RE: Resolving TextTrackCue issues

Hey Silvia,





This seems reasonable for Mozilla's implementation. Mozilla has been


following whatwg's track specification and the newest WebVTT spec.





However, we haven't implemented the abstraction yet. Instead, what


we have done is expose a single VTTCue class. So the only thing that


is different from the spec in Firefox that the user will see is that


VTTCue.prototype !== TextTrackCue.





We do intend to support the abstraction though when there are concrete


uses of it in the spec.





Rick

> From: silviapfeiffer1@gmail.com
> Date: Sat, 31 Aug 2013 17:26:51 +1000
> To: public-html@w3.org
> Subject: Resolving TextTrackCue issues
> 
> Hi all,
> 
> Recent changes to the TextTrackCue interface had led to a fork with
> the WHATWG spec [1] when resolving bug 21851 [2].
> 
> This caused extensive discussion on blink-dev [3] when an intent to
> implement was proposed.
> 
> In the W3C WG we recognize the need for a generic cue interface type
> with a constructor and a text attribute. It allows browsers to expose
> cues in text tracks of video or audio files for which browsers don't
> intend to implement parsers. It also allows JavaScript developers to
> create time-synchronized data for media elements in any format they
> require.
> 
> The discussion on blink-dev exposed that the currently specified
> solution of bug 21851 [2] in the HTML5 spec is flawed in several ways:
> 
> (1) TextTrackCue objects that are not fully abstract create hard to
> debug issues of backwards compatibility due to existing code that
> assumes "new TextTrackCue()" constructs a cue with VTT semantics;
> (2) in order to transition old TextTrackCue interface usage to "new
> VTTCue()", it is better to remove the existing TextTrackCue
> constructor causing hard failure (easily recognizable) instead of soft
> failure (more difficult to recognize);
> (3) the abstract TextTrackCue interface of the WHATWG is desirable for
> extensibility to non-text-based cue interfaces of the future;
> (4) the interface fork between the WHATWG and W3C spec should be removed.
> 
> An alternative resolution to bug 21851 [2] has previously been
> proposed and discussed: create a new interface that has the text
> attribute and the constructor and inherits from the abstract
> interface.
> 
> This will result in the following interfaces:
> 
> interface TextTrackCue : EventTarget {
>   readonly attribute TextTrack? track;
> 
>            attribute DOMString id;
>            attribute double startTime;
>            attribute double endTime;
>            attribute boolean pauseOnExit;
> 
>            attribute EventHandler onenter;
>            attribute EventHandler onexit;
> };
> 
> [Constructor(double startTime, double endTime, DOMString text)]
> interface GenericCue : TextTrackCue {
>            attribute DOMString text;
> };
> 
> Whether VTTCue will inherit from GenericCue or from TextTrackCue will
> be resolved in the TextTrack CG once this change has been applied to
> the HTML5 spec.
> 
> It is my understanding that this proposed change resolves all the
> above listed issues. I will therefore apply these changes next week
> unless there are any further concerns.
> 
> Regards,
> Silvia (as HTML spec editor).
> 
> [1] https://www.w3.org/Bugs/Public/show_bug.cgi?id=22903
> [2] https://www.w3.org/Bugs/Public/show_bug.cgi?id=21851
> [3] https://groups.google.com/a/chromium.org/d/msg/blink-dev/-VHGnuNNUxM/Yibbv2TgDoYJ
>