gistfile1.rb

# Install kicker on osx: sudo gem install kicker --source http://gemcutter.org
#
# Place a `.kick' file with this code in the root of your project, which contains the file to watch.
# Then start running kicker from the project root: $ kicker

module Mockup
  FILE_TO_WATCH = 'mockup-1.psd'
  
  def self.last_exported_version
    FileUtils.mkdir_p 'output'
    if version = Dir.glob("output/*.png").sort.last
      $1.to_i if version =~ /mockup-(\d+)\.png$/
    end
  end
  
  def self.version
    unless @version and File.exist?('output')
      @version = last_exported_version || 0
    end
    @version += 1
  end
  
  def self.create_export!
    execute "sips #{FILE_TO_WATCH} -s format png --out output/mockup-#{version}.png"
  end
  
  def self.call(files)
    create_export! if files.delete(FILE_TO_WATCH)
  end
end

process Mockup