user_helper.rb

module UserHelper
  attr_accessor :current_user
  
  class Credentials < Struct.new(:email, :password)
    def self.parse(arg)
      return arg if arg.is_a?(Credentials)
      new(arg)
    end
    
    def initialize(credentials)
      self.email, self.password = credentials.split('/')
    end
  end
  
  def sign_up_with(credentials)
    credentials = Credentials.parse(credentials)
    
    self.current_user = Factory(:user, 
      :email => credentials.email,
      :password => credentials.password,
      :password_confirmation => credentials.password)
    
    # hack the credentials into the user object so we can retrieve them later
    current_user.class_eval do
      attr_accessor :credentials
      def password
        credentials.password
      end
    end
    current_user.credentials = credentials
  end
  
  def sign_in
    sign_in_with current_user.credentials
  end
  
  def sign_in_with(credentials)
    credentials = Credentials.parse(credentials)
    visit new_user_session_path
    fill_in 'Email',    :with => credentials.email
    fill_in 'Password', :with => credentials.password
    click_button 'Sign in'
  end
end

World(UserHelper)