check_interface_against_canonical.rb

RSpec.shared_examples_for "check interface against canonical" do |canonical_interface, classes_to_check|
  classes_to_check.each do |class_to_check|
    example "#{class_to_check} conforms to the required interface", :aggregate_failures do
      interface_methods = canonical_interface.public_instance_methods(false)

      methods_to_check = class_to_check.public_instance_methods(false)
      diff = interface_methods - methods_to_check

      expect(diff).to be_empty, (<<~MESSAGE).split("\n").join(" ")
        Expected #{class_to_check} to be polymorphic with
        interface #{canonical_interface}, however the
        following methods were missing: #{diff}
      MESSAGE

      present_methods = interface_methods & methods_to_check
      present_methods.each do |method|
        expected_arity = canonical_interface.instance_method(method).arity
        actual_arity = class_to_check.instance_method(method).arity

        if actual_arity > -1 && expected_arity > -1
          expect(actual_arity).to eq(expected_arity), (<<~MESSAGE).split("\n").join(" ")
            Expected #{class_to_check}##{method.name} to have same arity
            as interface #{canonical_interface}##{method.name}
            #{expected_arity}, but it was #{actual_arity}.
          MESSAGE
        end
      end
    end
  end
end