Add new superclass_delegating_accessors. Similar to class inheritable attributes but with subtly different semantics. [Koz, tarmo]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@8056 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

4 files changed, 148 insertions, 0 deletions

diff --git a/activesupport/CHANGELOG b/activesupport/CHANGELOG
index 80f6c76401..4b22b6e7b2 100644
--- a/activesupport/CHANGELOG
+++ b/activesupport/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Add new superclass_delegating_accessors.  Similar to class inheritable attributes but with subtly different semantics. [Koz, tarmo]
+
 * Change JSON to encode %w(< > &) as 4 digit hex codes to be in compliance with the JSON spec.  Closes #9975 [josh, chuyeow, tpope]
 
 * Fix JSON encoding/decoding bugs dealing with /'s.  Closes #9990 [Rick, theamazingrando]
diff --git a/activesupport/lib/active_support/core_ext/class.rb b/activesupport/lib/active_support/core_ext/class.rb
index 83eb8135f4..44ad6c8c08 100644
--- a/activesupport/lib/active_support/core_ext/class.rb
+++ b/activesupport/lib/active_support/core_ext/class.rb
@@ -1,3 +1,4 @@
 require 'active_support/core_ext/class/attribute_accessors'
 require 'active_support/core_ext/class/inheritable_attributes'
 require 'active_support/core_ext/class/removal'
+require 'active_support/core_ext/class/delegating_attributes'
diff --git a/activesupport/lib/active_support/core_ext/class/delegating_attributes.rb b/activesupport/lib/active_support/core_ext/class/delegating_attributes.rb
new file mode 100644
index 0000000000..f5f0ef8779
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/class/delegating_attributes.rb
@@ -0,0 +1,40 @@
+# These class attributes behave something like the class
+# inheritable accessors.  But instead of copying the hash over at
+# the time the subclass is first defined,  the accessors simply
+# delegate to their superclass unless they have been given a 
+# specific value.  This stops the strange situation where values 
+# set after class definition don't get applied to subclasses.
+class Class
+  def superclass_delegating_reader(*names)
+    class_name_to_stop_searching_on = self.superclass.name.blank? ? "Object" : self.superclass.name
+    names.each do |name|
+      class_eval <<-EOS
+      def self.#{name}
+        if defined?(@#{name})
+          @#{name}
+        elsif superclass < #{class_name_to_stop_searching_on} && superclass.respond_to?(:#{name})
+          superclass.#{name}
+        end
+      end
+      def #{name}
+        self.class.#{name}
+      end
+      EOS
+    end
+  end
+
+  def superclass_delegating_writer(*names)
+    names.each do |name|
+      class_eval <<-EOS
+        def self.#{name}=(value)
+          @#{name} = value
+        end
+      EOS
+    end
+  end
+
+  def superclass_delegating_accessor(*names)
+    superclass_delegating_reader(*names)
+    superclass_delegating_writer(*names)
+  end
+end
\ No newline at end of file
diff --git a/activesupport/test/core_ext/class/delegating_attributes_test.rb b/activesupport/test/core_ext/class/delegating_attributes_test.rb
new file mode 100644
index 0000000000..59c0a6f63c
--- /dev/null
+++ b/activesupport/test/core_ext/class/delegating_attributes_test.rb
@@ -0,0 +1,105 @@
+require File.dirname(__FILE__) + '/../../abstract_unit'
+
+module DelegatingFixtures
+  class Parent
+  end
+
+  class Child < Parent
+    superclass_delegating_accessor :some_attribute
+  end
+
+  class Mokopuna < Child
+  end
+end
+
+class DelegatingAttributesTest < Test::Unit::TestCase
+  include DelegatingFixtures
+  attr_reader :single_class
+
+  def setup
+    @single_class = Class.new(Object)
+  end
+
+  def test_simple_reader_declaration
+    single_class.superclass_delegating_reader   :only_reader
+    # The class and instance should have an accessor, but there
+    # should be no mutator
+    assert single_class.respond_to?(:only_reader)
+    assert single_class.public_instance_methods.include?("only_reader")
+    assert !single_class.respond_to?(:only_reader=)
+  end
+
+  def test_simple_writer_declaration
+    single_class.superclass_delegating_writer   :only_writer
+    # The class should have a mutator, the instances shouldn't
+    # neither should have an accessor
+    assert single_class.respond_to?(:only_writer=)
+    assert !single_class.public_instance_methods.include?("only_writer=")
+    assert !single_class.public_instance_methods.include?("only_writer")
+    assert !single_class.respond_to?(:only_writer)
+  end
+
+  def test_simple_accessor_declaration
+    single_class.superclass_delegating_accessor :both
+    # Class should have accessor and mutator
+    # the instance should have an accessor only
+    assert single_class.respond_to?(:both)
+    assert single_class.respond_to?(:both=)
+    assert single_class.public_instance_methods.include?("both")
+    assert !single_class.public_instance_methods.include?("both=")
+  end
+
+  def test_working_with_simple_attributes
+    single_class.superclass_delegating_accessor :both
+    single_class.both= "HMMM"
+    assert_equal "HMMM", single_class.both
+    assert_equal "HMMM", single_class.new.both
+  end
+
+  def test_working_with_accessors
+    single_class.superclass_delegating_reader   :only_reader
+    single_class.instance_variable_set("@only_reader", "reading only")
+    assert_equal "reading only", single_class.only_reader
+    assert_equal "reading only", single_class.new.only_reader
+  end
+
+  def test_working_with_simple_mutators
+    single_class.superclass_delegating_writer   :only_writer
+    single_class.only_writer="written"
+    assert_equal "written", single_class.instance_variable_get("@only_writer")
+  end
+
+  def test_child_class_delegates_to_parent_but_can_be_overridden
+    parent = Class.new
+    parent.superclass_delegating_accessor :both
+    child = Class.new(parent)
+    parent.both= "1"
+    assert_equal "1", child.both
+
+    child.both="2"
+    assert_equal "1", parent.both
+    assert_equal "2", child.both
+
+    parent.both="3"
+    assert_equal "3", parent.both
+    assert_equal "2", child.both
+  end
+
+  def test_delegation_stops_at_the_right_level
+    assert_nil Mokopuna.some_attribute
+    assert_nil Child.some_attribute
+    Child.some_attribute="1"
+    assert_equal "1", Mokopuna.some_attribute
+  ensure
+    Child.some_attribute=nil
+  end
+  
+  def test_delegation_stops_for_nil
+    Mokopuna.some_attribute = nil
+    Child.some_attribute="1"
+    
+    assert_equal "1", Child.some_attribute
+    assert_nil Mokopuna.some_attribute
+  end
+
+end
\ No newline at end of file