Add before_remove_const callback to ActiveSupport::Dependencies.remove_unloadable_constants!

Signed-off-by: NXavier Noria <fxn@hashref.com>

activesupport/CHANGELOG

 *Rails 3.1.0 (unreleased)*
 
-* No changes
+* Added before_remove_const callback to ActiveSupport::Dependencies.remove_unloadable_constants! [Andrew White]
 
 
 *Rails 3.0.0 (August 29, 2010)*

activesupport/lib/active_support/dependencies.rb

@@ -511,7 +511,12 @@ def load_missing_constant(from_mod, const_name)
     end
 
     # Remove the constants that have been autoloaded, and those that have been
-    # marked for unloading.
+    # marked for unloading. Before each constant is removed a callback is sent
+    # to its class/module if it implements +before_remove_const+.
+    #
+    # The callback implementation should be restricted to cleaning up caches, etc.
+    # as the enviroment will be in an inconsistent state, e.g. other constants
+    # may have already been unloaded and not accessible.
     def remove_unloadable_constants!
       autoloaded_constants.each { |const| remove_constant const }
       autoloaded_constants.clear
@@ -636,6 +641,7 @@ def remove_constant(const) #:nodoc:
       parent = Inflector.constantize(names * '::')
 
       log "removing constant #{const}"
+      constantize(const).before_remove_const if constantize(const).respond_to?(:before_remove_const)
       parent.instance_eval { remove_const to_remove }
 
       return true

activesupport/test/dependencies_test.rb

@@ -574,6 +574,17 @@ def test_unloadable_should_return_change_flag
     end
   end
 
+  def test_unloadable_constants_should_receive_callback
+    Object.const_set :C, Class.new
+    C.unloadable
+    C.expects(:before_remove_const).once
+    assert C.respond_to?(:before_remove_const)
+    ActiveSupport::Dependencies.clear
+    assert !defined?(C)
+  ensure
+    Object.class_eval { remove_const :C } if defined?(C)
+  end
+
   def test_new_contants_in_without_constants
     assert_equal [], (ActiveSupport::Dependencies.new_constants_in(Object) { })
     assert ActiveSupport::Dependencies.constant_watch_stack.all? {|k,v| v.empty? }