Introduce class_attribute to declare inheritable class attributes. Writing an...

Introduce class_attribute to declare inheritable class attributes. Writing an attribute on a subclass behaves just like overriding the superclass reader method. Unifies and replaces most usage of cattr_accessor, class_inheritable_attribute, superclass_delegating_attribute, and extlib_inheritable_attribute.

activesupport/CHANGELOG

 *Rails 3.0 (pending)*
 
+* Introduce class_attribute to declare inheritable class attributes. Writing an attribute on a subclass behaves just like overriding the superclass reader method.  Unifies and replaces most usage of cattr_accessor, class_inheritable_attribute, superclass_delegating_attribute, and extlib_inheritable_attribute.  [Jeremy Kemper, Yehuda Katz]
+
 * Time#- with a DateTime argument behaves the same as with a Time argument, i.e. returns the difference between self and arg as a Float #3476 [Geoff Buesing]
 
 * YAML serialization for OrderedHash.  #3608 [Gregor Schmidt]

activesupport/lib/active_support/core_ext/class/attribute.rb

+require 'active_support/core_ext/object/metaclass'
+require 'active_support/core_ext/module/delegation'
+
+class Class
+  # Declare a class-level attribute whose value is inheritable and
+  # overwritable by subclasses:
+  #
+  #   class Base
+  #     class_attribute :setting
+  #   end
+  #
+  #   class Subclass < Base
+  #   end
+  #
+  #   Base.setting = true
+  #   Subclass.setting            # => true
+  #   Subclass.setting = false
+  #   Subclass.setting            # => false
+  #   Base.setting                # => true
+  #
+  # This matches normal Ruby method inheritance: think of writing an attribute
+  # on a subclass as overriding the reader method.
+  #
+  # For convenience, a query method is defined as well:
+  #
+  #   Subclass.setting?           # => false
+  def class_attribute(*attrs)
+    attrs.each do |attr|
+      metaclass.send(:define_method, attr) { }
+      metaclass.send(:define_method, "#{attr}?") { !!send(attr) }
+      metaclass.send(:define_method, "#{attr}=") do |value|
+        metaclass.send(:define_method, attr) { value }
+      end
+    end
+  end
+end

activesupport/test/core_ext/class/attribute_test.rb

+require 'abstract_unit'
+require 'active_support/core_ext/class/attribute'
+
+class ClassAttributeTest < ActiveSupport::TestCase
+  class Base
+    class_attribute :setting
+  end
+
+  class Subclass < Base
+  end
+
+  def setup
+    @klass = Class.new { class_attribute :setting }
+    @sub = Class.new(@klass)
+  end
+
+  test 'defaults to nil' do
+    assert_nil @klass.setting
+    assert_nil @sub.setting
+  end
+
+  test 'inheritable' do
+    @klass.setting = 1
+    assert_equal 1, @sub.setting
+  end
+
+  test 'overridable' do
+    @sub.setting = 1
+    assert_nil @klass.setting
+
+    @klass.setting = 2
+    assert_equal 1, @sub.setting
+
+    assert_equal 1, Class.new(@sub).setting
+  end
+
+  test 'query method' do
+    assert_equal false, @klass.setting?
+    @klass.setting = 1
+    assert_equal true, @klass.setting?
+  end
+
+  test 'no instance delegates' do
+    assert_raise(NoMethodError) { @klass.new.setting }
+    assert_raise(NoMethodError) { @klass.new.setting? }
+  end
+end