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Re: TreeWalker, NodeIterator

From: <keshlam@us.ibm.com>
Date: Tue, 6 Jun 2000 16:37:53 -0400
To: kboone@ebt.com
cc: "DOM (E-mail)" <www-dom@w3.org>
Message-ID: <852568F6.00715515.00@D51MTA03.pok.ibm.com>
>1.  TreeWalker.getParentNode() returns an ancestor, not necessarily the
>parent.  Should it not be renamed getAncestorNode() to reflect the
>semantics?

You've missed a subtle point: TreeWalker presents a filtered view of the
DOM tree. All navigational operations on a TreeWalker are in terms of that
filtered view, _not_ in terms of the "real" DOM it's walking over.

For example, consider:
     <a><skipMe><b/></skipMe><c/></a>

If the TreeWalker has a filter which skips the <skipMe/> element, it will
present a view of the tree as if <skipMe/> didn't exist:
     <a><b/><c/></a>

In this view, b's parent is a and its next-sibling is c -- even though
those aren't the relationships in the original document. In some sense,
that's the whole point of having a filtered view; it behaves more-or-less
like a standard DOM tree, but you see only those parts of the document
structure which are meaningful for the task at hand.


>2.  TreeWalker.getFirstChild()/getLastChild() apparently don't drill down,
>but only return "direct" offspring.  Can you confirm this/clarify the
>specification.

Again, think in terms of the filtered view. In the above example, if the
TreeWalker's current node is node a, TreeWalker.getFirstChild() will "drill
down" to node b -- because it's walking over the filtered view, where b
really is considered a's first child.


>The implementation of TreeWalker [especially in light of the "live" nature
>of TreeWalkers] would benefit from a detach() method, like that found in
>NodeIterator.

TreeWalker really shouldn't need a detach() method.

NodeIterator, with its maintain-current-position semantics, needs callbacks
when the DOM's structure changes. This imposes some computational overhead.
The detach() operation was added as a way of saying "I'm done with this
object" so the DOM can stop doing that work. (Think about NodeIterator in a
garbage-collected environment like Java; without detach(), abandoned
iterators would continue to consume cycles until the system got around to
garbage-collecting them.)

TreeWalker, on the other hand, uses "pure current node" semantics; all
operations occur in terms of a specific current node, and if you move that
current node the TreeWalker moves with it. That means that the TreeWalker
probably won't need callbacks from the DOM tree, which means there's no
computational cost for having one floating around after you're done with
it. Hence detach() should be unnecessary.


> That said, adding detach() to TreeWalker gives this interface
> a signature that would allow it to be an extension of NodeIterator.

That option was discussed. Our conclusion was that since the semantics of
TreeWalker and NodeIterator under document mutation were so different, it
really didn't make sense to derive one from the other. If anything, both
would derive from a common abstract traversal type... but our best guess is
that nobody will really need to view them that way.
Received on Tuesday, 6 June 2000 16:38:25 GMT

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