Re: Resolving TextTrackCue issues

+1


On Sat, Aug 31, 2013 at 1:26 AM, Silvia Pfeiffer
<silviapfeiffer1@gmail.com>wrote:

> 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
>
>