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Re: DOM Xpath comments

From: Jonas Sicking <sicking@bigfoot.com>
Date: Fri, 2 Nov 2001 01:48:38 +0100
Message-ID: <013001c16338$1cb18940$23ef42d5@kermit>
To: <www-dom@w3.org>
Arnold, Curt wrote:
> contextNode parameters:
>
> I would have expected EntityReference to have appeared in the list of
> acceptible context node types since it plays an equivalent role to
> Text or CDataSection.  Maybe it would be clearer if the
> prohibited node types were enumerated and why they were inappropriate.

Actually EntityReferences doesn't exist in the XPath data model, so I think
it's puposly left out. However the entityreferences children are inlined
(and merged with andjecent textnodes) so _they_ still exist in the tree. The
EntityReference it self doesn't exist though.

> XPathResult reuse:
>
> I believe that the attempt to enable XPathResult reuse does
> not provide a significant benefit and has several significant negative
> consequences.
>
> Having XPathResult implementations being mutable (which would
> be required for reuse) means that you cannot trust any XPathResult
> that has been publically exposed or passed as an argument to remain
> constant.  This would require defensive copying of XPathResults in
> many uses offsetting any benefit from minimizing XPathResult creation.
>
> Having XPathResult reusable requires (or at least strongly encourages)
> that there only be one implementation class for the XPathResult
> interface in an implementation.  Without the requirement for
> reuse, then an implementation could create distinct implementations for
> XPathResult corresponding to different result types.

This is exactly how TransforMiiX is designed (the XSLT/XPath engine used in
mozilla). We have one common interface similar to XPathResult and separate
implementations for each result type.

What we have had to do (Peter van der Beken has made a preliminary
implementation of DOM XPath) is to have a wrapper that holds an internal
object which can be of any result type. However this defeats the entire
purpose of reuse it's only the wrapper that gets reused, the internal object
is dropped and recreated on every evaulation.

One way to help this situation without dropping support for reuseing of
XPathResult objects would be to allow the implementations to return a new
XPathResult if it is not able to reuse the supplied XPathResult. OTOH this
might create interopability problems.

> Two step query:
>
> That the result set should be ordered or that a snapshot should be taken
is not communicated in the evaluate call, but on a subsequent call on the
XPathResult.  This prevents either requires the
> implementation to defer completely executing the query until the second
call (with the possibly of an interim document mutation) or forcing the
implementation to do an ordered snapshot at the initial
> evaluation call.
>
> One possible resolution would be to remove the NODE_SET_TYPE and replace
it with the
> four possible permutations: ORDERED_NODE_SNAPSHOT_TYPE,
> UNORDERED_NODE_SNAPSHOT_TYPE, ORDERED_NODE_ITERATOR_TYPE,
> UNORDERED_NODE_ITERATOR_TYPE.  That would allow the implementation to
> optionally schedule the execution of the query at the time of the evaluate
call.

I think that this sounds like a really good idea. We are just about to make
our XPath engine to produce sorted nodeset instead of unsorted ones, since
much time can be saved if the nodeset is sorted during evaluation rather
then after. This save would be totally lost if we are not told if the
nodeset is to be sorted until after evaluation.

> One possibility (but not a requirement) after doing this is that the
methods from
>  XPathSetSnapshot and XPathSetIterator could be merged into XPathResult.
> A resulting XPathResult interface might look like:
>
> public interface XPathResult
> {

[snip]

> }

While I don't like the getSetIterator or getSetSnapshot functions either, I
don't think putting it all in the same interface is ideal either. I propose
that XPathSetSnapshot and XPathSetIterator iterator inherit XPathResult:

interface XPathSetIterator : XPathResult {
  Node               nextNode()
                                        raises(DOMException);
};

interface XPathSetSnapshot : XPathResult {
  Node               item(in unsigned long index);
  readonly attribute unsigned long   length;
};


On a separate issue, I think that XPathSetIterator need some way to detect
if it still is valid without causing an exception to to be thrown. I propose
that an isStillValid() function is added to the iterator. The function
returns a boolean indicating if the nodeset is still valid. The function
returns false if a call to nextNode() would throw an INVALID_STATE_ERR
exception. The function does not indicate if a subsequent call to nextNode()
would return a non-null value or not.

/ Jonas Sicking
Received on Thursday, 1 November 2001 19:44:07 GMT

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