Hi,
After reading up on the HierarchicalVisitor pattern on C2
(http://c2.com/cgi/wiki?HierarchicalVisitorPattern) I realized that this
is what I had been looking for with the DescriptorVisitor that is in the
Qi4j SPI.
After some thinking I then implemented it as a generic interface like so:
public interface HierarchicalVisitor<NODE, LEAF, ThrowableType extends
Throwable>
extends Visitor<LEAF, ThrowableType>
{
boolean visitEnter( NODE visited )
throws ThrowableType;
boolean visitLeave( NODE visited )
throws ThrowableType;
}
---
and added it to all model classes instead of DescriptorVisitor. By not
using explicit visit methods in the visitor it becomes easier to add
more elements that are visited, but which are not necessarily exposed
through an SPI interface. The visitor has to do "if(instanceof)" checks
instead, and act on what is being visited.
With this in place I then realized that the dependency binding algorithm
could be rewritten as a visitor. So I have now done that and have
removed a lot of the unnecessary bind() methods that simply traversed
the model.
So, for the next release the ApplicationSPI.visitDescriptor() method is
gone, and instead there's an accept(HierarchicalVisitor) method in
ApplicationModelSPI instead. All usages of DescriptorVisitor have been
replaced with the above.
/Rickard
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