after_transition on: vs. after_transition to:

state_machine_sandpit.rb

require 'state_machine'

module Sandpit
  class Foo
    state_machine :state, initial: :created do
      event :start do
        transition created: :running
        transition running: same
      end
      event :stop do
        transition running: :stopped
      end

      after_transition on: :running do |foo, _|
        puts 'Congratulations on starting to run!'
      end

      after_transition on: :stopped do |foo, _|
        puts 'Thank goodness you stopped...'
      end
    end
  end

  class Bar
    state_machine :state, initial: :created do
      event :start do
        transition created: :running
        transition running: same
      end
      event :stop do
        transition running: :stopped
      end

      after_transition to: :running do |foo, _|
        puts 'Congratulations on starting to run!'
      end

      after_transition to: :stopped do |foo, _|
        puts 'Thank goodness you stopped...'
      end
    end
  end
end



foo = Sandpit::Foo.new
# => #<Sandpit::Foo:0x007fa3a230f780 @state="created">

bar = Sandpit::Bar.new
# => #<Sandpit::Bar:0x007fa3a232d280 @state="created">

foo.start
# => true

bar.start
# Congratulations on starting to run!
# => true
 
foo.stop
# => true
 
bar.stop
# Thank goodness you stopped...
# => true

cheers @kgunbin for the explanation...

on: is for events
to: is for states