CPKeyValueBinding -reverseSetValueFor: -> extract a method for getting the source value for a given binding. the goal is to facilitate subclassing.

AppKit/CPKeyValueBinding.j

@@ -190,7 +190,7 @@ var CPBindingOperationAnd = 0,
     var destination = [_info objectForKey:CPObservedObjectKey],
         keyPath = [_info objectForKey:CPObservedKeyPathKey],
         options = [_info objectForKey:CPOptionsKey],
-        newValue = [_source valueForKeyPath:aBinding];
+        newValue = [self valueForBinding:aBinding];
 
     newValue = [self reverseTransformValue:newValue withOptions:options];
 
@@ -199,6 +199,11 @@ var CPBindingOperationAnd = 0,
     [self unsuppressSpecificNotificationFromObject:destination keyPath:keyPath];
 }
 
+- (id)valueForBinding:(CPString)aBinding
+{
+    return [_source valueForKeyPath:aBinding];
+}
+
 - (void)observeValueForKeyPath:(CPString)aKeyPath ofObject:(id)anObject change:(CPDictionary)changes context:(id)context
 {
     if (!changes)