Registers a live observer with this event.
Registers a live observer with this event. The method is called with the
observing function which receives the event's update messages, and the
method generates an opaque Observer instance, which may be used to
remove the observer eventually (through the observer's remove method),
or to add the observer to other events of the same type (using the
observer's add method).
Note that the caller should not call add
on the resulting observer to register this event, as this is already
done as part of the call to react.
Like react, but passing in a transaction as first function argument.