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Re: DOM L2 comments, various

From: <keshlam@us.ibm.com>
Date: Fri, 1 Oct 1999 16:42:26 -0400
cc: www-dom@w3.org
Message-ID: <852567FD.0071BF32.00@D51MTA03.pok.ibm.com>
Dave, I'm not sure of the status of your past questions about event handling --
I'm also implementing this and stumbled across a few ot the same issues -- but a
few comments follow based on my own best understanding. Note that I could be
completely wrong; I wasn't involved in designing this chapter!

Re 6.2, "there ought to be _one_ mechanism whereby event reporting isn't subject
to interference." -- I think the best current answer to that need is to have the
target node's event handler (which can't be blocked) or a capture handler on the
tree's root (which ought to run before capturing gets turned off) respond to the
event by sending out a separate custom event which, presumably, nobody else is
going to discard before your "unblockable" handlers hear it.

Someone proposed the concept of "event listener sets", wherein the "prevent"
calls would only block other listeners which registered as members of the same
set. The current inclination seems to be to defer this suggestion to a future
release so we have time to think about its implications on behavior and
performance.

Re 6.3.1 and the Swing event conventions --  Swing can get away with this
because Swing doesn't attempt to pass generic events; specific events are tied
to specific registration calls and specific listeners. Since we intend to let
application-specific events pass through our engine, we can't really hardcode
event types into the dispatch/listener APIs.

That's also part of the answer to your question about applications defining
their own events: That _is_ supposed to be possible, though I'm not sure it can
be done fully portably. No specific prefix is assigned for that use; it was
deemed better to let folks negotiate their own conventions -- up to and
including namespace-like URIs if absolutely necessary -- with the DOM* space
reserved for those defined by the DOM spec.

6.2.2: The intent is indeed that preventCapture() only shortcircuits the Capture
phase. Good catch.

6.6.4 DOMSubtreeModified: I believe the intent is that any document mutation
generates its specific event and also "eventually" triggers this event... but
there may not be a 1:1 relationship. If you take a node that's already in the
tree and give it a new parent (which is a removal plus an insertion, each
generating its own event), the hope is that a sufficiently clever DOM might
generate only a single DOMSubtreeModified message for the combined operation,
sent to the lowest common ancestor of the old and new parents. It's hoped that
this might reduce the load if "realtime" response is not required. The
definition of "sufficiently clever", and what granularity the batching occurs
on, seem to have been left to the implementation.

6.6.4 DOMNodeRemovedFrom/InsertedIntoDocument: There were a few use cases
advanced where folks had active processes associated with nodes, which only
wanted to run while the node was part of the Document. All I can suggest is not
dispatching these unless there's a listener for them.

______________________________________
Joe Kesselman  / IBM Research
Received on Friday, 1 October 1999 16:42:46 GMT

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