Template Method を Ruby で

Template Method は、振る舞いに関するデザインパターンの一つで、アルゴリズムの骨組みを基底クラスで定義し、サブクラスではアルゴリズムの全体的な構造は残したまま、ステップを上書きします。

Template Method の詳細

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使用例： Template Method パターンは、 Ruby コードではよく見かけます。フレームワークの利用者に継承を用いて標準機能を拡張する単純な方法を提供するために、開発者がよく利用します。

見つけ方： 基底クラスにあるメソッドの一つが、他の抽象または空くうのメソッドをたくさん呼んでいる場合、 Template Method を識別できます。

概念的な例

この例は、 Template Method デザインパターンの構造を説明するためのものです。以下の質問に答えることを目的としています：

どういうクラスからできているか？
それぞれのクラスの役割は？
パターンの要素同士はどう関係しているのか？

main.rb: 概念的な例

# The Abstract Class defines a template method that contains a skeleton of some
# algorithm, composed of calls to (usually) abstract primitive operations.
#
# Concrete subclasses should implement these operations, but leave the template
# method itself intact.
class AbstractClass
  # The template method defines the skeleton of an algorithm.
  def template_method
    base_operation1
    required_operations1
    base_operation2
    hook1
    required_operations2
    base_operation3
    hook2
  end

  # These operations already have implementations.

  def base_operation1
    puts 'AbstractClass says: I am doing the bulk of the work'
  end

  def base_operation2
    puts 'AbstractClass says: But I let subclasses override some operations'
  end

  def base_operation3
    puts 'AbstractClass says: But I am doing the bulk of the work anyway'
  end

  # These operations have to be implemented in subclasses.
  def required_operations1
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  # @abstract
  def required_operations2
    raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
  end

  # These are "hooks." Subclasses may override them, but it's not mandatory
  # since the hooks already have default (but empty) implementation. Hooks
  # provide additional extension points in some crucial places of the algorithm.

  def hook1; end

  def hook2; end
end

# Concrete classes have to implement all abstract operations of the base class.
# They can also override some operations with a default implementation.
class ConcreteClass1 < AbstractClass
  def required_operations1
    puts 'ConcreteClass1 says: Implemented Operation1'
  end

  def required_operations2
    puts 'ConcreteClass1 says: Implemented Operation2'
  end
end

# Usually, concrete classes override only a fraction of base class' operations.
class ConcreteClass2 < AbstractClass
  def required_operations1
    puts 'ConcreteClass2 says: Implemented Operation1'
  end

  def required_operations2
    puts 'ConcreteClass2 says: Implemented Operation2'
  end

  def hook1
    puts 'ConcreteClass2 says: Overridden Hook1'
  end
end

# The client code calls the template method to execute the algorithm. Client
# code does not have to know the concrete class of an object it works with, as
# long as it works with objects through the interface of their base class.
def client_code(abstract_class)
  # ...
  abstract_class.template_method
  # ...
end

puts 'Same client code can work with different subclasses:'
client_code(ConcreteClass1.new)
puts "\n"

puts 'Same client code can work with different subclasses:'
client_code(ConcreteClass2.new)

output.txt: 実行結果

Same client code can work with different subclasses:
AbstractClass says: I am doing the bulk of the work
ConcreteClass1 says: Implemented Operation1
AbstractClass says: But I let subclasses override some operations
ConcreteClass1 says: Implemented Operation2
AbstractClass says: But I am doing the bulk of the work anyway

Same client code can work with different subclasses:
AbstractClass says: I am doing the bulk of the work
ConcreteClass2 says: Implemented Operation1
AbstractClass says: But I let subclasses override some operations
ConcreteClass2 says: Overridden Hook1
ConcreteClass2 says: Implemented Operation2
AbstractClass says: But I am doing the bulk of the work anyway

Template Method を Ruby で

概念的な例

main.rb: 概念的な例

output.txt: 実行結果

他言語での Template Method