app.rb

require 'drb'

# ruby-serial was crashing Shoes so run serial code in another process using DRb to 
# communicate the state of the controller.
pid = Process.fork { exec('ruby', 'controller.rb') }

Shoes.app :width => 938, :height => 768, :resizable => false do
  
  # Start the DRb client.
  DRb.start_service
  @controller = DRbObject.new nil, 'druby://localhost:7777'
  
  background 'images/background.png'    
  
  x = y = _x = _y = nil
  screen_width, screen_height = 670, 477
  default_pen_color = '#555'
  
  @sketch = flow do
    stroke default_pen_color
    translate 138, 135
    
    keypress do |key|
      case key
      when 'c'
        clear
      when 'q'
        # Quitting the app by any other method will leave the background thread behind.
        Process.kill('HUP', pid)
        exit
      when '1'..'9'
        strokewidth key.to_i
      when "\e"
        strokewidth 1
        stroke default_pen_color
      when 'r'
        stroke red
      when 'b'
        stroke blue
      when 'g'
        stroke green
      when 'y'
        stroke yellow
      end
    end

    animate do
      # Clear the screen if the tile switch is activated.
      clear if @controller.t == 1
      
      x = @controller.x/1024.0 * screen_width
      y = @controller.y/1024.0 * screen_height
      
      if(_x and _y and (x != _x or y != _y))
        append do
          line _x, _y, x, y
        end
      end
      
      _x, _y = x, y
    end
  end
end

controller.rb

#!/usr/bin/env ruby -w
require 'drb'
require 'serialport'

port = '/dev/tty.usbserial-A70061Un'
baud_rate = 9600
data_bits = 8
stop_bits = 1
parity = SerialPort::NONE

sp = SerialPort.open(port, baud_rate, data_bits, stop_bits, parity)

# Simple class to represent the state of the controller.
class ControllerState
  attr_accessor :x, :y, :t
end

state = ControllerState.new

DRb.start_service 'druby://:7777', state
puts DRb.uri

input = value = ''

while true do
  c = sprintf("%c", sp.getc)
  print c
  
  if %w(x y t).include?(c)
    state.send "#{input}=", value.to_i unless input.empty?
    input = c
    value = ''
  else
    value << c
  end
end

# Never reached...
DRb.thread.join
sp.close