Subscribe for watching mutations on a stored thing. If this function is called on a thing which is not stored, and therefore has no #ID, then a value_err() is raised.
This method returns
nil but always triggers an init event, even when an earlier call to this function has been made.
This function only works with socket connections. When using this function with an HTTP API request, it has no effect.
This function does not generate an event.
This code shows how to use
Return value in JSON format: