/* Callbacks that can be registered. */
      template <typename T> CallbackKey
OnSendComponentUpdate(const SendComponentCallback<T> &, bool);

template <typename T> CallbackKey
OnSendCommandRequest(const SendCommandRequestCallback<T> &, bool);

template <typename T>
CallbackKey
OnSendCommandResponse(const SendCommandResponseCallback<T> &, bool);

CallbackKey
OnSendCreateEntityRequest(const SendCreateEntityRequestCallback &, bool);

CallbackKey
OnSendEntityQueryRequest(const SendEntityQueryRequestCallback &, bool);

CallbackKey
OnSendReserveEntityIdRequest(const SendReserveEntityIdRequestCallback &, bool);

CallbackKey
OnSendReserveEntityIdsRequest(const SendReserveEntityIdsRequestCallback &, bool);

CallbackKey
OnSendComponentInterestRequest(const SendComponentInterestRequestCallback &, bool);

/* Methods to invoke. */
template <typename T> void SendComponentUpdate(...);
template <typename T> void SendCommandRequest(...);
template <typename T> void SendCommandResponse(...);
void SendCreateEntityRequest(...);
void SendEntityQueryRequest(...);
void SendReserveEntityIdRequest(...);
void SendReserveEntityIdsRequest(...);
void SendComponentInterestRequest(...);

/* Uninstall a callback. */
void Remove(CallbackKey);