Radical Proxy experiment for JRubyArt

Here I create a custom (abstract) Proxy class as a java extension for JRubyArt, which when appropriately initialized, gives implementing classes access to processing pre(), draw() and post() loops. With example usage (by a custom Array class), note the empty draw loop.... It probably doesn't matter whether the particle gets updated in pre(), draw() or post() loops. See original vanilla processing code in action here as js version. Note how each module/particle has it's own copy of the update and draw code in those versions. Whereas in JRubyArt a single object, the custom array has this code, and the particle is just a simple Struct.

package processing.core;

/**
 *
 * @author Martin Prout
 */
public abstract class Proxy {

    private final PApplet app;

    /**
     * Useful accessors
     */
    public int width, height;

    /**
     *
     * @param app Applet
     */
    public Proxy(PApplet app) {
        this.app = app;
        this.width = app.width;
        this.height = app.height;
        setActive(true);
    }

    /**
     * Extending classes must implement
     */
    public abstract void pre();

    /**
     * Extending classes must implement
     */
    public abstract void draw();

    /**
     * Extending classes must implement
     */
    public abstract void post();

    private void setActive(boolean active) {
        if (active) {
            this.app.registerMethod("pre", this);
            this.app.registerMethod("draw", this);
            this.app.registerMethod("post", this);
            this.app.registerMethod("dispose", this);
        } else {
            this.app.unregisterMethod("pre", this);
            this.app.unregisterMethod("draw", this);
            this.app.unregisterMethod("post", this);
        }
    }

    /**
     * Simple signature
     * @param col
     */
    public void background(int col) {
        this.app.background(col);
    }

    /**
     * Simple signature
     * @param col
     */
    public void fill(int col) {
        this.app.fill(col);
    }

    /**
     * Simple signature
     * @param col
     */
    public void stroke(int col) {
        this.app.stroke(col);
    }

    /**
     * Access applet if we must
     * @return
     */
    public PApplet app() {
        return this.app;
    }

    /**
     * required for processing
     */
    public void dispose() {
        setActive(false);
    }
}

# Demonstrates possible syntax for creating a custom array of objects.

UNIT = 40
attr_reader :custom_array

def setup
  size 640, 360
  wide_count = width / UNIT
  height_count = height / UNIT
  @custom_array = CustomArray.new(self)
  height_count.times do |i|
    wide_count.times do |j|
      custom_array.add_object(j * UNIT, i * UNIT, UNIT / 2, UNIT / 2, rand(0.05..0.8))
    end
  end
  no_stroke
end

def draw
end

# The Particle object

Particle = Struct.new(:x, :y, :mx, :my, :size, :speed, :xdir, :ydir)

require 'forwardable'

# The custom Array (that can access pre, post and draw loops by reflection)
# Also Proxy has access to background(int), fill(int) and stroke(int)
class CustomArray < Java::ProcessingCore::Proxy
  extend Forwardable
  def_delegators(:@objs, :each, :<<)
  include Enumerable

  attr_reader :app

  def initialize(app)
    @app = app
    @objs = []
  end

  def add_object(mx, my, x, y, speed)
    self << Particle.new(x.to_i, y.to_i, mx, my, UNIT, speed, 1, 1)
  end

  def post
    each do |obj|
      update_x obj
      next unless obj.y >= UNIT || obj.x <= 0
      obj.ydir *= -1
      obj.y += obj.ydir
    end
  end

  def update_x(obj)
    obj.x += obj.speed * obj.xdir
    return if (0..UNIT).cover? obj.x
    obj.xdir *= -1
    obj.x += obj.xdir
    obj.y += obj.ydir
  end

  def pre
  end

  def draw
    background(0)
    fill(255)
    each do |obj|
      app.ellipse(obj.mx + obj.x, obj.my + obj.y, 6, 6)
    end
  end
end

JRubyArt is a development branch of ruby-processing, that is currently going its own way (ie somewhat different implementation)

