public ulong Add(EntityId entityId, Action <T> callback)
{
if (!indexedCallbacks.TryGetValue(entityId, out var callbacks))
{
callbacks = new CallbackCollection <T>();
indexedCallbacks.Add(entityId, callbacks);
}
lastCallbackKey++;
callbackKeyToEntityId.Add(lastCallbackKey, entityId);
callbacks.Add(lastCallbackKey, callback);
return(lastCallbackKey);
}