user_defined_visibility.rb

class Module

  def wrap_package_private name
    wraped_method = instance_method(name)
    define_method(name) do |*args|
      # check
      puts "checking package_private: #{name}"
      if true
        e = NoMethodError.new("package private method is called from outside of the package")
        e.set_backtrace caller
        raise e
      end
      # call original
      wraped_method.bind(self).call(*args)
    end
  end
  private :wrap_package_private

  def package_private *methods
    if methods.length > 0
      methods.each do |m|
        wrap_package_private m
      end
    else
      set_current_visibility :package_private
    end
  end

  def set_current_visibility visi
    @current_visibility = visi
    @caller_of_current_visibility_set_method = caller[2..-1]
  end
  private :set_current_visibility

  def method_added name
    if @current_visibility
      current_caller = caller[1..-1]
      if current_caller == @caller_of_current_visibility_set_method
        case @current_visibility
        when :package_private
          wrap_package_private name
        end
      end
    end
  end

  alias __public__ public
  def public *args
    @current_visibility = nil
    __public__ *args
  end
  alias __protected__ protected
  def protected *args
    @current_visibility = nil
    __protected__ *args
  end
  alias __private__ private
  def private *args
    @current_visibility = nil
    __private__ *args
  end
end

class Neko
  def public_method1
    puts "public_method1"
  end

  package_private
  def package_private_method1
    puts "package_private_method1"
  end
  def package_private_method2
    puts "package_private_method2"
  end

  public
  def public_method2
    puts "public_method2"
  end

end

class Neko
  def public_method3
    puts "public_method3"
  end
end

Neko.new.public_method1
Neko.new.public_method2
Neko.new.public_method3
Neko.new.package_private_method1
Neko.new.package_private_method2