
+            make-observer
+            alist->observer
+            observer?
+            observer-events
+            observer-callbacks
+            observer-on
+            observer-off
+            observer-clear
+            observer-trigger))
+
+(define-record-type <observer>
+  (%make-observer events)
+  observer?
+  (events observer-events))
+
+(define (make-observer)
+  "Return a new observer."
+  (%make-observer (make-hash-table)))
+
+(define (alist->observer alst)
+  "Return a new observer that registers the callbacks for events in
+the given alist. Each pair in the alist should map one event type to
+one callback procedure. For multiple event handlers of the same type,
+use multiple pairs."
+  (let ((observer (make-observer)))
+    (for-each (lambda (e) (observer-on observer (car e) (cdr e))) alst)
+    observer))
+
+(define (observer-callbacks observer event-type)
+  "Return a list of callback procedures for the given EVENT-TYPE. The
+null list is returned if there are no callbacks for EVENT-TYPE."
+  (or (hash-ref (observer-events observer) event-type)
+      '()))
+
+(define (observer-on observer event-type proc)
+  "Register PROC as a callback for the given EVENT-TYPE."
+  (hash-set! (observer-events observer)
+             event-type
+             (cons proc (observer-callbacks observer event-type))))
+
+(define (observer-off observer event-type proc)
+  "Unregister PROC as a callabck for the given EVENT-TYPE."
+  (hash-set! (observer-events observer)
+             event-type
+             (delete proc (observer-callbacks observer event-type))))
+
+(define (observer-clear observer event-type)
+  "Unregister all callbacks for EVENT-TYPE."
+  (hash-remove! (observer-events observer) event-type))
+
+(define (observer-trigger observer event-type . args)
+  "Call all callbacks for EVENT-TYPE with the given ARGS."
+  (for-each (cut apply <> args)
+            (observer-callbacks observer event-type)))