Refactoring: Refactorings/Extract Class

Classes should have crisp abstraction and a few clear responsibilities. When the classes get too big and complicated, you should create a new class to handle the extra responsibilities.

Mechanics

1. Decide how to split the responsibilities of the class
2. Create a new class to express the split-off responsibilities. Rename the old class to better describe if necessary
3. Make a link from the old to the new class (might need a back link, but not unless absolutely necessary)
4. Use Move Field on each field you want to move
5. Test after each move
6. Use Move Method to move methods from old to new class. Start with low-level methods (called rather than calling) and build to higher level.
7. Test after each move
8. Review and reduce interfaces of each class (try to remove back link if possible)

Example

Before

class Person
  attr_reader :name
  attr_accessor :office_area_code, :office_number
  def telephone_number
    '(' + @office_area_code + ') ' + @office_number
  end
  # ...
end

After

class Person
  attr_reader :name
  def initialize
    @office_telephone = TelephoneNumber.new
  end

  def telephone_number
    @office_telephone.telephone_number
  end

  def office_telephone
    @office_telephone
  end
end

class TelephoneNumber
  attr_accessor :area_code, :number
  def telephone_number
    '(' + area_code + ') ' + number
  end
end