attr_reader_guard.rb

module AttrReaderGuard
  extend ActiveSupport::Concern

  class InvalidError < ArgumentError
    PREFIX = '[attr_reader_guard!]:'

    def initialize(subject, predicate)
      super [PREFIX, subject, predicate].join(' ')
    end
  end

  included do
    cattr_reader :attr_reader_guards, default: ->{ {} }
  end

  class_methods do
    def attr_reader_guard!(name, &matcher)
      unless block_given?
        raise InvalidError, name, 'has no matcher.'
      end

      @attr_reader_guards ||= {}
      @attr_reader_guards[name.to_sym] = matcher
      attr_reader name.to_sym
    end
  end

  def initialize(**attributes)
    super(**attributes)

    attributes.each do |name, value|
      matcher = attr_reader_guards.fetch(name.to_sym)
      unless matcher.call(value)
        raise InvalidError, name, 'does not satisfy the pattern'
      end

      instance_variable_set("@#{name}", value)
    rescue KeyError
      raise InvalidError, name, "is not accepted for #{self.class} (unknown attribute)"
    end
  end
end

resize_images.rb

class ResizeImages
  include AttrReaderGuard

  attr_reader_guard!(:width) { it in Integer | /^\d+$/ }
  attr_reader_guard!(:height) { it in Integer | /^\d+$/ }
  attr_reader_guard!(:input_images) { it in [String, String, String] }
  attr_reader_guard!(:object_fit) { it in 'cover' | 'contain' }

  def width = super.to_i
  def height = super.to_i
  def header_image = input_images.first
  def body_image = input_images.second
  def footer_image = input_images.third
end