Question

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:

Artist
 - id
 - name
 - .. more scalar values ..
 - worksOfArt <-- this is a collection as WorkOfArt instances


WorkOfArt
 - id
 - name
 - .. more scalar values ..
 - artists <-- this is a collection of Artist instances

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:

VisitableAggregateInterface
{
    public function getVisitableChildren(); // this would return only visitable children
}

And then let both Artist and WorkOfArt extend an abstract class like this:

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.
    */
}

The goal is ultimately to end up with a concrete Visitor that will write out an XML file similar to this:

<?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>

Please advice: Am I going in the right direction here? Because the getVisitableChildren() interface feels a bit quirky to me. Should I perhaps maybe even ditch the visitor pattern altogether and take a different approach?

Thanks.

Answer 1

You don’t actually state what the relation between artist and artwork is. If, as I suspect, both mean the Artist made the WorkOfArt, then you don’t have a recursive data structure, so you don’t need a visitor pattern.

I would simply “brute force” it. Something like this (untested)

echo "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\"?>
<artgallery>
    <artists>\n";
Foreach (Artists as Artist) {
   echo "        <artist>
            <id>1</id>
            <name></name>
            <worksOfArt>\n";
   ForEach (Artist.worksOfArt as Work) {
      $refid = ????;
      echo "<workOfArt refid=\"$refid"/>\n";
   }
   echo "        </worksOfArt>
        </artist>\n";
}
echo " </artists>
    <worksOfArt>";
Foreach (WorkOfArt as work) {
   ForEach (work.Artists as Artist) {

   }
}

Then just put the rest of echo statements into the above

BTW – you have a typo in the question. The close of and need /s added.