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An applet is a Java class that inherits from java.applet.Applet
.
The applet can be downloaded and run in a Java-capable web-browser.
To generate an applet from a Scheme program, write the Scheme
program with appropriate definitions of the functions ‘init’,
‘start’, ‘stop’ and ‘destroy’. You must declare these
as zero-argument functions with a <void>
return-type.
Here is an example, based on the scribble applet in Flanagan’s "Java Examples in a Nutshell" (O’Reilly, 1997):
(define-private last-x 0) (define-private last-y 0) (define (init) :: void (let ((applet (this))) (applet:addMouseListener (object (java.awt.event.MouseAdapter) ((mousePressed e) (set! last-x (e:getX)) (set! last-y (e:getY))))) (applet:addMouseMotionListener (object (java.awt.event.MouseMotionAdapter) ((mouseDragged e) (let ((g (applet:getGraphics)) (x (e:getX)) (y (e:getY))) (g:drawLine last-x last-y x y) (set! last-x x) (set! last-y y))))))) (define (start) :: void (format #t "called start.~%~!")) (define (stop) :: void (format #t "called stop.~%~!")) (define (destroy) :: void (format #t "called destroy.~%~!"))
You compile the program with the ‘--applet’ flag in addition to the normal ‘-C’ flag:
java kawa.repl --applet -C scribble.scm
You can then create a ‘.jar’ archive containing your applet:
jar cf scribble.jar scribble*.class
Finally, you create an ‘.html’ page referencing your applet
and its support jar
s:
<html><head><title>Scribble testapp</title></head> <body><h1>Scribble testapp</h1> You can scribble here: <br> <applet code="scribble.class" archive="scribble.jar, kawa-2.93.jar" width=200 height=200> Sorry, Java is needed.</applet> </body></html>
The problem with using Kawa to write applets is that the Kawa .jar
file is quite big, and may take a while to download over a network connection.
Some possible solutions:
.jar
any classes your
applet doesn’t need.
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