gem install statesman-events

class TaskStateMachine
  include Statesman::Machine
  include Statesman::Events

  state :unstarted, initial: true
  state :started
  state :finished
  state :cancelled

  event :start do
    transition from: :unstarted,  to: :started
  end

  event :finish do
    transition from: :started,    to: :finished
  end

  event :cancel do
    transition from: :unstarted,  to: :cancelled
    transition from: :started,    to: :cancelled
  end

  event :restart do
    transition from: :finished,   to: :started
    transition from: :cancelled,  to: :started
  end
end

class Task < ActiveRecord::Base
  delegate :current_state, :trigger, :available_events, to: :state_machine

  def state_machine
    @state_machine ||= TaskStateMachine.new(self)
  end
end

task = Task.new

task.current_state          # => "unstarted"
task.trigger(:start)        # => true/false
task.current_state          # => "started"
task.available_events       # => [:finish, :cancel]

ExampleMachine.event(:some_event) do
  transition from: :some_state,       to: :another_state
  transition from: :some_other_state, to: :yet_another_state
end

instance.trigger(:some_event)

instance.trigger!(:some_event)

instance.available_events