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DOM Mutation Events Replacement: When to deliver mutations

From: Rafael Weinstein <rafaelw@google.com>
Date: Wed, 10 Aug 2011 17:44:32 -0700
Message-ID: <CABMdHiTkabvoNi+20+kjDzPOvXsw7RskwsuZNEWhXS5GEjzvWw@mail.gmail.com>
To: Webapps WG <public-webapps@w3.org>
Although everyone seems to agree that mutations should be delivered
after the DOM operations which generated them complete, the question
remains:

  When, exactly, should mutations be delivered?

The four options I'm aware of are:

1) Immediately - i.e. while the operation is underway. [Note: This is
how current DOM Mutation events work].

2) Upon completion of the "outer-most" DOM operation. i.e. Immediately
before a the lowest-on-the-stack DOM operation returns, but after it
has done all of its work.

3) At the end of the current Task. i.e. immediately before the UA is
about to fetch a new Task to run.

4) Scheduled as a future Task. i.e. fully async.

-------

Discussion:

Options 1 & 4 are don't seem to have any proponents that I know of, so briefly:

Option 1, Immediately:

Pro:
-It's conceptually the easiest thing to understand. The following *always* hold:
  -For calling code: When any DOM operation I make completes, all
observers will have run.
  -For notified code: If I'm being called, the operation which caused
this is below me on the stack.

Con:
-Because mutations must be delivered for some DOM operations before
the operation is complete, UAs must tolerate all ways in which script
may invalidate their assumptions before they do further work.


Option 4, Scheduled as a future Task:

Pro:
-Conceptually easy to understand
-Easy to implement.

Con:
-It's too late. Most use cases for mutation observation require that
observers run before a paint occurs. E.g. a widget library which
watches for special attributes. Script may create a <div
class="FooButton"> and an observer will react to this by decorating
the div as a FooButton. It is unacceptable (creates visual
artifacts/flickering) to have the div be painted before the widget
library has decorated it as a FooButton.

Both of these options appear to be non-starters. Option 1 has been
shown by experience to be an unreasonable implementation burden for
UAs. Option 4 clearly doesn't handle properly important use cases.

-------

Options 2 & 3 have proponents. Since I'm one of them (a proponent),
I'll just summarize the main *pro* arguments for each and invite those
who wish (including myself), to weigh in with further support or
criticism in follow-on emails.


Option 2: Upon completion of the "outer-most" DOM operation.

Pro:
-It's conceptually close to fully synchronous. For simple uses
(specifically, setting aside the case of making DOM operations within
a mutation callback), it has the advantages of Option 1, without its
disadvantages. Because of this, it's similar to the behavior of
current Mutation Events.

Option 3: At the end of the current Task.

Pro:
-No code is at risk for having its assumptions invalidated while it is
trying to do work. All participants (main application script,
libraries which are implemented using DOM mutation observation) are
allowed to complete whatever work (DOM operations) they wish before
another participant starts doing work.
Received on Thursday, 11 August 2011 00:44:56 GMT

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