Added new API to xmodule(descriptor). get_children_locations and has_dynamic_children.

common/lib/xmodule/xmodule/abtest_module.py

@@ -47,11 +47,9 @@ class ABTestModule(XModule):
 
     def get_shared_state(self):
         return json.dumps({'group': self.group})
-
-    def displayable_items(self):
-        child_locations = self.definition['data']['group_content'][self.group]
-        children = [self.system.get_module(loc) for loc in child_locations]
-        return [c for c in children if c is not None]
+    
+    def get_children_locations(self):
+        return self.definition['data']['group_content'][self.group]
 
 
 # TODO (cpennington): Use Groups should be a first class object, rather than being
@@ -158,3 +156,7 @@ class ABTestDescriptor(RawDescriptor, XmlDescriptor):
                 group_elem.append(etree.fromstring(child.export_to_xml(resource_fs)))
 
         return xml_object
+    
+    
+    def has_dynamic_children(self):
+        return True

common/lib/xmodule/xmodule/x_module.py

@@ -219,13 +219,28 @@ class XModule(HTMLSnippet):
         Return module instances for all the children of this module.
         '''
         if self._loaded_children is None:
-            child_locations = self.definition.get('children', [])
+            child_locations = self.get_children_locations()
             children = [self.system.get_module(loc) for loc in child_locations]
             # get_module returns None if the current user doesn't have access
             # to the location.
             self._loaded_children = [c for c in children if c is not None]
 
         return self._loaded_children
+    
+    def get_children_locations(self):
+        '''
+        Returns the locations of each of child modules.
+        
+        Overriding this changes the behavior of get_children and
+        anything that uses get_children, such as get_display_items.
+        
+        This method will not instantiate the modules of the children
+        unless absolutely necessary, so it is cheaper to call than get_children
+        
+        These children will be the same children returned by the
+        descriptor unless descriptor.has_dynamic_children() is true.
+        '''
+        return self.definition.get('children', [])
 
     def get_display_items(self):
         '''
@@ -489,6 +504,18 @@ class XModuleDescriptor(Plugin, HTMLSnippet):
             self,
             metadata=self.metadata
         )
+    
+    
+    def has_dynamic_children(self):
+        """
+        Returns True if this descriptor has dynamic children for a given
+        student when the module is created.
+        
+        Returns False if the children of this descriptor are the same
+        children that the module will return for any student. 
+        """
+        return False
+        
 
     # ================================= JSON PARSING ===========================
     @staticmethod