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Re: Scheduling multiple types of end-of-(micro)task work

From: Jonas Sicking <jonas@sicking.cc>
Date: Thu, 18 Oct 2012 16:16:14 -0700
Message-ID: <CA+c2ei-hniSBOFTENYaeJHh=KHE-HUqgkBv6RRAKumhWLP1Jog@mail.gmail.com>
To: James Robinson <jamesr@google.com>
Cc: Alan Stearns <stearns@adobe.com>, "olli@pettay.fi" <olli@pettay.fi>, Rafael Weinstein <rafaelw@google.com>, Webapps WG <public-webapps@w3.org>, Ian Hickson <ian@hixie.ch>, Ryosuke Niwa <rniwa@webkit.org>, Ojan Vafai <ojan@chromium.org>
On Thu, Oct 18, 2012 at 3:34 PM, James Robinson <jamesr@google.com> wrote:
> On Thu, Oct 18, 2012 at 3:19 PM, Alan Stearns <stearns@adobe.com> wrote:
>> On 10/18/12 2:51 PM, "Olli Pettay" <Olli.Pettay@helsinki.fi> wrote:
>> >On 10/19/2012 12:08 AM, Rafael Weinstein wrote:
>> >> CSS Regions regionLayoutUpdate brings up an issue I think we need to
>> >> get ahead of:
>> >>
>> >>    https://www.w3.org/Bugs/Public/show_bug.cgi?id=16391
>> >>
>> >> For context:
>> >> --------
>> >> Mutation Observers are currently spec'd in DOM4
>> >>
>> >>      http://dom.spec.whatwg.org/#mutation-observers
>> >>
>> >> and delivery timing is defined in HTML
>> >>
>> >>
>>
>> >> >>http://www.whatwg.org/specs/web-apps/current-work/#perform-a-microtask-ch
>> >>eckpoint
>> >>
>> >> The timing here is described as a "microtask checkpoint" and is
>> >> conceptually "deliver all pending mutation records immediately after
>> >> any script invocation exits".
>> >>
>> >> TC-39 has recently approved Object.observe
>> >>
>> >>      http://wiki.ecmascript.org/doku.php?id=harmony:observe
>> >
>> >(Not sure how that will work with native objects.)
>> >
>> >
>> >>
>> >> for inclusion in ECMAScript. It is conceptually modeled on Mutation
>> >> Observers, and delivers all pending change records immediately
>> >> *before* the last script stack frame exits.
>> >>
>> >> Additionally, although I've seen various discussion of dispatching DOM
>> >> Events with the microtask timing, CSS regionLayoutUpdate is the first
>> >> I'm aware of to attempt it
>> >>
>> >>      http://dev.w3.org/csswg/css3-regions/#region-flow-layout-events
>> >
>> >
>> >Could you explain why microtasks are good for this case?
>> >I would have expected something bound to animation frame callback
>> >handling,
>> >or perhaps just tasks (but before next layout flush or something).
>>
>> In the spec bug discussion, it was suggested that we use end-of-task or
>> end-of-microtask timing. When I looked at these options, it seemed to me
>> that the regionLayoutUpdate event was somewhat close in intent to
>> MutationObservers. So between those two options, I picked microtask. If
>> there's a better place to trigger the event, I'm happy to make a change to
>> the spec.
>>
>> The current wording may be wrong for separate reasons anyway. The event is
>> looking for layout changes. For instance, if the geometry of a region in
>> the region chain is modified, and this causes either (a) overflow in the
>> last region in the chain or (b) the last region in the chain to become
>> empty, then we want the event to trigger so that a script can add or
>> remove regions in the chain to make the content fit correctly. If a task
>> in the event queue caused the change, then the microtask point after that
>> task is probably too soon to evaluate whether the event needs to fire. And
>> if that was the last task in the queue, then there may not be another
>> microtask happening after layout has occurred.
>>
>> So what I need is an appropriate timing step for responding to layout
>> changes. Any suggestions?
>
>
> I think events based off of layout are a terrible idea and there is no good
> timing for them.  The regions case is a good example of why not to have
> them.  If you need javascript to respond to DOM changes then mutation
> observers are the primitive to use.  If you just want to get callbacks at a
> good time to update visual effects use requestAnimationFrame().

Does that mean that you think events like onresize and onscroll are
bad? It's an honest question since I can definitely see your argument.

Though I do wonder if layout events can be useful in cases where you
don't want to be called every time the DOM changes (MutationObservers)
or on every time the screen is painted (requestAnimationFrame).

You could use those two, specifically the latter, to replace onscroll
and onresize. But it might end up using a lot more CPU power if you
have to check the current scroll position or screen size on every
painted frame. Likewise it might use a lot more CPU power if you have
to check layout primitives on every DOM mutation.

People have been asking for resize events for individual elements for
a very long time (I think it might even be implemented in some
non-Gecko browsers). This obviously wouldn't be needed if people used
MutationObservers and requestAnimationFrame, or MutationObservers and
page-wide onresize. However it might save a lot of CPU power and make
authoring a whole lot simpler if you could get called only when it was
actually needed.

I don't know enough about CSS regions to know if the same thing
applies there. But I would imagine there are layout changes where
this applies.

/ Jonas
Received on Thursday, 18 October 2012 23:17:12 GMT

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