That depends on what version of Pharo you use.  
In Pharo 1.4 you can use SystemChangeNotifier to do something like this:

SystemChangeNotifier uniqueInstance
notify: toSomeObject
ofSystemChangesOfItem: #class
change: #Added
using: #someMethod:.


then if a new class is added to the system, SystemChangeNotifier will call 
#someMethod: on toSomeObject, passing an Event to #someMethod:. You can browse 
the code on SystemChangeNotifier to see all the notification you can use, and 
the package "System-Change Notification" to see the events you can receive.

In Pharo 2.0 you can do the same thing using SystemAnnouncer:

SystemAnnouncer uniqueInstance
on: ClassAdded
send: #someMethod:
to: toSomeObject.


You can see the different announcements you can use on browsing the package 
"System-announcements". In this example #someMethod: will receive an 
announcement.
--  
Daniel Galdames


El domingo, 14 de octubre de 2012 a las 20:14, Miguel Enrique Campusano Araya 
escribió:

> I want to do some logic when there is a change on the system, like when a new 
> class is loaded or when a method change. I don't want to put this logic 
> explicit on the methods of the system who do that. I would like to know if i 
> could subscribe to those special events (method change, new method, changes 
> on a class and so on) so I can do what I want outside those methods.
> I heard about the Announcements but I don't know if Announcements could help 
> with this problem.
>  
> Can anyone give me some highlights to do those things?
>  
> Thanks!  
> (I'm really trying with my english, hope it's understandable)  
>  
>  
> --  
> Miguel Enrique Campusano Araya
> Estudiante de Ingeniería Civil en Computación
> Universidad de Chile
>  

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