[rules-users] property change listener for collections

[Mark Proctor]

the old/new value is part of the PropertyChangeSupport spec, however 
drools actually ignores all that, all it cares about is the source object:
    public void propertyChange(final PropertyChangeEvent event) {
        final Object object = event.getSource();

        try {
            FactHandle handle = getFactHandle( object );
            if ( handle == null ) {
                throw new FactException( "Update error: handle not found 
for object: " + object + ". Is it in the working memory?" );
            }
            update( handle,
                    object );
        } catch ( final FactException e ) {
            throw new RuntimeDroolsException( e.getMessage() );
        }
    }

However do be aware that if you are asserting collections into drools 
4.0.x you have some serious mutability issues, with regards to shadow 
facts. Search for previous posts on this in core and dev and edson has 
recently written a detailed blog on this at http://blog.athico.com.

Andreas Langegger wrote:
> hello,
>
> when I have a bean and I want to use a property change listener for 
> collection members, how can I tell the listener the old state of the 
> collection?
>
> This is the code for simple fields from the docs:
>
> public void setState(final String newState) {
>     String oldState = this.state;
>     this.state = newState;
>     this.changes.firePropertyChange( "state",
>                                       oldState,
>                                       newState );
> }
>
> and now I want to hook in the firePropertyChange for the following 
> method which adds some elements to a set:
>
> protected Set<Var> knownVariables = new HashSet<Var>();
> public void addKnownVariable(String var) {
>     this.knownVariables.add(var);   
>     changes.firePropertyChange("knownVariables", OLDSTATE??!, 
> knownVariables);
> }
>
> Will I have to copy the whole collection, switch the complete field 
> collection object that I can supply the "old state"?
>
> thanks,
> Andy
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