simple_state_machine.rb

# Usage:
#
# class Mail
#   include SimpleStateMachine
#
#   self.initial_state = 'unread'
#   self.transitions_map = {
#     read:   {from: 'unread', to: 'read'},
#     unread: {from: 'any',   to: 'unread'},
#     delete: {from: 'any',   to: 'deleted'},
#     spam:   {from: 'any',   to: 'spam'}
#   }
#
#   attr_accessor :state
# end
#
# mail = Mail.new
# mail.state # => 'unread'
# mail.read  # => true
# mail.state # => 'read'
# mail.read_allowed? # => false
# mail.read! # => RuntimeError: Invalid transition from `read` by `read`
#
module SimpleStateMachine
  def state
    fail NotImplementedError
  end

  def state=(value)
    fail NotImplementedError
  end

  def initialize(*)
    super
    self.state = self.class.initial_state if self.class.initial_state
  end

  def self.included(klass)
    klass.extend ClassMethods
  end

  module ClassMethods
    attr_accessor :initial_state

    def transitions_map=(transitions_map)
      transitions_map.each do |transition_name, options|
        from = Array(options.fetch(:from))
        to = options.fetch(:to)
        define_method("#{transition_name}_allowed?") do
          !([state, 'any'] & from).empty?
        end
        define_method(transition_name) do
          return unless send("#{transition_name}_allowed?")
          self.state = to
          true
        end
        define_method("#{transition_name}!") do
          return true if send(transition_name)
          raise "Invalid transition from `#{state}` by `#{transition_name}`"
        end
      end
    end
  end
end