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