Custom Contact Listener for pbox2d and ruby-processing

require 'pbox2d'
require_relative 'lib/custom_listener'
require_relative 'lib/particle'
require_relative 'lib/boundary'

attr_reader :box2d, :particles, :wall

def setup
  size 400, 400
  @box2d = Box2D.new(self)
  box2d.create_world
  box2d.add_listener(CustomListener.new)
  @particles = []
  @wall = Boundary.new(box2d, width / 2, height - 5, width, 10)
end

def draw
  background(255)

  if (rand < 0.1)
    particles << Particle.new(box2d, rand(width), 20, rand(4..8))
  end
  particles.each{ |p| p.display(self) }
  particles.reject!(&:done)
  wall.display(self)
end

Custom Listener

class CustomListener
  include ContactListener

  def begin_contact(cp)
    # Get both fixtures
    f1 = cp.getFixtureA
    f2 = cp.getFixtureB
    # Get both bodies
    b1 = f1.getBody
    b2 = f2.getBody
    # Get our objects that reference these bodies
    o1 = b1.getUserData
    o2 = b2.getUserData
    return unless (o1.respond_to?(:change) && o2.respond_to?(:change))
    o1.change
    o2.change
  end

  def end_contact(cp)
  end

  def pre_solve(cp, m)
  end

  def post_solve(cp, ci)
  end

end

Boundary class

CENTER ||= Java::ProcessingCore::PConstants::CENTER

class Boundary
  include PB
  attr_reader :box2d, :x, :y, :w, :h, :b
  def initialize(b2d, x, y, w, h)
    @box2d, @x, @y, @w, @h = b2d, x, y, w, h
    sd = PolygonShape.new
    box2dW = box2d.scale_to_world(w / 2)
    box2dH = box2d.scale_to_world(h / 2)
    # We're just a box
    sd.setAsBox(box2dW, box2dH);


    # Create the body
    bd = BodyDef.new
    bd.type = BodyType::STATIC;
    bd.position.set(box2d.processing_to_world(x,y))
    @b = box2d.create_body(bd)

    # Attached the shape to the body using a Fixture
    b.create_fixture(sd, 1)
    b.setUserData(self)
  end

  # Draw the boundary, if it were at an angle we'd have to do something fancier
  def display(app)
    app.fill(0)
    app.stroke(0)
    app.rectMode(CENTER)
    app.rect(x,y,w,h)
  end
end

Particle class

class Particle
  include PB
  attr_accessor :body
  attr_reader :box2d, :radius, :col

  def initialize(b2d, x, y, r)
    @box2d, @x, @y, @radius = b2d, x, y, r
    # This function puts the particle in the Box2d world
    make_body(x, y, radius)
    @col = -5263441
    body.setUserData(self)
  end

  # This function removes the particle from the box2d world
  def kill_body
    box2d.destroy_body(body)
  end

  # Change color when hit
  def change
    @col = -65536 # red
  end

  # Is the particle ready for deletion?
  def done
    # Let's find the screen position of the particle
    pos = box2d.body_coord(body)
    # Is it off the bottom of the screen?
    return false unless (pos.y > box2d.height + radius * 2)
    kill_body
    true
  end

  def display(app)
    # We look at each body and get its screen position
    pos = box2d.body_coord(body)
    # Get its angle of rotation
    a = body.get_angle
    app.push_matrix
    app.translate(pos.x, pos.y)
    app.rotate(a)
    app.fill(col)
    app.stroke(0)
    app.stroke_weight(1)
    app.ellipse(0, 0, radius * 2, radius * 2)
    # Let's add a line so we can see the rotation
    app.line(0, 0, radius, 0)
    app.pop_matrix
  end

