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.

1 files changed, 36 insertions, 0 deletions

diff --git a/activesupport/lib/active_support/core_ext/class/attribute.rb b/activesupport/lib/active_support/core_ext/class/attribute.rb
new file mode 100644
index 0000000000..d74219cb93
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/class/attribute.rb
@@ -0,0 +1,36 @@
+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