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Re: Should MutationObservers be able to observe work done by the HTML parser?

From: Jonas Sicking <jonas@sicking.cc>
Date: Thu, 6 Sep 2012 15:51:59 -0700
Message-ID: <CA+c2ei9w5B9EDOhLNpYn7icNVDQ9aXVKwFKtYjftsof39RBDVw@mail.gmail.com>
To: Adam Klein <adamk@chromium.org>
Cc: Ian Hickson <ian@hixie.ch>, Mihai Parparita <mihaip@chromium.org>, Ryosuke Niwa <rniwa@webkit.org>, WebApps WG <public-webapps@w3.org>, Anne van Kesteren <annevk@annevk.nl>, Olli Pettay <Olli.Pettay@helsinki.fi>, Olli@pettay.fi, Rafael Weinstein <rafaelw@chromium.org>
On Thu, Sep 6, 2012 at 3:14 PM, Adam Klein <adamk@chromium.org> wrote:
> On Wed, Aug 29, 2012 at 4:05 PM, Ian Hickson <ian@hixie.ch> wrote:
>>
>> On Fri, 15 Jun 2012, Ryosuke Niwa wrote:
>> > On Fri, Jun 15, 2012 at 6:15 PM, Mihai Parparita wrote:
>> > >
>> > > I used MutationObservers for the first time last night (in Chrome),
>> > > and I was surprised by this behavior. I was working on something that
>> > > transmogrified one node into another, so having observers file during
>> > > parsing would have helpful. Otherwise something like:
>> > >
>> > > <script>var observer = new MutationObserver(/* observer that
>> > > manipulates <foo> tags */);</script>
>> > > <body>
>> > > ....
>> > > <foo></foo>
>> > > <script>
>> > >  /* code that acts on foo tags */
>> > > </script>
>> > >
>> > > If I have to use DOMContentLoaded, then the code in the second
>> > > <script> block would get a chance to operate on <foo> before my
>> > > observer had had a chance to transmogrify it.
>> >
>> > That is a slightly different issue.
>> >
>> > There is no guarantee that your observer is called before the second
>> > script element's script is executed. The only way for that to happen is
>> > if the parser yielded to the event loop immediately after parsing the
>> > foo element but before executing the script element.
>>
>> The spec actually does require that the UA "provide a stable state" before
>> processing <script>s, which invokes the relevant part of the event loop.
>> If mutation observers were to fire during parse, it would require those to
>> fire too (it currently does not).
>
> In my testing, Gecko doesn't behave this way: MutationRecords are delivered
> at the end of any encountered <script> tags (at the end-of-microtask,
> essentially), rather than before they run. If delivery-during-parse is how
> we end up going, spec-wise, I think it's important for the use-cases that we
> deliver before each script runs.

Agreed. This sounds like a bug in our implementation. I filed

https://bugzilla.mozilla.org/show_bug.cgi?id=789315

/ Jonas
Received on Thursday, 6 September 2012 22:52:58 GMT

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