snippet.rb

class User
  def initialize()
    @context = Context.new
  end
  def state
    @context.state
  end
end

class Context
  attr_accessor :state
  def initialize
    @state = Pending.new(self)
  end
end

class State
  attr_accessor :name
  def initialize(context)
    @context = context
    @name = self.class.name.downcase.to_sym
  end
  def to_s
    name
  end
end

class Pending < State
  def active!
    @context.state = Active.new(@context)
  end
end

class Active < State
  def inactive!
    @context.state = Inactive.new(@context)
  end
end

class Inactive < State
  def delete!
    @context.state = Delete.new(@context)
  end
end

class Delete < State
  def pending!
    @context.state = Pending.new(@context)
  end
end


if __FILE__ == $PROGRAM_NAME
   require "rubygems" 
   require "spec" 
   describe User do
     before(:each) do
       @user = User.new
     end
     it "should have initial state" do
       @user.state.name.should eql(:pending)
     end
     it "should transform to new state" do
       @user.state.active!
       @user.state.name.should eql(:active)
     end
     it "should not allow invalid transition" do
       @user.state.delete!
       @user.state.name.should eql(:pending)
     end
   end 
end