Implementing visitor pattern on aggregate objects

[yazari]

I'm struggling with applying the visitor pattern on some objects that have scalar members and at the same time aggregate members (collections).These are the objects that I have:\[code\]Artist - id - name - .. more scalar values .. - worksOfArt <-- this is a collection as WorkOfArt instancesWorkOfArt - id - name - .. more scalar values .. - artists <-- this is a collection of Artist instances\[/code\]As you can see, the structure will also be recursive, but that's of later concern to me. ;-)My question is: what is the best way to implement the visitor pattern, that allows me visit the objects and also only their visitable children (the collections).I thought creating an interface like this:\[code\]VisitableAggregateInterface{ public function getVisitableChildren(); // this would return only visitable children}\[/code\]And then let both Artist and WorkOfArt extend an abstract class like this:\[code\]VisitableAggregateAbstract implements VisitableAggregateInterface{ public function accept( Visitor $visitor ) { $visitor->visit( $this ); foreach( $this->getVisitableChildren() as $visitableChild ) { $visitableChild->accept( $visitor ); } } /* VisitableAggregateInterface::getVisitableChildren() will be implemented by Artist and WorkOfArt and will only return visitable children (like collections), and not scalar values. */}\[/code\]The goal is ultimately to end up with a concrete Visitor that will write out an XML file similar to this:\[code\]<?xml version="1.0" encoding="utf-8" standalone="yes"?><artgallery> <artists> <artist> <id>1</id> <name></name> <worksOfArt> <workOfArt refid="11"/> <workOfArt refid="12"/> </worksOfArt> <artist> <artists> <worksOfArt> <workOfArt> <id>11</id> <artists> <artist refid="1"/> </artists> <name></name> <info><![CDATA[some info]]></info> </workOfArt> <workOfArt> <id>12</id> <artists> <artist refid="1"/> </artists> <name></name> <info><![CDATA[some info]]></info> </workOfArt> </worksOfArt></artgallery>\[/code\]Please advice: Am I going in the right direction here? Because the \[code\]getVisitableChildren()\[/code\] interface feels a bit quirky to me. Should I perhaps maybe even ditch the visitor pattern altogether and take a different approach?Thanks.