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[whatwg] <video> resource selection algorithm and NETWORK_NO_SOURCE

From: Silvia Pfeiffer <silviapfeiffer1@gmail.com>
Date: Mon, 26 Jul 2010 10:58:07 +1000
Message-ID: <AANLkTim0PzvkqPGhZ=dG6D0o8n0akj1ZNbBYTOLzzUmq@mail.gmail.com>
On Sun, Jul 25, 2010 at 3:31 PM, Maciej Stachowiak <mjs at apple.com> wrote:

>
> On Jul 23, 2010, at 7:16 AM, Philip J?genstedt wrote:
>
> Silvia made we aware of discrepancy in how browsers implement the resource
> selection algorithm, see forwarded message. It's my assessment that Opera is
> the only browser following the spec. I've filed this bug with Mozilla:
>
> https://bugzilla.mozilla.org/show_bug.cgi?id=581355
>
> I've also reported bugs in Chrome and Safari, but can't see where they
> ended up.
>
> The reason I'm writing this email is that apparently everyone but myself
> has a different interpretation of the spec, so perhaps this is something we
> need to discuss. Does any other browser ever set the state NETWORK_NO_SOURCE
> at all? I speculated that perhaps other browsers aren't very strict about
> which parts of the algorithm are run synchronously and not, but even
> checking the networkState after a setTimeout it still isn't
> NETWORK_NO_SOURCE.
>
> Test case: http://people.opera.com/philipj/2010/07/23/networkState.html
>
> Please fix implementation or spec :)
>
>
> 1) Which behavior is more useful?
> 2) Sylvia's original issue was with play() - should we ensure that any time
> you call play(), it will cause the media resource to start playing once
> loaded? That seems like the real spec bug.
>

The problem there with play in Opera was that the @src was changed, but
Opera doesn't run the resource selection algorithm to load it for play(),
but instead expects to run load() first. The description of play() however
clearly states as a first step:
1. If the media
element<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element>'s
networkState<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#dom-media-networkstate>attribute
has the value
NETWORK_EMPTY<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#dom-media-network_empty>,
invoke the media
element<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element>'s
resource selection
algorithm<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#concept-media-load-algorithm>
.

This is why we are discussing whether the networkState needs to be
NETWORK_EMPTY or NETWORK_NO_SOURCE after initialisation of a media element.

Incidentally, that has effects on other elements, too, such as for <source>
it is stated:
If a source<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#the-source-element>element
is inserted as a child of a media
element<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element>that
has no
src<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#attr-media-src>attribute
and whose
networkState<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#dom-media-networkstate>has
the value
NETWORK_EMPTY<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#dom-media-network_empty>,
the user agent must invoke the media
element<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element>'s
resource selection
algorithm<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#concept-media-load-algorithm>
.

And for @src it is said:
If a src<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#attr-media-src>attribute
of a media
element<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element>is
set or changed, the user agent must invoke the media
element<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element>'s
media element load
algorithm<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#media-element-load-algorithm>.
(*Removing* the
src<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#attr-media-src>attribute
does not do this, even if there are
source<http://www.whatwg.org/specs/web-apps/current-work/complete/video.html#the-source-element>elements
present.)

That is kinda strange, too, because really when @src is changed, resource
selection has to be run first rather than media element load.

The easiest way to get this consistent is to keep the meaning of
NETWORK_EMPTY and NETWORK_NO_SOURCE and make sure everything else is made
consistent with that, i.e.
* change the first state of the resource selection to use NETWORK_EMPTY
* change the first action of @src to run the resource selection algorithm
rather than the resource load algorithm

Any other changes would cause a lot more follow-on changes IMO.

Cheers,
Silvia.
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