abstract class DataChangeListenersMixin<T> {

 void _markChanged(T key, changeSet);
 void _notify({author});
 /**
  * Internal Set of data objects removed from Collection that still have DataListener listening.
  */
 Set<T> _removedObjects = new Set();
 /**
  * Internal set of listeners for change events on individual data objects.
  */
 final Map<dynamic, StreamSubscription> _dataListeners =
     new Map<dynamic, StreamSubscription>();

 /**
  * Removes listeners to all objects which have been removed and stacked in [_removedObjects]
  */
 void _onBeforeNotify() {
   // if this object was removed and then re-added in this event loop, don't
   // destroy onChange listener to it.
   for(T key in _removedObjects.toList()) {
     _removeOnDataChangeListener(key);
   }
   _removedObjects.clear();
 }

 /**
  * Starts listening to changes on [dataObj].
  */
 void _addOnDataChangeListener(T key, DataReference dataObj) {
   if (_dataListeners.containsKey(dataObj)) return;

   _dataListeners[key] = dataObj.onChangeSync.listen((changeEvent) {
     _markChanged(key, changeEvent['change']);
     _notify(author: changeEvent['author']);
   });
 }

 /**
  * Stops listening to changes on [dataObj]
  * Second possibility is to add to [_removedObjects] and call [_onBeforeNotify]
  */
 void _removeAllOnDataChangeListeners() {
   for(T key in _removedObjects.toList()) {
     _removeOnDataChangeListener(key);
   }
 }

 void _removeOnDataChangeListener(T key) {
   if (_dataListeners.containsKey(key)) {
     _dataListeners[key].cancel();
     _dataListeners.remove(key);
   }
 }
}