specification_pattern_basics.rb

module Spec
  module Generic
    class And
      def initialize(specs)
        @specifications = specs
      end
      
      def is_satisfied_by?(subject)
        @specifications.all? do |spec|
          spec.is_satisfied_by?(subject)
        end
      end
    end
    
    class Or
      def initialize(specs)
        @specifications = specs
      end
      
      def is_satisfied_by?(subject)
        @specifications.any? do |spec|
          spec.is_satisfied_by?(subject)
        end
      end
    end
    
    class Not
      def initialize(spec)
        @spec = spec
      end
      
      def is_satisfied_by?(subject)
        !@spec.is_satisfied_by?(subject)
      end
    end
  end
  
  class DividableBy
    def initialize(divisor)
      @divisor = divisor
    end
    
    def is_satisfied_by?(subject)
      subject % @divisor == 0
    end
  end
  
  class IsInteger
    def is_satisfied_by?(subject)
      subject.is_a?(Integer)
    end
  end
  
  #example of custom composite specification.
  #subject must be dividable by 4 and not be an integer
  #or is dividable by 3
  class MyMagicSpecification
    def initialize(divisor, alt_divisor)
      @divisor = divisor
      @alt_divisor = alt_divisor
    end
    
    def is_satisfied_by?(subject)
      specification.is_satisfied_by?(subject)
    end
    
    private
    
    def specification
      Generic::Or.new([dividable_not_integer, alt_dividable])
    end
    
    def dividable_not_integer
      Generic::And.new([DividableBy.new(@divisor),
                        Generic::Not.new(IsInteger.new)])
    end
    
    def alt_dividable
      DividableBy.new(@alt_divisor)
    end
  end
end

my_magic_spec = Spec::MyMagicSpecification.new(4,3)
my_magic_spec.is_satisfied_by?(4)   # => false
my_magic_spec.is_satisfied_by?(4.0) # => true
my_magic_spec.is_satisfied_by?(12)  # => true