// requires: non-null callback method or listener class
// effects: adds the callback to the list of layer cbs
public void AddLayerChangedCallback(LayerChangedCallback callback)
{
_layerChangeCbs.Add(callback);
}
// requires: non-null callback method or listener class
// effects: removes the callback from the list of layer cbs; noop if not found
public void RemoveLayerChangedCallback(LayerChangedCallback callback)
{
_layerChangeCbs.Remove(callback);
}
