JRuby

java interfaces

JRuby classes can mixin Java interfaces as modules in order to implement them.

class SomeFlexibleClass
  include java.lang.Runnable
  include java.lang.Comparable
end

JRuby does a wonderful thing with Ruby’s blocks, where a block can be converted to the appropriate Java interface. It works by converting the block to a Proc, and then decorating it with a proxy object that runs the block for any method called on the interface. Runnable is the prime example here. You should try this yourself, also see using callbacks hype library examples for more sophisticated stuff.

button = javax.swing.JButton.new "Press me!"
button.add_action_listener { |event| event.source.text = "I done been pressed." }

Read more about java interface etc. Otherwise see wiki

java reflection

In vanilla processing, you can create libraries in java that can access the sketch pre, draw, post, mouseEvent and keyEvent methods by java reflection, but you must register these methods with the PApplet. We can also register custom methods (but sadly only those without parameters, which is why we found another way of dealing with selectInput see chooser, movieEvent see video_event).

Here we show you how to registerMethods in a ruby class by java reflection in JRuby (requires the !become_java method).

#!/usr/bin/env jruby
require 'picrate'
require_relative 'register_send'

class ReflectionVoodoo < Processing::App
  def settings
    size 300, 200
    # pixel_density(2) # for HiDpi screens
    # smooth # see https://processing.org/reference/smooth_.html
  end

  def setup
    sketch_title 'Reflection Voodoo'
    RegisterSend.new self
    no_loop
  end

  def draw
    fill 0, 0, 200
    ellipse(170, 115, 70, 100)
  end
end

ReflectionVoodoo.new

Here is the class that does the reflection (it needs to become java so that self gets recognized as a java object)

require 'jruby/core_ext' # required to add become_java! method

# This class demonstrates how to use 'reflection' methods in ruby-processing
# NB: the class must become a java object to get registered. This is an
# advanced feature in vanilla processing, mainly used by libraries.
class RegisterSend
  attr_reader :parent

  def initialize(parent)
    @parent = parent
    parent.java_send :registerMethod, [java.lang.String, java.lang.Object], :draw, self
    parent.java_send :registerMethod, [java.lang.String, java.lang.Object], :pre, self
  end

  def pre
    puts 'before draw'
    parent.background(100)
  end

  def draw
    puts 'at begin draw...'
    parent.fill(200, 100)
    parent.ellipse(150, 100, 200, 60)
  end
  # putting become_java! here works OK
  become_java!
end

Note how the transparent ellipse is placed on top of the solid ellipse, so the reflection element must get drawn after the original sketch element.

Java Extensions for JRuby

JRuby is a 100% Java implementation of the Ruby programming language. It is Ruby for the JVM. So it is quite possible to write your own ruby code in java, you might do this to better integrate jruby with java or to write your own ruby classes (modules etc) in java see Method Signatures and Annotations in JRuby extensions and jruby-examples.