Template Method Pattern

The Template Method Pattern consists of a method that is used as a default value. It is then overwritten in classes that inherit or include it:

class Bicycle
  attr_reader :size, :chain, :tire_size

  def initialize(args={})
    @size = args[:size]
    @chain = args[:chain] || default_chain
    @tire_size = args[:tire_size] || default_tire_size
  end

  def default_chain # <- common default
    '10-speed'
  end
end

This way classes that inherit from Cycle can overwrite default_chain, removing the need of super calls and achieve less coupling:

</pre>
class MountainBike < Bicycle
  def default_chain
    '5-speed'
  end
end

Seen in POODR.


Null Object Pattern

Bad:

ids = ['pig', '', 'sheep']

animals = ids.map {|id| Animal.find(id)}

animals.each do |animal|
  puts animal.nil? ? 'no animal' : animal.name
end

Good:

animals = ids.map{ |id| GuaranteedAnimal.find(id) }
animals.each{ |animal| puts animal.name }

class GuaranteedAnimal
  def self.find(id)
    Animal.find(id) || MissingAnimal.new
  end
end

class MissingAnimal
  def name
    'no animal'
  end
end

Seen in the video of Sandi Metz in RailsConf 2015: