Digester and attribute matching?

["Chen, Tim" <Ti...@NielsenMedia.com>]

I saw that in the list archives it was not supported but
1) is it planned to be able to support something like 
digester.addCallMethod("foo[@bar=\"blah"\]", "foo"); ?
2) is there a way around it currently?
I see that all the rules have a way to see the attributes before processing
but I don't know how to stop that rule from processing.

-Tim

-------------------------------------
Gin-Ting Chen
Gin.Chen@NielsenMedia.com
813-366-4291

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Re: Digester and attribute matching?

[robert burrell donkin <ro...@blueyonder.co.uk>]

On 8 Apr 2004, at 17:36, Chen, Tim wrote:

> I saw that in the list archives it was not supported but
> 1) is it planned to be able to support something like
> digester.addCallMethod("foo[@bar=\"blah"\]", "foo"); ?

(simon's covered this pretty well. a long time ago scott knew most 
about this so if he's still around, now would be a good time for him to 
jump in ;)

> 2) is there a way around it currently?
> I see that all the rules have a way to see the attributes before 
> processing
> but I don't know how to stop that rule from processing.

if stopping a rule firing on the basic of attributes, that can be done 
by a wrapper Rule implementation. this should test the attributes and 
only execute the rule if they match your expectations. the only wrinkle 
is that you'll probably need to push the implementation onto the stack. 
here's the type of thing i mean (not tested and with badly named 
methods):

public interface AttributesApprover {
	public boolean approved(Attributes attributes);
}

public class doNothingRule extends Rule {}

public class AttributesFilter extends Rule {
	private ArrayStack ruleStack = new ArrayStack();
	private AttributesApprover approver;
	private Rule rule;
	public void begin(String namespace, String name, Attributes 
attributes) {
		if (approver.approve(attributes)) {
			ruleStack.push(rule);
		} else {
			ruleStack.push(new DoNothingRule());
		}
		
		((Rule)ruleStack.peek()).begin(namespace, name, attributes);
	}

	public void body(String namespace, String name, String text) 
{		((Rule)ruleStack.peek()).body(namespace, name, text);
	}

	public void end(String namespace, String name,) 
{		((Rule)ruleStack.pop()).end(namespace, name,);
	}
}

- robert


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Re: Digester and attribute matching?

[Simon Kitching <si...@ecnetwork.co.nz>]

On Fri, 2004-04-09 at 04:36, Chen, Tim wrote:
> I saw that in the list archives it was not supported but
> 1) is it planned to be able to support something like 
> digester.addCallMethod("foo[@bar=\"blah"\]", "foo"); ?
> 2) is there a way around it currently?
> I see that all the rules have a way to see the attributes before processing
> but I don't know how to stop that rule from processing.

No, there is no support for this kind of conditional matching in
Digester.

I can see that it would be nice for digester to have it, but I wouldn't
have a clue how it could be implemented (efficiently and elegantly). It
isn't planned any time in the near future, because as far as I know,
none of the current Digester developers have a need for this feature.
I've got it as a bullet-point for consideration for Digester-2.0. but
that could be quite a while away (if ever).

But that's where the power of "open source" comes in - if you want the
feature, please feel free to implement it. If you can come up with some
proof-of-concept code that shows how this could be done in Digester,
then I (and I'm sure others) would be very interested. I am happy to
(try to) answer any questions you have if you do decide to tackle this.
Note, however, that spending 5 minutes to write some pseudo-code
probably won't motivate any of the existing regular developers to then
implement it for you :-).

I got involved with Digester because I wanted an extra feature,
implemented it and submitted it as a patch. So from experience I can say
that contributions *are* welcome.

If all you want is to prevent CallMethodRule instances from firing if
certain attributes have certain values, then you *may* get away with
subclassing CallMethodRule, and storing a boolean flag on the rule
instance. Set the flag to true/false in begin(), according to the
attribute values, and then  in body/end methods, just test the flag and
skip the call if the flag is set. Note that this is not a proper
general-purpose solution, as it can fail horribly if you use that
CallMethodRule with patterns containing wildcards. But if that is not
the case for your application, then you can ignore this issue.

Regards,

Simon


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