Guardfile

require 'childprocess'

guard 'shell' do
  watch %r{^app/(.+)\.rb$} do |m|
    `killall rake`

    # Why this:
    # - spawn a child process to avoid locking Guard
    # - make sure that the child process has stdout and stdin otherwise it crashes
    # - bonus point: get REPL access in the simulator!
    process = ChildProcess.build('rake')
    process.io.inherit!
    process.start    
  end
end

install.txt

gem install guard
gem install guard-shell
gem install rb-fsevent
gem install childprocess

guard -i