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Re: getElementsByTagName

From: Joseph Kesselman/Watson/IBM <keshlam@us.ibm.com>
Date: Wed, 18 Oct 2000 15:38:27 -0400
To: Stanley Guan <Stanley.Guan@oracle.com>
Cc: www-dom@w3.org, lauren@sqwest.bc.ca
Message-ID: <OF35758DCF.686AB522-ON8525697C.00698724@pok.ibm.com>

>getElementsByTagName will return a new NodeList object containing all the
>matched Elements.  According to spec., NodeList objects in the DOM are
>live.

That is a strict requirement for compliance.

YES, it requires that NodeList somehow be made aware of changes in the
subtree it covers.

YES, it means that the index number for a given node may change whenever
the document changes.

And YES, it is tough to implement this efficiently. In my experience so
far, you have to settle for making common cases efficient and accept a
performance hit on the less common cases... which requires that DOM authors
decide which cases are common for the applications they expect to serve.


The following is personal opinion:


If you're just trying to use getElementsByTagName, my general advice is
"Consider alternatives." I generally try to steer users  toward the DOM
Level 2 Traversal mechanisms, or  toward an explicit tree-walk if they're
coding in DOM Level 1. Even though NodeIterator requires similar
notification when the document changes, its recovery mechanism is a lot
simpler and less expensive. TreeWalker's recovery is trivial since it just
stays with its current node, and most user-written tree walks are equally
simpleminded.

If folks must use getElementsByTagName, I usually recommend they consider
loop-until-null solutions rather than getting the length in advance. This
may allow the DOM to perform some optimization.

Other DOMs may behave differently, but I think these approaches are
generally the way to bet unless you know otherwise.


Of course if folks don't care about performance, and/or are sure they will
not alter the DOM during the NodeList's lifetime,  the answer may be "Use
whatever call or object best expresses your intent, and hope that the DOM's
authors considered your usage pattern one of those worth optimizing for."

______________________________________
Joe Kesselman  / IBM Research
Received on Wednesday, 18 October 2000 15:39:13 GMT

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