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Re: Details of MutationObserver delivery order

From: Olli Pettay <Olli.Pettay@helsinki.fi>
Date: Sun, 11 Mar 2012 23:45:26 +0200
Message-ID: <4F5D1CF6.20505@helsinki.fi>
To: Adam Klein <adamk@chromium.org>
CC: www-dom@w3.org, Anne van Kesteren <annevk@opera.com>, Olli@pettay.fi
On 03/07/2012 06:52 PM, Adam Klein wrote:
> http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html#concept-mo-invoke
> describes an algorithm for delivering MutationRecords to
> MutationObservers. In particular, it describes an order of delivery, and
> I wonder if tweaking it a little bit would make it simpler to implement.
> Note that I don't think the particular order is of much importance: it's
> just important that there is a well-defined order.  In particular, there
> are two cases I'm worried about:
>
> 1. Assume observers A, B, and C (created in the order A, B, C). Say that
> at the beginning of the algorithm, only A and C have non-empty queues.
> But during A's callback, it mutates DOM that causes a record to be added
> to B's queue.
>
> 2. Assume an observer A with a non-empty queue. During its callback, it
> creates a new observer B, starts B observing, and mutates DOM that adds
> a record to both A's and B's queue.
>
> By the spec, case (1) would result in the delivery order A-B-C. And (2)
> would be A-B-A.
>
> In the WebKit implementation, though, only the "active" observers (those
> with records in their queues) are kept in a list (this makes it fast in
> the common case that there's no delivery necessary). This makes our
> algorithm more like this:
>
> I. Make a copy of the existing "active" list, clear the list, and then
> iterate over the copy.
> II. When that iteration is complete, the active list is checked again;
> if it's non-empty, go back to step I.
>
> When applied to the cases above, (1) results in the order A-C-B (B
> doesn't get notified until the next time around the loop), and (2)
> results in A-A-B (again, the newly-added observer doesn't get notified
> until the second time through the loop).
>
> Thoughts?
Sounds ok.


> Also, if the above is too hard to follow, I have test-cases
> that demonstrate these cases, and could point at those instead.
>
> - Adam
Received on Sunday, 11 March 2012 21:46:26 GMT

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