Add all the A tasks

Task/Animation/Racket/animation.rkt

@@ -0,0 +1,40 @@
+#lang racket/gui
+
+;; One of 'left or 'right
+(define direction 'left)
+
+;; Set up the GUI
+(define animation-frame%
+  (class frame%
+    (super-new [label "Animation"])
+    ;; reverse direction on a click
+    (define/override (on-subwindow-event win evt)
+      (when (send evt button-down?)
+        (set! direction
+              (if (eq? direction 'left)
+                  'right
+                  'left))))))
+
+(define frame (new animation-frame%))
+(define msg (new message%
+                 [label "Hello World! "]
+                 [parent frame]))
+
+;; Timer callback to adjust the message
+(define (callback)
+  (define old-label (send msg get-label))
+  (define len (string-length old-label))
+  (if (eq? direction 'left)
+      (send msg set-label
+            (string-append (substring old-label 1)
+                           (substring old-label 0 1)))
+      (send msg set-label
+            (string-append (substring old-label (- len 1) len)
+                           (substring old-label 0 (- len 1))))))
+
+;; Set a timer and go
+(define timer (new timer%
+                   [notify-callback callback]
+                   [interval 500]))
+
+(send frame show #t)