Extract Value Object

Value Objects are objects that represent a value (such as a dollar amount) rather than a unique, identifiable entity (such as a particular user).

Value objects often implement information derived from a primitive object, such as the dollars and cents from a float, or the user name and domain from an email string.

Uses

• Prevent duplicated code from making the same observations of primitive objects throughout the code base.
• Remove large classes by splitting out query methods associated with a particular variable.
• Make the code easier to understand by fully encapsulating related logic into a single class, following the single responsibility principle.
• Eliminate divergent change by extracting code related to an embedded semantic type.

Example

InvitationsController is bloated with methods and logic relating to parsing a string that contains a list of email addresses:

# app/controllers/invitations_controller.rb
def recipient_list
  @recipient_list ||= recipients.gsub(/\s+/, '').split(/[\n,;]+/)
end

def recipients
  params[:invitation][:recipients]
end

We can extract a new class to offload this responsibility:

# app/models/recipient_list.rb
class RecipientList
  include Enumerable

  def initialize(recipient_string)
    @recipient_string = recipient_string
  end

  def each(&block)
    recipients.each(&block)
  end

  def to_s
    @recipient_string
  end

  private

  def recipients
    @recipient_string.to_s.gsub(/\s+/, '').split(/[\n,;]+/)
  end
end

# app/controllers/invitations_controller.rb
def recipient_list
  @recipient_list ||= RecipientList.new(params[:invitation][:recipients])
end

Next Steps

• Search the application for duplicated code related to the newly extracted class.
• Value objects should be immutable. Make sure the extracted class doesn’t have any writer methods.