Hey, this is motion-state-machine — a state machine gem designed for RubyMotion for iOS.

It features:

  • A simple, nice-looking definition syntax
  • Reaction to sent events, defined timeouts and global NSNotifications
  • Transition guards and actions
  • State entry / exit actions
  • Internal transitions that don't leave the machine's current state
  • Optional verbose log output for easy debugging
  • Grand Central Dispatch-awareness — uses GCD queues for synchronization

Defining a state machine looks like this:

fsm = start_state: :awake

fsm.when :awake do |state|
  state.on_entry { puts "I'm awake, started and alive!" }
  state.transition_to :sleeping, on:  :finished_hard_work,
  state.die on: :too_hard_work

See below for more examples and usage instructions or consult the RubyDoc page on RubyGems.


Undefined states and visual glitches in applications with complex UIs can be a hassle, especially when the UI is animated and the app has to handle asynchronous data retrieved in the background.

Well-defined UI state machines avoid these problems while ensuring that asynchronous event handling does not lead to undefined results (a.k.a. bugs).

MacRuby and Cocoa don't provide a simple library to address this — motion-state-machine should fill the gap for RubyMotion developers.


  1. If not done yet, add bundler gem management to your RubyMotion app. See for an explanation how.

  2. Add this line to your application's Gemfile:

   gem 'motion-state-machine'
  1. Execute:
   $ bundle


The following example shows how to initialize and define a state machine:

fsm = start_state: :working, verbose: true

This initializes a state machine. Calling fsm.start! would start the machine in the defined start state :working. Using :verbose activates debug output on the console.

Defining states and transitions

After initialization, you can define states and transitions:

fsm.when :working do |state|

  state.on_entry { puts "I'm awake, started and alive!" }
  state.on_exit { puts "Phew. That was enough work." }

  state.transition_to :sleeping,
    on:      :finished_hard_work,
    if:      proc { really_worked_enough_for_now? },
    action:  proc { puts "Will go to sleep now." }

  state.die on: :too_hard_work


This defines…

  1. An entry and an exit action block, called when entering/exiting the state :working.

  2. a transition from state :working to :sleeping, happening when calling fsm.event(:finished_hard_work).

Before the transition is executed, the state machine asks the :if guard block if the transition is allowed. Returning false in this block would prevent the transition from happening.

If the transition is executed, the machine calls the given :action block.

  1. another transition that terminates the state machine when calling fsm.event(:too_hard_work). When terminated, the state machine stops responding to events.

Note that a transition from a state to itself can be internal: Entry/exit actions are not called on execution in this case.

Handling events, timeouts and NSNotifications

Transitions can be triggered…

  • by calling the state machine's #event method (see above).

  • automatically after a given timeout:

  fsm.when :sleeping do |state|
      state.transition_to :working, after: 20

(goes back to :working after 20 seconds in state :sleeping)

  • when a NSNotification is posted: ruby fsm.when :awake do |state| state.transition_to :in_background, on_notification: UIApplicationDidEnterBackgroundNotification end

How fast is it?

The implementation is designed for general non-performance-intensive purposes like managing UI state behavior. It might be too slow for parsing XML, realtime signal processing with high sample rates and similar tasks.

Anyway, it should be able to handle several thousand events per second on an iOS device.


Feel free to fork the project and send me a pull request if you would like me to integrate your bugfix, enhancement, or feature.

You can easily add new triggering mechanisms — they can be implemented in few lines by subclassing the Transition class (see the implementation of NotificationTransition for an example).

I'm also open for suggestions regarding the interface design.

To contribute,

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

If the feature has specs, I will probably merge it :)