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[whatwg] Restarting the media element resource fetch algorithm after "load" event

From: Eric Carlson <eric.carlson@apple.com>
Date: Thu, 08 Oct 2009 10:42:14 -0700
Message-ID: <1A8D8C33-84CC-4E97-AB7E-793F681FD199@apple.com>

On Oct 8, 2009, at 5:32 AM, Philip J?genstedt wrote:

> On Thu, 08 Oct 2009 12:10:01 +0200, Robert O'Callahan <robert at ocallahan.org 
> > wrote:
>
>> Another issue is that it's not completely clear to me what is meant  
>> by
>> "While the user agent might still need network access to obtain  
>> parts of the
>> media resource<http://www.whatwg.org/specs/web-apps/current-work/#media-resource 
>> >..."
>> What if there is data in the resource that we don't need in order to
>> play through normally, but which might be needed in some special  
>> situations
>> (e.g., enabling subtitles, or seeking using an index), and we  
>> optimize to
>> not load that data unless/until we need it? In that case would we  
>> never
>> reach NETWORK_LOADED?
>
> As I understand it, NETWORK_LOADED means that all bytes of the  
> resource have been loaded, regardless of whether they will be used  
> or not. Are there any formats that would actually allow not  
> downloading parts of the resource in a meaningful way?

   Yes. A disabled track in an MPEG-4 or QuickTime file is not  
rendered so the data is not used when presenting the movie. Media data  
for an enabled but invisible video track (eg. size 0x0, or not within  
the visible region) or an enabled but muted audio track isn't  
technically needed for the presentation either.


> Subtitles and indexes are too small to bother, and multiplexed audio/ 
> video tracks can hardly be skipped without zillions of HTTP Range  
> requests. It seems to me that kind of thing would have to be done  
> either with a server side media fragment request (using the 'track'  
> dimension) or with an external audio/video track somehow synced to  
> the master track (much like external subtitles).
>
   I don't agree that this is necessarily best done on a server. Some  
file formats include tables with the location of every sample, so a  
media engine that uses range requests anyway can easily read just the  
data needed. It might be wise for such an engine to optimize the size  
of chunks read from the server, but that is an implementation detail.

   Also remember that "multiplexed" is a relative term, different  
chunking/interleaving schemes make sense for different media types and  
use cases so not all multiplexed files interleave data in small chunks.


>> In general NETWORK_LOADED and the "load" event seem rather useless  
>> and
>> dangerous IMHO. If you're playing a resource that doesn't fit in  
>> your cache
>> then you'll certainly never reach NETWORK_LOADED, and since authors  
>> can't
>> know the cache size they can never rely on "load" firing. And if  
>> you allow
>> the cache discarding behavior I described above, authors can't rely  
>> on data
>> actually being present locally even after "load" has fired.

   If data can be evicted from the cache you can never reach  
NETWORK_LOADED because "Network connectivity could be lost without  
affecting the media playback".


>> I suspect many
>> authors will make invalid assumptions about "load" being sure to  
>> fire and
>> about what "load" means if it does fire. Does anyone have any use  
>> cases that
>> "load" actually solves?
>
   I also agree that the 'load' event and the NETWORK_LOADED state are  
not terribly useful and will likely cause a great deal of confusion  
for developers. We have have seen a number of cases where experienced  
web developers have used the 'load' event when they should have used  
the 'canplaythough', and I fear that this will be a common mistake.


> I agree, sites that depend on the load event sites will likely break  
> randomly for file sizes that usually barely fit into the cache of  
> the browser they were tested with. If browsers are conservative with  
> bandwidth and only send the load event when it's true, I think we  
> will have less of a problem however.
>

   I don't agree that it will be any less of a problem if browsers are  
conservative, users will still not *ever* be able to depend on the  
'load' event firing (except perhaps for local files).


> Note that the load event isn't strictly needed, waiting for a  
> progress event with loaded==total would achieve the same thing.
>
   Actually, a progress event with loaded==total tells you even less  
than the 'load' event because it doesn't guarantee that the data won't  
be evicted from the cache.


>  Aesthetically, however, I think it would be strange to not have the  
> load event.

   I am not worried about the aesthetics of not having the event.  I  
am somewhat concerned about existing content that uses it (including  
many of the WebKit layout tests :-( ), but I think we will be better  
off in the long run if we get rid of the event and network state now.

eric

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