How to get the changes to an object in PreUpdate?

[Andrew Willerding]

I can't seem to find documentation, or better yet some examples for 4.0 
or 4.1 that explains how to check for either a single property change or 
how to get a list of the property changes in an onPreUpdate callback. 
Unless I'm missing something obvious (entirely likely ;-) ) the 
Lifecycle events documentation doesn't seem to mention any way to do 
this.  Is this something that I need to manually track in the application?

Also, what should I do if I want to reject the changes inside of an 
onPreUpdate callback?  Do I need to do an explicit rollback or simply 
throw a CayenneRuntimeException of some sort?  I'm guessing it's an 
explicit rollback.

Thanks,

Andrew