  # Here's our function that adds the particle to the Box2D world
  def make_body(x, y, r)
    # Define a body
    bd = BodyDef.new
    # Set its position
    bd.position = box2d.processing_to_world(x, y)
    bd.type = BodyType::DYNAMIC
    @body = box2d.create_body(bd)
    # Make the body's shape a circle
    cs = CircleShape.new
    cs.m_radius = box2d.scale_to_world(r)
    fd = FixtureDef.new
    fd.shape = cs
    # Parameters that affect physics
    fd.density = 1
    fd.friction = 0.01
    fd.restitution = 0.3
    # Attach fixture to body
    body.create_fixture(fd)
    body.set_angular_velocity(rand(-10.0..10))
  end
end

Using the pbox2d gem in JRubyArt

Things are little bit more complicated for JRubyArt (compared with ruby-processing) as I am currently refusing to use the Processing::Proxy sugar (that allows you in part to mimic the inner classes of vanilla processing) in JRubyArt as it is kind of bad practice to expose variables and methods willy nilly.

require 'jruby_art'
require 'pbox2d'
require_relative 'lib/custom_shape'

class Polygons < Processing::App
  # Basic example of falling rectangles
  attr_reader :box2d, :boundaries, :polygons

  def setup
    size(640, 360)
    # Initialize box2d physics and create the world
    @box2d = Box2D.new(self)
    box2d.create_world
    # We are setting a custom gravity
    box2d.gravity(0, -20)
    # Create Arrays
    @polygons = []
    @boundaries = []
    # Add a bunch of fixed boundaries
    boundaries << Boundary.new(box2d, width / 4, height - 5, width / 2 - 50, 10, 0)
    boundaries << Boundary.new(box2d, 3 * width / 4, height - 50, width / 2 - 50, 10, 0)
    boundaries << Boundary.new(box2d, width - 5, height / 2, 10, height, 0)
    boundaries << Boundary.new(box2d, 5, height / 2, 10, height, 0)
  end

  def draw
    background(255)
    # Display all the boundaries
    boundaries.each { |wall| wall.display(self) }
    # Display all the polygons
    polygons.each { |poly| poly.display(self) }
    # polygons that leave the screen, we delete them
    # (note they have to be deleted from both the box2d world and our list
    polygons.reject!(&:done)
  end

  def mouse_pressed
    polygons << CustomShape.new(box2d, mouse_x, mouse_y, height)
  end
end

Polygons.new(title: 'Polygons')

CLOSE = Java::ProcessingCore::PConstants::CLOSE   # these are just ints
CENTER = Java::ProcessingCore::PConstants::CENTER

class CustomShape
  include PB
  # We need to keep track of a Body and a width and height
  attr_reader :body, :box2d, :height

  # Constructor
  def initialize(b2d, x, y, height)
    # Add the box to the box2d world
    @box2d, @height = b2d, height
    make_body(PB::Vec2.new(x, y))
  end

  # This function removes the particle from the box2d world
  def kill_body!
    box2d.destroy_body(body)
  end

  # Is the particle ready for deletion?
  def done
    # Let's find the screen position of the particle
    pos = box2d.body_coord(body)
    # Is it off the bottom of the screen?
    return false unless pos.y > height
    kill_body!
    true
  end

  # Drawing the box
  def display(app)
    # We look at each body and get its screen position
    pos = box2d.body_coord(body)
    # Get its angle of rotation
    a = body.get_angle
    f = body.get_fixture_list
    ps = f.get_shape
    app.rect_mode(CENTER)
    app.push_matrix
    app.translate(pos.x, pos.y)
    app.rotate(-a)
    app.fill(175)
    app.stroke(0)
    app.begin_shape
    # For every vertex, convert to pixel vector
    ps.get_vertex_count.times do |i|
      v = box2d.vector_to_processing(ps.get_vertex(i))
      app.vertex(v.x, v.y)
    end
    app.end_shape(CLOSE)
    app.pop_matrix
  end

