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Re: The DOM is not a model, it is a library!

From: Ray D. Whitmer <rayw@imall.com>
Date: Thu, 7 Oct 1999 20:23:18 -0600 (MDT)
To: "Stephen R. Savitzky" <steve@rsv.ricoh.com>
cc: Michael Champion <michael_champion@ameritech.net>, DOM Mailing List <www-dom@w3.org>
Message-ID: <Pine.GSO.4.05.9910071945411.12988-100000@sol.imall.com>
On 7 Oct 1999, Stephen R. Savitzky wrote:

>1. Live nodelists are trivial to implement badly, but useless in practice. 

Live nodelists are trivial to implement so that they are efficient as long
as the tree is not mutated during use.  The live node list is much more
efficient for this case than a static one would be because you are not
forced to build the list.  Often the user may only access the first node,
too, and it is wastefull to traverse the whole document before they
actually reference the node.

The big problem with TreeWalker is that it can be used in a wide variety
of ways, not all of which are optimized in the same way.

Based on my implementation, I give my users a suggestion not use the count 
(keep going until a null is returned) and copy node refs to a safe place if 
they want to keep them around or mutate the document -- in Java, a simple 
for loop adding elements to a vector.  It serves them well.

>   Specific fix: add operations to DocumentTraversal to construct a
>   NodeIterator from a NodeList and from a NamedNodeMap.  This allows the
>   problematic NodeList objects to exist only transiently before being
>   replaced by something efficient. 

I would be in favor of this if I didn't think it were already covered by
iterators and tree walkers -- I still could be convinced by some use cases.  
I think the users will have to change their code one way or the other to get 
something better than NodeList for cases where this does not perform well.

>2. The new ownerElement attribute of Attr is a major nuisance because it
>   means it's impossible to share default values from the DTD.
>   Specific fix:  permit (or require) ownerElement to return null from an
>   Attr for which specified is false. 

This is not new with the addition of the ownerElement.  Please, explain to
me how you can possibly share the Attr node object directly under the level
1 API.  When the user does a setNodeValue on the attribute, are you going to
change the value of that attribute for all elements which happened to have
the old value in the tree?  If you are not, then you have to create at least
a temporary object to hold the attribute's position in the tree.  There was
never anything stopping you from sharing most of the attribute, but you need
some placeholder, which also makes it trivially possible to implement
ownerElement, because if you modify an attribute node, the system has to
know well which element it is going to affect.

FWIW, there is no requirement in DOM that the Java == function identify that
two nodes are the same node.  You can create temporaries on the fly that
get garbage collected.  This point may not be clear in the spec, but it
needs to be, and was always hard-fought to keep it that way.  Many
implementations don't have any attribute nodes in the hierarchy unless
someone requests them, in which case they are constructed on the fly.
And most users never bother to request them as nodes, but rather as
strings, so you can just keep shared data and never worry about this.

>3. Requiring the children of an EntityReference to exist and be identical to
>   the children of the corresponding Entity is horrendous. 
>   Specific fix: make this a feature.  Better, allow it to be enabled or
>   disabled by the application at the Document level.  It's much better for
>   many applications if they look up the Entity.  Turning on
>   expandEntityReferences in a TreeWalker would do this automatically
>   without requiring the application to do anything.   
>   It would be necessary to make it clear that, when a TreeWalker descends
>   into the ``children'' of an EntityReference, parentNode of those children
>   might refer to the Entity rather than to the EntityReference. 

This has always bothered me, and I think it might be possible to fix it in
this way.  I like this issue and the proposed solution.

>4. The requirement that the entire document be accessible is onerous for
>   applications with limited memory and no external storage.  One could not,
>   for example, put a DOM on a Palm Pilot. 
>   Workaround: enable a TreeWalker (perhaps under the control of some new
>   attribute) to return a shallow clone of the current node rather than the
>   node itself; add another attribute to make the TreeWalker unidirectional.
>   This isn't as good as a complete set of tree traversers and constructors
>   that can completely insulate an application from the Nodes, but it's
>   better than nothing. 

Returning the shallow clone is easy to do with the current API.  Lots of
different lazy loading schemes are also possible.

I don't see the need to make the thing unidirectional -- I think that most
constraints can be addressed in other ways.  I fully plan on having a lazy-
loading DOM that works well for palm pilots.  This is easy with the current
API.  And it doesn't rely on TreeWalker.  I'd be happy to discuss a variety 
of techniques...

>5. No way for an application to attach its own information to the tree.
>   Specific fix: add a new attribute to Node:
>	attribute NamedNodeMap applicationData;
>   It must be explicitly permissible for the items in the map to come from
>   any mixture of owner documents, and indeed from any mixture of DOM
>   implementations (this might have to be a feature).  This would allow an
>   application to construct a Document containing sharable metadata with
>   which it can then decorate some other Document with the application, not
>   the DOM, controlling whether sharing is to occur.
>   Allowing application data to come from different DOM implementations
>   would allow an application to decorate a document from some third-party
>   library with its own extended nodes if it had them.

I strongly agree that it is nice to be able to attach data to a node of the
DOM, and this could easily turn out to be one of the first things we add for 
level 3, as soon as we are able to address the legitimate concerns.

I am opposed to a single attribute, because it means that when you have
multiple DOM applications working on the same tree and using this for
different extension purposes, they collide.  Why limit extensions
this way?  I also think there are times when this data needs to be
cloned/copied with the node.  Otherwise, cut and paste can lose important
data, so a small accomodation is needed during node cloning, IMO.

One other concern is shared data.  My DOM shares everything whenever it
is the same.  For example, all similar whitespace is a single object
in my DOM.  All identical attributes are a shared object, and so on.
Attaching unique data makes this more difficult, but I think I can solve 
it, so I am for the proposal.
Received on Thursday, 7 October 1999 22:24:05 UTC

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