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

From: Rafael Weinstein <rafaelw@google.com>
Date: Thu, 18 Oct 2012 14:08:28 -0700
Message-ID: <CABMdHiSwc9QPO39U3-AdEbJxZsUKDOJ0WU44F5ZE-qG_1Up05g@mail.gmail.com>
To: Webapps WG <public-webapps@w3.org>
Cc: Olli@pettay.fi, Jonas Sicking <jonas@sicking.cc>, Ian Hickson <ian@hixie.ch>, stearns@adobe.com, Ryosuke Niwa <rniwa@webkit.org>, Ojan Vafai <ojan@chromium.org>
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-checkpoint

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

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

[I think this is wrong, and I'm hoping this email can help nail down
what will work better].

-------

Strawman:

I'd like to propose a mental model for how these types of work get
scheduled. Note that my guiding principles are consistent with the
original design of the the end-of-(micro)task timing:

-Observers should be delivered to async, but "soon"

-Best efforts should be made to prevent future events from running in
a world where pending observer work has not yet been completed.


Delivery cycles:

1) Script (Object.observe) delivery. This is conceptually identical to
Mutation Observers.

http://wiki.ecmascript.org/doku.php?id=harmony:observe#deliverallchangerecords

2) DOM (Mutation Observers) delivery.

http://dom.spec.whatwg.org/#mutation-observers

3) End-of-task queue.

This would be a new construct. Conceptually it would be a task queue
like other task queues, except that its purpose is to schedule
end-of-task work. Running it causes events to be dispatched in order
until the queue is empty.


Scheduling:

A) Immediately before any script invocation returns to the browser
(after the last stack frame exits), run (1). This can be purely a
concern of the script engine and spec'd independent of HTML & DOM4.

B) Immediately after any script invocation returns to the browser
(microtask checkpoint), run (2). Note that delivering to each observer
creates a new script invocation, at the end of which, (1) will run
again because of (A).

C) Immediately before the UA completes the current task, run (2). This
is necessary incase DOM changes have occurred outside of a script
context (e.g. an input event triggered a change), and is already
implemented as part of DOM Mutation Observers.

D) Run (3). Note that each script invocation terminates in running (1)
because of (A), then (2) because of (B).
Received on Thursday, 18 October 2012 21:08:55 GMT

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