module Skr::Concerns

    # @see ClassMethods
    module AttrAccessorWithDefault
        extend ActiveSupport::Concern

        module ClassMethods

            # defines a attr_accessor with a default value
            # @param name [ Symbol ] name of the attribute
            # @param default [ Object, lambda, Proc ] if a value is given, be aware that it will be shared between instances
            # @example
            #
            #     Shared = Struct.new(:str)
            #     class AttrTestClass
            #         include AttrAccessorWithDefault
            #         attr_accessor_with_default :non_copying, ->{ Shared.new("a default string") }
            #         attr_accessor_with_default :shared, Shared.new("a default string")
            #     end
            #     a = AttrTestClass.new
            #     b = AttrTestClass.new
            #     a.non_copying.str                   #=> "a default string"
            #     a.non_copying.str  = "new_string"   #=> "new string"
            #     b.non_copying.str                   #=> "a default string"
            #
            #     a.shared.str                   #=> "a default string"
            #     b.shared.str                   #=> "a default string"
            #     a.shared.str = "new string"    #=> "new string"
            #     b.shared.str                   #=> "new string"

            def attr_accessor_with_default( name, default )
                attr_writer name
                attr_reader_with_default( name, default )
            end

            def attr_reader_with_default( name, default )
                module_eval do
                    define_method( name ) do
                        class << self; self; end.class_eval do
                            attr_reader( name )
                        end
                        if instance_variables.include? "@#{name}"
                            instance_variable_get( "@#{name}" )
                        else
                            instance_variable_set( "@#{name}", default.is_a?(Proc) ? default.call : default )
                        end
                    end
                end
            end
        end
    end


end