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

From: Rafael Weinstein <rafaelw@google.com>
Date: Thu, 30 Aug 2012 03:44:15 -0700
Message-ID: <CABMdHiRe4jXeZzuH_hFKfoFnkTfCxKdJQKDpnbGmcUgJKFb=sg@mail.gmail.com>
To: WebApps WG <public-webapps@w3.org>
When does Gecko invoke observers during page construction? Are any of
the goals we set for the processing model still retained during load,

-"I get to complete my work before paint"
-"Future script invocations don't see the world in an inconsistent
state because observations have yet to be delivered"

For example, if I wanted to implement an observer which polyfiled web
components and did fix-up on elements with known attribute values as
they are loaded and rendered, am I guaranteed that the user will never
see undecorated elements during load? ... that any <script> that runs
can assume "components" they retrieve via querySelector have been

On Wed, Aug 29, 2012 at 9:47 PM, Jonas Sicking <jonas@sicking.cc> wrote:
> On Thu, Aug 30, 2012 at 12:57 AM, Olli Pettay <Olli.Pettay@helsinki.fi> wrote:
>> On 08/30/2012 02:05 AM, Ian Hickson wrote:
>>>> On Fri, Jun 15, 2012 at 4:35 PM, Adam Klein <adamk@google.com> wrote:
>>>>> This code alerts in Firefox but not in Chrome:
>>>>> <!DOCTYPE html>
>>>>> <body>
>>>>>    <script>
>>>>>      var observer = new MutationObserver(function(r) {
>>>>>        alert(r);
>>>>>      });
>>>>>      observer.observe(document.body, {childList: true, subtree: true});
>>>>>    </script>
>>>>>    <p>Hello, World</p>
>>>>> </body>
>>>>> In WebKit's implementation, we had assumed that MutationObservers were
>>>>> meant to observe changes after page load (and I personally thought
>>>>> that we'd specced it that way, by putting it in DOM4, not HTML). But
>>>>> it seems the Mozilla implementors made a different assumption. But
>>>>> what should happen?
>>>>> IMHO, it may not be worth the gain may not be worth the possible
>>>>> performance degradation. If script wants to find out what the parser
>>>>> put on the page, it should wait for DOMContentLoaded. But I can
>>>>> imagine a use case where script might want to find out about the
>>>>> parser's work during load.
>>>>> In any case, we should try to come to a decision about this, since
>>>>> this seems to be the one major divergence between the existent
>>>>> implementations of MutationObservers.
>>> The spec used to say "DOM mutation events must not fire for changes caused
>>> by the UA parsing the document". I've updated this to also mention
>>> mutation observers.
>> Why? Getting MutationObserver notifications during parsing is what I'd
>> expect
>> the API to provide (and that is implemented in Gecko).
>>> 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).
>>> On Tue, 26 Jun 2012, Adam Klein wrote:
>>>> I take it from your reply that you and I had the same view of what's
>>>> specced in DOM4. That is, that MutationObservers are not specified to be
>>>> notified of actions taken by the parser. Given that fact, it seems that
>>>> either the spec should be changed (and by "spec" here I think the
>>>> required changes are in HTML, not DOM), or Firefox's implementation
>>>> ought to be changed.
>>>> Anne, Ian, Olli, Jonas, your thoughts?
>>> I've updated HTML to explicitly say mutation observers don't fire when
>>> parsing, in the same way that it says not to fire mutation events.
>> The point is that MutationObserver can handle parsing case just fine.
>> It doesn't have similar problems what Mutation Events have.
> Indeed. I have the same questions. The API doesn't have same
> performance and re-entrancy problems that caused us to disable
> mutation events.
> If we don't fire mutation observers during parsing it means that you
> basically can't rely on mutation observers at all until after the
> DOMContentLoaded event has fired. In fact, you can't rely on them even
> past then since if another page is loaded in an <iframe> and nodes are
> moved from that iframe to your document, you won't be notified about
> changes done by the parser to those nodes.
> So this change removes the ability to guarantee that you can track
> mutations done to a document, which was one of the big advantages that
> MutationObservers had over mutation events. I.e. mutation events only
> let you track mutations to your document as long as those mutations
> "behaved well", i.e. weren't done from inside mutation observers. With
> this we add a similar requirement to mutation observers that they are
> only reliable as long as pages "behave well" and don't move nodes from
> a document which is being parsed.
> Just like mutation events solved the use cases of many authors,
> mutation observers will solve the use cases of many authors even with
> this change. However it will miss edge cases which I think is really
> unfortunate.
> / Jonas
Received on Thursday, 30 August 2012 10:44:43 GMT

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