module Tins::Annotate

def annotate(name)
  singleton_class.class_eval do
    define_method(name) do |annotation|
      instance_variable_set "@__annotation_#{name}__", annotation
    end
    define_method("#{name}_of") do |method_name|
      __send__("#{name}_annotations")[method_name]
    end
    define_method("#{name}_annotations") do
      if instance_variable_defined?("@__annotation_#{name}_annotations__")
        instance_variable_get "@__annotation_#{name}_annotations__"
      else
        instance_variable_set "@__annotation_#{name}_annotations__", {}
      end
    end
    old_method_added = instance_method(:method_added)
    define_method(:method_added) do |method_name|
      old_method_added.bind(self).call method_name
      if annotation = instance_variable_get("@__annotation_#{name}__")
        __send__("#{name}_annotations")[method_name] = annotation
      end
      instance_variable_set "@__annotation_#{name}__", nil
    end
  end
end