Computed attributes

computed_attributes.rb

module ComputedAttributes
  module ClassMethods
    def compute(name, components, &block)
      getter = -> do
        value = instance_variable_get("@_#{name}")
        unless value
          value = instance_exec(&block)
          instance_variable_set("@_#{name}", value)
        end
        value
      end
      define_method(name, &getter)
      components.each do |attribute|
        setter = ->(value) do
          instance_variable_set("@_#{name}", nil)
          super(value)
        end
        define_method("#{attribute}=", &setter)
      end
    end
  end
end

class Person < Struct.new(:first_name, :last_name)
  extend ComputedAttributes::ClassMethods

  compute :full_name, [:first_name, :last_name] { "#{first_name} #{last_name}" }
end

me = Person.new('John', 'Doe')
puts me.full_name
me.first_name = 'Jane'
puts me.full_name
me.last_name = 'Smith'
puts me.full_name