private void fireCellSelectionChangeNotification() {
		if ( _locked) {
			return;
		}
		Set<CellKey> changedKeys = new HashSet<CellKey>();
		for(CellKey oldKey : _oldCellKeys) {
			if(!isCellKeySelected(oldKey)) {
				changedKeys.add(oldKey);
			}
		}
		for(CellKey selectedKey : _selectedCellKeys) {
			if ( !_oldCellKeys.contains( selectedKey)) {
				changedKeys.add( selectedKey);
			}
		}
		if(changedKeys.size() != 0) {
			for(SelectionListener listener : _listeners) {
				listener.cellSelectionChanged(this, changedKeys);
			}
		}

		_oldCellKeys = new HashSet<CellKey>(_selectedCellKeys);
	}