Ruby : methods to change a class

class_macros.rb

#using extend to modify a class
#extend will add an class method we will use this class method to create instance method with 'define_method' dynamiclly

require 'test/unit'

module CheckedAttributes
    def self.included(klass)
        klass.extend ClassMethods # here we created an class method
    end
    module ClassMethods
        def attr_checked(attribute,&block)
            define_method "#{attribute}=" do |value|
                raise 'Invalid attribute' unless block.call(value)
                instance_variable_set("@#{attribute}",value)
            end

            define_method attribute do
                instance_variable_get("@#{attribute}")
            end
        end
    end
end

class Person
    include CheckedAttributes
    attr_checked :age do |v|
        v >= 18
    end
end

class TestCheckedAttributes < Test::Unit::TestCase
    def setup
        @bob = Person.new
    end
    def test_accepts_valid_values
        @bob.age = 20
        assert_equal 20,@bob.age
    end
    def test_refuses_invalide_values
        assert_raises RuntimeError, 'Invalid attribute' do
            @bob.age = 17
        end
    end
end

modify_class.rb

 
#open class override 

#extend clas

#define class in class's singleton_class

#using eval

#using class_val