Peter Cooper's jRuby Pong, Refactored

ball.rb

require 'java'
require 'lwjgl.jar'
require 'slick.jar'

java_import org.newdawn.slick.Image

require 'pong/image_context'
require 'pong/collidable_attributes'

module Pong
  class Ball
    include Pong::ImageContext
    include CollidableAttributes
    attr_accessor :x, :y, :velocity, :angle

    def initialize
      @image = Image.new('ball.png')
      reset
    end

    def reset
      @x = 200
      @y = 200
      @angle = 45     
      @velocity = 0.3
    end

    def move(delta)
      @x += @velocity * delta * Math.cos(@angle * Math::PI / 180)
      @y -= @velocity * delta * Math.sin(@angle * Math::PI / 180)
    end
  end
end

collidable_attributes.rb

module Pong
  # Used for collision detection of the ball to the container and the ball to the paddle.
  # This module assumes the existence of dimension attributes (x, y, width, height)
  #
  # @TODO: Extract this into GameUtils
  module CollidableAttributes
    def is_colliding_with? entity
      x >= entity.x && x <= (entity.x + entity.width) && y.round >= (entity.y - height)
    end

    def colliding_with_container? container
      %w{left right top bottom}.each do |direction|
        return true if container_collision(container, direction)
      end
      return false
    end

    def container_collision(entity, edge)
      case edge
      when 'left'
        return x < 0
      when 'right'
        return x > entity.width - width
      when 'top'
        return y < 0
      when 'bottom'
        return y > entity.height
      else
        return false
      end
    end
  end
end

image_context.rb

module Pong
  # Images in Slick2D work differently than...well, any other game library I've worked with. Instead of
  # explicitly binding images to the window context, it implicitly binds them by only allowing a single
  # container to be instantiated.
  #
  # ImageContext handles delegation of coordinates and sizing of an object to the image context.  In Pong,
  # we use this for the Pong and Paddle
  module ImageContext
    attr_accessor :image

    def width
      @image.width 
    end

    def height
      @image.height
    end

    # Assumes `x` and `y` methods in the included method
    def draw
      @image.draw(x, y)
    end
  end
end

main.rb

$:.push File.expand_path('../lib', __FILE__)

require 'java'
require 'lwjgl.jar'
require 'slick.jar'
require 'pong'

# Screwing with the Java library integration
java_import org.newdawn.slick.AppGameContainer

# Slick2D isolates the game class from the window context that renders it
# This is different from Gosu that mixes it within the Gosu::Window class
app = AppGameContainer.new(Pong::Game.new)
app.set_display_mode(640, 480, false)
app.start

paddle.rb

require 'java'
require 'lwjgl.jar'
require 'slick.jar'

java_import org.newdawn.slick.Image

require 'pong/image_context'
require 'pong/collidable_attributes'

module Pong 
  class Paddle
    include Pong::ImageContext
    include CollidableAttributes
    attr_accessor :x, :y, :velocity

    def initialize
      @image = Image.new('paddle.png')
      @y = 400
      reset
    end

    def reset
      @x = 200
      @velocity = 0.3
    end

    def move(delta)
      @x += velocity * delta
    end
  end
end

pong.rb

require 'java'
require 'lwjgl.jar'
require 'slick.jar'
require 'pong/ball'
require 'pong/paddle'

java_import org.newdawn.slick.BasicGame
java_import org.newdawn.slick.GameContainer
java_import org.newdawn.slick.Graphics
java_import org.newdawn.slick.Image
java_import org.newdawn.slick.Input
java_import org.newdawn.slick.SlickException

module Pong
  # We subclass BasicGame so we can let Slick2D handle the rendering and main game loop
  # This means we need to define `render`, `init`, and `update`.
  class Game < BasicGame
    def initialize
      super('RubyPong') 
    end
    
    # This method is called continuously.  Rule of thumb: Always redraw the screen from blank.  You do not 
    # because you do not know if `update` will have been called.
    def render(container, graphics)
      @bg.draw(0, 0)
      @ball.draw
      @paddle.draw
      graphics.draw_string('RubyPong (ESC to exit)', 8, container.height - 30)
    end

    # Establishes the connection between container and game state. Creates our game objects. We have `init`
    # AND `initialize` because Java sucks at naming...
    # 
    # @param [AppContainer] - The AppContainer state
    def init(container)
      @bg = Image.new('bg.png')
      @ball = Pong::Ball.new
      @paddle = Pong::Paddle.new
    end

    # This is your game logic method.  This will be called continuously in the main game loop and should be
    # the entry point for input handling and game state change.  Do NOT put rendering logic in the update method
    # due to the nature of update vs render running potentially simultaneously
    # 
    # @param [AppContainer] - The AppContainer game state
    # @param [Float] - The time since last update call
    def update(container, delta)
      handle_input(container, delta)
      @ball.move(delta)
      collision_detection(container)
    end
    
    # Handles input to move the paddle left/right and game exit. 
    # Controls: [LEFT/A] for left, [RIGHT/D] for right, [ESC] to exit
    # 
    # Input handler.  Every key is mapped to a constant in the Input class.  This method is not an override, but
    # it's a good idea to isolate your input handler from your game logic as it will be the second most bloated
    # point for game logic
    # 
    # @param [AppContainer] - The AppContainer game state
    # @param [Float] - Time since last update call
    def handle_input(container, delta)
      input = container.get_input
      container.exit if input.is_key_down(Input::KEY_ESCAPE)

      @paddle.move(-delta) if (input.is_key_down(Input::KEY_LEFT) || input.is_key_down(Input::KEY_A)) && !@paddle.container_collision(container, 'left')
      @paddle.move(delta) if (input.is_key_down(Input::KEY_RIGHT) || input.is_key_down(Input::KEY_D)) && !@paddle.container_collision(container, 'right')
    end

    # Collision detection for the ball staying within the window container
    #
    # @param [container] - the AppContainer object state
    def collision_detection(container)
      if @ball.colliding_with_container? container
        @ball.angle = (@ball.angle + 90) % 360
      end

      if @ball.container_collision(container, 'bottom')
        @paddle.reset
        @ball.reset
      end

      if @ball.is_colliding_with? @paddle
        @ball.angle = (@ball.angle + 90) % 360
      end  
    end
  end
end