Documentation and more tests for Lotus::Utils::ClassAttribute

lib/lotus/utils/class_attribute.rb

@@ -1,5 +1,7 @@
 module Lotus
   module Utils
+    # Inheritable class level variable accessors.
+    # (see Lotus::Utils::ClassAttribute::ClassMethods)
     module ClassAttribute
       def self.included(base)
         base.class_eval do
@@ -8,23 +10,76 @@ def self.included(base)
       end
 
       module ClassMethods
-        def class_attribute(*names)
+        # Public: Defines a class level accessor for the given attribute(s).
+        #
+        # A value set for a superclass is automatically available by their
+        # subclasses, unless a different value is explicitely set within the
+        # inheritance chain.
+        #
+        # @param attributes [Symbol] a single or multiple attribute name(s)
+        #
+        # Examples:
+        #
+        #   require 'lotus/utils/class_attribute'
+        #
+        #   class Vehicle
+        #     include Lotus::Utils::ClassAttribute
+        #     class_attribute :engines, :wheels
+        #
+        #     self.engines = 0
+        #     self.wheels  = 0
+        #   end
+        #
+        #   class Car < Vehicle
+        #     self.engines = 1
+        #     self.wheels  = 4
+        #   end
+        #
+        #   class Airplane < Vehicle
+        #     self.engines = 4
+        #     self.wheels  = 16
+        #   end
+        #
+        #   class SmallAirplane < Airplane
+        #     self.engines = 2
+        #     self.wheels  = 8
+        #   end
+        #
+        #   Vehicle.engines # => 0
+        #   Vehicle.wheels  # => 0
+        #
+        #   Car.engines # => 1
+        #   Car.wheels  # => 4
+        #
+        #   Airplane.engines # => 4
+        #   Airplane.wheels  # => 16
+        #
+        #   SmallAirplane.engines # => 2
+        #   SmallAirplane.wheels  # => 8
+        def class_attribute(*attributes)
           (class << self; self; end).class_eval do
-            attr_accessor *names
+            attr_accessor *attributes
           end
 
-          @class_attributes ||= Set.new
-          @class_attributes.merge(names)
+          class_attributes.merge(attributes)
         end
 
+        # Internal: Overrides `Class#inherited`.
         def inherited(subclass)
-          @class_attributes.each do |attr|
-            value = send(attr).dup rescue nil
+          class_attributes.each do |attr|
+            value = send(attr)
+            value = value.dup rescue value
             subclass.send("#{attr}=", value)
           end
 
           super
         end
+
+        private
+        # Internal: accessor for class attributes.
+        def class_attributes
+          @class_attributes ||= Set.new
+        end
       end
     end
   end

test/class_attribute_test.rb

@@ -11,7 +11,47 @@ class ClassAttributeTest
     end
 
     class SubclassAttributeTest < ClassAttributeTest
-      self.functions = [:x, :y]
+      class_attribute :subattribute
+      self.functions    = [:x, :y]
+      self.subattribute = 42
+    end
+
+    class SubSubclassAttributeTest < SubclassAttributeTest
+    end
+
+    class Vehicle
+      include Lotus::Utils::ClassAttribute
+      class_attribute :engines, :wheels
+
+      self.engines = 0
+      self.wheels  = 0
+    end
+
+    class Car < Vehicle
+      self.engines = 1
+      self.wheels  = 4
+    end
+
+    class Airplane < Vehicle
+      self.engines = 4
+      self.wheels  = 16
+    end
+
+    class SmallAirplane < Airplane
+      self.engines = 2
+      self.wheels  = 8
+    end
+  end
+
+  after do
+    [:ClassAttributeTest,
+     :SubclassAttributeTest,
+     :SubSubclassAttributeTest,
+     :Vehicle,
+     :Car,
+     :Airplane,
+     :SmallAirplane].each do |const|
+      Object.send :remove_const, const
     end
   end
 
@@ -32,4 +72,33 @@ class SubclassAttributeTest < ClassAttributeTest
     SubclassAttributeTest.values = [3,2]
     ClassAttributeTest.values.must_equal([1])
   end
+
+  # it 'if the subclass value changes it affects subclasses' do
+  #   values = [3,2]
+  #   SubclassAttributeTest.values = values
+  #   SubclassAttributeTest.values.must_equal(values)
+  #   SubSubclassAttributeTest.values.must_equal(values)
+  # end
+
+  it 'if the subclass defines an attribute it should not be available for the superclass' do
+    -> { ClassAttributeTest.subattribute }.must_raise(NoMethodError)
+  end
+
+  it 'if the subclass defines an attribute it should be available for its subclasses' do
+    SubSubclassAttributeTest.subattribute.must_equal 42
+  end
+
+  it 'preserves values within the inheritance chain' do
+    Vehicle.engines.must_equal 0
+    Vehicle.wheels.must_equal  0
+
+    Car.engines.must_equal 1
+    Car.wheels.must_equal  4
+
+    Airplane.engines.must_equal 4
+    Airplane.wheels.must_equal  16
+
+    SmallAirplane.engines.must_equal 2
+    SmallAirplane.wheels.must_equal  8
+  end
 end