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Re: DOM L2 comments, various

From: David Brownell <david-b@pacbell.net>
Date: Fri, 08 Oct 1999 14:03:01 -0700
Message-ID: <37FE5C05.AB4BF38E@pacbell.net>
To: www-dom@w3.org
Well, today's the day "last call" ends.  So here are a few more
comments I'd queued up.  This should be the last set of new "last
call" comments from me.

- Dave

1.3	DocumentType ... this shouldn't expose _only_ the external DTD
	subset.  If it exposes that, it should expose the internal DTD
	subset too -- else it's deleting half the key information!!

	FIX BY:  exposing the other half of the DTD.
	- Add "readonly attribute DOMString internalSubset" to
	  DocumentType, containing the literal text of the internal
	  subset.  (The very same kind of text found in the resource
	  identified by the systemId URI ...)

	- Add an "in DOMString internalSubset" parameter to the
	  DOMImplementation.createDocumentType() method.

	If you don't fix this, then remove the external DTD subset info;
	having one half of the information isn't a good idea.

7.1	While I appreciate the intent, I'd like to see the words "simple
	and efficient" struck from the description.  Or at least see them
	made to apply only to the application code, not the implementation.

	These aren't "simple" since they must interact with selected node
	deletions from their backing document.  They aren't "efficient"
	since that interaction increases the cost of those deletions;
	I've measured a 12%-15% increase.  These iterators push costs out
	of the application into the DOM -- so applications that wouldn't
	normally incur those costs are slowed down, getting no benefit.

7.1	I'd like to see some motivation of the "liveness" here.  It's clear
	to me that most servers don't need or want "liveness".  Given that
	there is a cost to being sensitive to deletions, why must all
	implementations pay that cost?  I'd probably like these APIs if I
	didn't have to pay that cost.

7.1.1	The explanations in and have made a step forward
	by adding the notion of a "reference node", but they still have
	that confusing notion of a position "between" two nodes.

	Can we get those explanations made _purely_ in terms of the
	reference node?  For the majority of implementations, using an
	object to represent each node in the DOM tree, representing a
	position "between" nodes is an unnatural act.  Not so for
	anything using a reference node -- and of course, from the user
	perspective, all they can ever see is nodes, so an explanation in
	terms of nodes (not between-ness) can't help but be clearer.

	I think that references to "position" can be turned into references
	to an internal flag that seems to be necessary.  "Before the
	reference node" is "direction flag is left"; and "after the reference
	node" is "direction flag is right".  Then one can fully define
	the behaviors of nextNode() and previousNode() ... since if the
	direction flag isn't going in the right direction, all they do is
	toggle the flag and return the reference node (right??).  Note that
	such a flag is almost suggested in the first para of; it's
	clearly not newly introduced state.	I found the explanation (not quite algorithmic) to be problematic.

	For example, it states at the end that the reference node is at
	a particular position when the iterator is first created.

	Hmm, I didn't implement it that way, yet from what I can tell
	my code is fully conformant.  So the text is either incorrect or
	it's incomplete, or both.

	Similarly it defines the reference node to be the last node
	returned -- which clearly can't cover the initial state, where
	no value has been returned.	More of the same -- unclear.

	2nd para, "changes to the iterated list" ... what changes is the
	DOM document (a tree!) backing the iterator.  The whole point of
	this is to make the iterated list react to such changes, since
	that list can't be directly manipulated.

	6th para, "If the reference node is before the iterator, which
	is usually the case after nextNode() has been called" ... the
	iterator is an object outside the DOM tree.  Surely you mean
	"position", not iterator.  Similar in paragraph 8.  (But again,
	I'd like to see reference to "position" removed.  Those two can
	be simple references to the implicit direction flag.)

	"One special case arises..."  Hmm, danger sign!!  There should be
	a corresponding special case for the _other_ end of the list ...
	the text is clearly incomplete, it doesn't capture the problem's
	esential symmetry.  (These cases should have one general rule to
	cover them, of course!)	1st para penultimate sentence, "the reference node is the last node
	in the list, whether or not it is visible".  Strike that.  It's in
	fundamental conflict with the definition of the reference node as
	the last node returned.

7.2	A familiar GC issue rears its ugly head.  Because NodeIterator (and
	also TreeWalker) require deletions of particular nodes (or their
	ancestors) to affect the iterator, this specification has REQUIRED
	the backing tree to be coupled to the iterators/walkers traversing it.

	However, there is no way to break that coupling once it's been set
	up.  Accordingly, iterators can never be GC'd until the node which
	establishes that coupling becomes garbage.  (No, it's not OK to force
	all DOM Level 2 implementations to use Java2 "weak references", or to
	preclude using traversal in other platforms without such support, or
	even to say that you can't use .)

	FIX BY:  add a new method to NodeIterator (and TreeWalker) which
	is defined more or less as follows:

	void finished ()
		Marks the traversal object as no longer needed,
		permitting the DOM to release all resources that
		are associated with it.  Subsequent calls to nextNode
		and previousNode() will report an exception.

	Of course, COM (and JavaScript implementations built on it, such
	as Mozilla's) can play refcounting tricks and arrange to have the
	last "real" reference release those resources.  But finished() will
	not hurt those implementations, either, and can help avoid the need
	for such tricks -- always healthy.

	I know this GC issue got discussed earlier in the context of
	problems providing an acceptably performant "live NodeList".
	It can be more or less sidestepped with a NodeList since there's
	no notion of "current" node there -- you can implement NodeList
	liveness by tracking a document mutation count, and if it changes
	then doing the getElementByTagName() search over again from the
	beginning.  Slow as all get-out, but correct.  (Some implementations
	have proprietary parser integration, and only restart if the change
	wasn't from a parser "appending" more data after existing nodes.)

7.2	TreeWalker is exactly a NodeIterator with some new methods, and should
	be defined as such:  inherit from NodeIterator.  Repeating that much
	text is a bad idea.  Ether differences are intended, so the text
	should be very different; or they're not, in which case all the identical
	interface features should be shared through the sharing mechanism
	(inheritance of interface).
Received on Friday, 8 October 1999 17:03:43 UTC

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