Routing

tip

Remember the number 1 rule: 1 pattern of an event object = 1 handler

To assign more, you need to clone (EventClone) the original object or use other pattern or topic.

Since the notebook is usually a playground, you need to have an opportunity to undo actions, when you reevaluate the cell. Therefore there is concept of explicit routing of one event to some other handlers.

For example you have a generator of events (does not necessary has to be a button)

generator[uid_String] := With[{btn = InputButton["Click"]},
	(* we use input button only for demonstration purposes *)
	(* one can fire any EventObject manually using EventFire[] *)

	EventHandler[btn, Function[Null,
		EventFire[uid, "some data"];
	]];
	
	btn
]

here we used the representation of events object as a string uid (it is also valid) and fire it using a button (which also returns EventObject).

Now we need an arbitrary handler

handler[uid_String, func_:Print] := With[{ev = EventClone[uid]},
	(* clean up, if the cell has been destroyed *)
	EventHandler[ResultCell[], {"Destroy" -> Function[Null, Delete[ev]]}];

	(* assign Print for instance *)
	EventHandler[ev, func];
]

info

You do not need to clone EvaluationCell or ResultCell to assign many handlers to it. it is cloned automatically once appeared in EventHandler

The cool thing, that we can have many of those handlers attached to the same event, since it clones it every-time and removes handler function, when you reevaluate the cell. Let us see it on the following example

generator["random-word-you-like"]
handler["random-word-you-like"]

and in some other cell, if you like you can add other handler to the same event

handler["random-word-you-like", FrontSubmit[Alert[#]]&]

By pressing the button, it will call the whole list of attached handlers.

This approach comes very handy, when you are constructing complex animation on Slides