How to find listeners attached to an object

Hi,

I thought this would be a trivial task, but I cannot find the way to do this. Given an object in your workspace, is there an easy way to get handles to all the listeners that have been defined on this object?

cheers, Daniel

[Joakim Magnusson]

Why do you want to get handles to all listeners for an object?

[Adam Danz]

Related: how to determine if event has a listener

https://www.mathworks.com/help/matlab/matlab_oop/are-there-listeners-for-this-event.html

[Guillaume]

This reminds me that the other day I was playing around with meta.class and its two events InstanceCreated and InstanceDestroyed and attaching listeners without keeping the listener handles around. I thought I'd just delete the meta.class object when I'm done and that'll destroy the listeners.

Oops! meta.class objects are singleton (undocumented!) and kept around even if you delete your instance (undocumented!) so here I was with lots of listeners firing, no way to list the listeners (as per this thread) and no way to destroy them either,

clear classes is the only way to get rid of meta.class objects once they've been created.

[Adam]

From a code design perspective the original question does seem to head down a route that is not generally advisable. An object that allows listeners to be attached should not ever need to know what those listeners are. The example Adam Danz gave is understandable, to know if there are listeners at all, but an object knowing actually what is listening to it seems like a case of knowing far too much.

It would be akin to a radio host being able to find out the names of everyone listening to his show!

That's not to say I haven't also fallen into numerous traps with listeners firing and giving me pages of errors for objects that no longer exist, but whose listener still exists, etc, etc, but it should never be up to the object being listened to to directly interact with those doing the listening. Otherwise some more concrete form of two-way object interaction should be used I would think, as listeners are inherently a one-way communication.

[Adam Danz]

I wish there were a way to h=findall('listeners') or h=findall(obj,'listeners') or something like that because some listeners are created by functions that do not return object handles and I don't know of any other way to get the listener handles (although I've only searched briefly).

[addendum]

To add context to my previous comment that brought this 5+ year thread back to life, I am (was; partially gave up) trying to troubleshoot this problem:

https://www.mathworks.com/matlabcentral/answers/489831-yyaxis-not-working-in-app-designer

Answer 1

[Harry Dymond]

Found the answer on StackOverflow: any object with listeners attached will have an undocumented property .AutoListeners__, which is a cell array of listener handles. The cell array does not appear to get "pruned" if listeners are deleted, so .AutoListeners__{x} could contain a handle to a deleted listener. If the object has never had listeners created for any of its properties, .AutoListeners__ will not exist.

[Adam Danz]

Good find, Harry Dymond!

[DNF]

As far as I can tell this does not return all listeners. Listeners to some undocumented events do not show up. If you attach a listener to a window for its WindowMouseMotion event, then you cannot find that in the .AutoListeners__ array.

Answer 2

[Joakim Magnusson]

Can't you save your listeners when created like this? :

handles.listener1 = addlistener(...);
handles.listener2 = addlistener(...);
etc...

Or that wont work for you?

[Daniel]

Thanks, but unfortunately no.

This is exactly what I wanted to avoid. I don't want to pass the variable 'handles' across my functions and methods, as this would force me to a severe code rewriting and restructuring.

I have the impression that listing all objects listening to something should be easy (that is what Matlab itself does when broadcasting a "notify"), but looks like the program does not offer this functionality to users.

[Joakim Magnusson]

No i couldn't find anything like that either.