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

From: Olli Pettay <Olli.Pettay@helsinki.fi>
Date: Wed, 12 Sep 2012 08:21:26 +0200
Message-ID: <505029E6.3090805@helsinki.fi>
To: Jonas Sicking <jonas@sicking.cc>
CC: Adam Klein <adamk@chromium.org>, 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>, Rafael Weinstein <rafaelw@chromium.org>
On 09/10/2012 02:33 PM, Jonas Sicking wrote:
> On Fri, Sep 7, 2012 at 12:59 PM, Adam Klein <adamk@chromium.org> wrote:
>> On Thu, Sep 6, 2012 at 3:51 PM, Jonas Sicking <jonas@sicking.cc> wrote:
>>> 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:
>>>>> 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
>>
>> As Olli points out on that bug, providing a stable state doesn't
>> perform a microtask checkpoint (nor invoke
>> MutationObservers). Given the note attached to the "provide a stable
>> state" section, "A synchronous section never mutates the DOM, runs any
>> script, or has any other side-effects", it seems that running observer
>> callbacks whenever asked to provide a stable state is unlikely to be
>> the right thing.
>>
>> If the HTML parser is updated to enqueue mutations, it seems like
>> there also needs to be an addition to the steps run when inserting
>> preparing a <script>, unless Olli and I are missing something.
>
> Sorry. I didn't mean to say that the spec was calling for any
> particular behavior here. What I meant was that I agree that the most
> sensible behavior would be to flush notifications before and after
> executing each <script>.
>
> Hence I think the spec should call for that behavior, and that
> implementations should implement that.
>
> / Jonas
>


It is not clear to me we need to change the spec
(except that it should say mutation records are created during parsing).
If someone wants to get mutation records earlier than
end-outermost-microtask-or-end-of-innermost-task, there
is always takeRecords().

-Olli
Received on Wednesday, 12 September 2012 06:22:11 GMT

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