Refactoring: Refactorings/Extract Surrounding Method

Two methods contain identical code, save for the middle of the methods. Ruby supports yielding to calling code blocks

Mechanics

1. Use Extract Method on one piece of duplication. Name it after the duplicated behavior (this will become our surrounding method.

   Note: extracted surrounding method will still perform unique behavior for now

2. Test
3. Modify calling method to pass block parameter to surrounding method. Copy unique logic from surrounding method into the block.
4. Replace unique logic in extracted method with yield keyword
5. Identify any variables in the surrounding method that are needed by the unique logic and pass them as parameters in the call to yield
6. Test
7. Modify other methods that can now use the new surrounding method.

Example

Before

class Person
  attr_reader :mother, :children, :name

  def initialize(name, date_of_birth, date_of_death=nil, mother=nil)
    @name, @mother = name, mother
    @date_of_birth, @date_of_death = date_of_birth, date_of_death
    @children = []
    @children.add_child(self) if @mother
  end

  def add_child(child)
    @children << child
  end

  def number_of_living_descendants
    children.inject(0) do |count, child|
      count += 1 if child.alive?
      count + child.number_of_living_descendants
    end
  end

  def number_of descendants_named(name)
    children.inject(0) do |count, child|
      count += 1 if child.name == name
      count + child.number_of_descendants_named(name)
    end
  end

  def alive?
    @date_of_death.nil?
  end
end

After

class Person
  attr_reader :mother, :children, :name

  def initialize(name, date_of_birth, date_of_death=nil, mother=nil)
    @name, @mother = name, mother
    @date_of_birth, @date_of_death = date_of_birth, date_of_death
    @children = []
    @children.add_child(self) if @mother
  end

  def add_child(child)
    @children << child
  end

  def number_of_living_descendants
    count_descendants_matching { |descendant| descendant.alive? }
  end

  def number_of descendants_named(name)
    count_descendants_matching { |descendant| descendant.name == name }
  end

  def count_descendants_matching(&block)
    children.inject(0) do |count, child|
      count += 1 if yield child
      count + child.count_descendants_matching(&block)
    end
  end

  def alive?
    @date_of_death.nil?
  end
end