  # This function adds the rectangle to the box2d world
  def make_body(center)
    # Define a polygon (this is what we use for a rectangle)
    sd = PB::PolygonShape.new
    vertices = []
    vertices << box2d.vector_to_world(PB::Vec2.new(-15, 25))
    vertices << box2d.vector_to_world(PB::Vec2.new(15, 0))
    vertices << box2d.vector_to_world(PB::Vec2.new(20, -15))
    vertices << box2d.vector_to_world(PB::Vec2.new(-10, -10))
    sd.set(vertices.to_java(Java::OrgJbox2dCommon::Vec2), vertices.length)
    # Define the body and make it from the shape
    bd = PB::BodyDef.new
    bd.type = PB::BodyType::DYNAMIC
    bd.position.set(box2d.processing_to_world(center))
    @body = box2d.create_body(bd)
    body.create_fixture(sd, 1.0)
    # Give it some initial random velocity
    body.set_linear_velocity(Vec2.new(rand(-5.0..5), rand(2.0..5)))
    body.set_angular_velocity(rand(-5.0..5))
  end
end

class Boundary
  include PB
  attr_reader :box2d, :b, :x, :y, :w, :h
  def initialize(b2d, x, y, w, h, a)
    @box2d, @x, @y, @w, @h = b2d, x, y, w, h
    # Define the polygon
    sd = PB::PolygonShape.new
    # Figure out the box2d coordinates
    box2d_w = box2d.scale_to_world(w / 2)
    box2d_h = box2d.scale_to_world(h / 2)
    # We're just a box
    sd.set_as_box(box2d_w, box2d_h)
    # Create the body
    bd = PB::BodyDef.new
    bd.type = PB::BodyType::STATIC
    bd.angle = a
    bd.position.set(box2d.processing_to_world(x, y))
    @b = box2d.create_body(bd)
    # Attached the shape to the body using a Fixture
    b.create_fixture(sd, 1)
  end

  # Draw the boundary, it doesn't move so we don't have to ask the Body for location
  def display(app)
    app.fill(0)
    app.stroke(0)
    app.stroke_weight(1)
    app.rect_mode(CENTER)
    a = b.get_angle
    app.push_matrix
    app.translate(x, y)
    app.rotate(-a)
    app.rect(0, 0, w, h)
    app.pop_matrix
  end
end

Creating a gem wrapper for jbox2d in ruby-processing

The mechanism for including Daniel Shiffmans PBox2D for processing into ruby-processing is unnecessarily complicated, and I had always thought there should be an easier way. So here it is jbox2d wrapped as a gem using some jruby sugar (and avoiding the nasty reflection nonsense that Dan used, although under the hood jruby may do something similar).
The ruby-processing sketch

require 'pbox2d'
require_relative 'lib/particle_system'
attr_reader :box2d, :boundaries, :systems

def setup
  size(400,300)
  @box2d = Box2D.new(self)
  box2d.create_world
  # We are setting a custom gravity
  box2d.gravity(0, -20)

  # Create Arrays
  @systems = []
  @boundaries = []
  # Add a bunch of fixed boundaries
  boundaries << Boundary.new(box2d, 50, 100, 300, 5, -0.3)
  boundaries << Boundary.new(box2d, 250, 175, 300, 5, 0.5)
end

def draw
  background(255)
  # Run all the particle systems
  if systems.size > 0
    systems.each do |system|
      system.run
      system.add_particles(box2d, rand(0..2))
    end
  end
  # Display all the boundaries
  boundaries.each(&:display)
end

def mouse_pressed
  # Add a new Particle System whenever the mouse is clicked
  systems << ParticleSystem.new(box2d, 0, mouse_x, mouse_y)
end

The associated (local) library

require 'forwardable'

module Runnable
  def run
    reject! { |item| item.done }
    each { |item| item.display }
  end
end

class ParticleSystem
  include Enumerable, Runnable
  extend Forwardable
  def_delegators(:@particles, :each, :reject!, :<<, :empty?)
  def_delegator(:@particles, :empty?, :dead?)

