1  require 'active_support/core_ext/kernel/singleton_class'
   2  require 'active_support/core_ext/module/remove_method'
   3 
   4  class Class
   5    # Declare a class-level attribute whose value is inheritable and
   6    # overwritable by subclasses:
   7    #
   8    #   class Base
   9    #     class_attribute :setting
  10    #   end
  11    #
  12    #   class Subclass < Base
  13    #   end
  14    #
  15    #   Base.setting = true
  16    #   Subclass.setting            # => true
  17    #   Subclass.setting = false
  18    #   Subclass.setting            # => false
  19    #   Base.setting                # => true
  20    #
  21    # This matches normal Ruby method inheritance: think of writing an attribute
  22    # on a subclass as overriding the reader method.
  23    #
  24    # For convenience, a query method is defined as well:
  25    #
  26    #   Subclass.setting?           # => false
  27    #
  28    # Instances may overwrite the class value in the same way:
  29    #
  30    #   Base.setting = true
  31    #   object = Base.new
  32    #   object.setting          # => true
  33    #   object.setting = false
  34    #   object.setting          # => false
  35    #   Base.setting            # => true
  36    #
  37    # To opt out of the instance writer method, pass :instance_writer => false.
  38    #
  39    #   object.setting = false  # => NoMethodError
  40    def class_attribute(*attrs)
  41      instance_writer = !attrs.last.is_a?(Hash) || attrs.pop[:instance_writer]
  42 
  43      attrs.each do |name|
  44        class_eval <<-RUBY, __FILE__, __LINE__ + 1
  45          def self.#{name}() nil end
  46          def self.#{name}?() !!#{name} end
  47 
  48          def self.#{name}=(val)
  49            singleton_class.class_eval do
  50              remove_possible_method(:#{name})
  51              define_method(:#{name}) { val }
  52            end
  53          end
  54 
  55          def #{name}
  56            defined?(@#{name}) ? @#{name} : singleton_class.#{name}
  57          end
  58 
  59          def #{name}?
  60            !!#{name}
  61          end
  62        RUBY
  63 
  64        attr_writer name if instance_writer
  65      end
  66    end
  67  end