W3C home > Mailing lists > Public > public-media-capture@w3.org > August 2012

RE: MediaStreams and src attributes

From: Travis Leithead <travis.leithead@microsoft.com>
Date: Wed, 22 Aug 2012 18:48:31 +0000
To: Adam Barth <w3c@adambarth.com>, "public-media-capture@w3.org" <public-media-capture@w3.org>
CC: Robert O'Callahan <robert@ocallahan.org>, Anant Narayanan <anant@mozilla.com>
Message-ID: <9768D477C67135458BF978A45BCF9B383842ABE5@TK5EX14MBXW602.wingroup.windeploy.ntdev.microsoft.com>
> From: Adam Barth [mailto:w3c@adambarth.com]
> Eric Rescorla mentioned to me in Firefox you can directly assign a
> MediaStream to the src property of a media element.  I had assumed that
> meant Firefox was implicitly calling createObjectURL in order to serialize the
> MediaStream into a DOMString that could be represented in the src attribute
> of the media element.  However, according to some experiments and to
> <http://lists.w3.org/Archives/Public/public-webrtc/2012Mar/0036.html>,
> what's actually going on is that assigning a MediaStream to the src property
> stops that property from reflecting the underlying DOM attribute.
> The main problem with this approach is that it breaks the semantics of the src
> property.  Previously, the src property of HTMLMediaStream was a normal
> reflection property, like countless other reflection properties throughout the
> DOM, including many commonly used src properties on other DOM
> interfaces.  This is problematic for two
> reasons:
> 1) Breaking the reflection semantics of the src property just for
> HTMLMediaElement is inconsistent with how the rest of the DOM works.
> It's unclear why HTMLMediaElement should have a special, magical src
> property that works different from the src properties of
> HTMLImageElement, HTMLScriptElement, HTMLIFrameElement, etc, etc, etc.
>  For example, Anne van Kesteren has argued (in a slightly different
> context) "the more we tear down what little consistency is left, the harder it
> will be for people to learn and fully grasp what exactly it is we have here." [1]
> 2) In working with web developers, I've often found that many people do
> not have a clear conceptual model of the difference between reflection
> properties and DOM attributes.  They use the two interchangeably and don't
> really conceptualize them as separate concepts.  I take that to be a success of
> the reflection mechanism: it works so seamlessly that folks don't need to
> worry about it.  Breaking the reflection semantics in this case will confuse and
> frustrate these developers because they won't understand why assigning to
> the src property works but setting the src attribute does not.
> If you agree with me that we shouldn't break the reflection semantics of the
> src attribute, there appear to be at least two alternatives:
> A) Instead of breaking the src property, we can introduce a new property, for
> example "stream", with the type MediaStream.  Rather than assigning to the
> src property, developers would instead assign to the stream property, which
> would not have reflection semantics.
> B) We can serialize the MediaStream into a DOMString so that it can be
> reflected into the src attribute.  As discussed previously, we wouldn't want
> this serialization to be implicit because we want to help developers avoid
> resource leaks.  The natural approach here is to use createObjectURL to
> explicitly serialize the MediaStream into a URL, which can then be assigned to
> the src property and reflected into the src attribute.


Agree with everything here. Note that the current spec [1] does not define the direct-assignment in the way that implementors have chosen. I'd strongly recommend that existing implementations of this approach be updated to adhere to the current draft spec.

[1] http://dev.w3.org/2011/webrtc/editor/getusermedia.html
Received on Wednesday, 22 August 2012 18:49:03 UTC

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