  attr_reader :x, :y

  def initialize(bd, num, x, y)
    @particles = []          # Initialize the Array
    @x, @y = x, y            # Store the origin point  
    num.times do
      self << Particle.new(bd, x, y)
    end
  end

  def add_particles(bd, n)
    n.times do
      self << Particle.new(bd, x, y)
    end
  end
end

# A Particle
require 'pbox2d'

class Particle
  include Processing::Proxy, PB
  TRAIL_SIZE = 6
  # We need to keep track of a Body

  attr_reader :trail, :body, :box2d

  # Constructor
  def initialize(b2d, x, y)
    @box2d = b2d
    @trail = Array.new(TRAIL_SIZE, [x, y])
    # Add the box to the box2d world
    # Here's a little trick, let's make a tiny tiny radius
    # This way we have collisions, but they don't overwhelm the system
    make_body(PB::Vec2.new(x, y), 0.2)
  end

  # This function removes the particle from the box2d world
  def kill_body
    box2d.destroy_body(body)
  end

  # Is the particle ready for deletion?
  def done
    # Let's find the screen position of the particle
    pos = box2d.body_coord(body)
    # Is it off the bottom of the screen?
    return false unless (pos.y > $app.height + 20)
    kill_body
    true
  end

  # Drawing the box
  def display
    # We look at each body and get its screen position
    pos = box2d.body_coord(body)
    # Keep track of a history of screen positions in an array
    (TRAIL_SIZE - 1).times do |i|
      trail[i] = trail[i + 1]
    end
    trail[TRAIL_SIZE - 1] = [pos.x, pos.y]
    # Draw particle as a trail
    begin_shape
    no_fill
    stroke_weight(2)
    stroke(0, 150)
    trail.each do |v|
      vertex(v[0], v[1])
    end
    end_shape
  end

  # This function adds the rectangle to the box2d world
  def make_body(center, r)
    # Define and create the body
    bd = PB::BodyDef.new
    bd.type = PB::BodyType::DYNAMIC
    bd.position.set(box2d.processing_to_world(center))
    @body = box2d.create_body(bd)
    # Give it some initial random velocity
    body.set_linear_velocity(PB::Vec2.new(rand(-1.0..1), rand(-1.0..1)))
    # Make the body's shape a circle
    cs = PB::CircleShape.new
    cs.m_radius = box2d.scale_to_world(r)
    fd = PB::FixtureDef.new
    fd.shape = cs
    fd.density = 1
    fd.friction = 0  # Slippery when wet!
    fd.restitution = 0.5
    # We could use this if we want to turn collisions off
    # cd.filter.groupIndex = -10
    # Attach fixture to body
    body.create_fixture(fd)
  end
end

class Boundary
  include Processing::Proxy, PB
  attr_reader :box2d, :b, :x, :y, :w, :h

  def initialize(b2d, x, y, w, h, a)
    @box2d, @x, @y, @w, @h = b2d, x, y, w, h
    # Define the polygon
    sd = PB::PolygonShape.new
    # Figure out the box2d coordinates
    box2d_w = box2d.scale_to_world(w / 2)
    box2d_h = box2d.scale_to_world(h / 2)
    # We're just a box
    sd.set_as_box(box2d_w, box2d_h)
    # Create the body
    bd = PB::BodyDef.new
    bd.type = PB::BodyType::STATIC
    bd.angle = a
    bd.position.set(box2d.processing_to_world(x, y))
    @b = box2d.create_body(bd)
    # Attached the shape to the body using a Fixture
    b.create_fixture(sd, 1)
  end

  # Draw the boundary, it doesn't move so we don't have to ask the Body for location
  def display
    fill(0)
    stroke(0)
    stroke_weight(1)
    rect_mode(CENTER)
    a = b.get_angle
    push_matrix
    translate(x, y)
    rotate(-a)
    rect(0, 0, w, h)
    pop_matrix
  end
end