Re: [css3-animation] why content property not animatable?

On Wed, Oct 8, 2014 at 4:26 PM, Sylvain Galineau <galineau@adobe.com> wrote:

>
> On Oct 8, 2014, at 10:35 AM, Tab Atkins Jr. <jackalmage@gmail.com> wrote:
>
> > On Thu, Oct 2, 2014 at 3:43 AM, Tim Larson <Tim.Larson@telventdtn.com>
> wrote:
> >> Background: I had the idea recently to implement a simple "busy
> spinner" using text. Cycling through a sequence like the characters U+25F0
> through U+25F3, or even 0 through 9, seems like a good way to convey this
> idea. Using text eliminates the need for downloading an image, and this
> would also theoretically be possible in a console UA.
> >>
> >> So after I'd whipped up the @keyframes declaration to try it out, I was
> surprised to notice that the content property is not animatable. Why is
> that? Of course, one could code it up in JS, but it would be so much
> simpler to do it declaratively, this ought to be a CSS solution.
> >>
> >> I've been trying to find the answer to why some properties can be
> animated but others cannot. Perhaps some just wouldn't make sense, but this
> one does.
> >
> > Originally we only made properties animatable if they lived in a
> > reasonable metric space - we needed some way to compute distance and
> > represent intermediate values at all points of the animation.  This
> > covers everything whose value is a number or number-like things (such
> > as colors).
> >
> > We've planned for some time to allow all properties to be animatable,
> > even when they don't have a metric.  Instead, they'd just switch from
> > their start to end value halfway through the timing-function progress.
> > This is still the plan of record, but nobody's tried out the
> > implementation yet to see if there are any compat issues that we'd
> > have to address.
> >
> What Tab said; we can't really 'animate' from any content property value
> to any other as in "Let's compute all the intermediate values to go smoothy
> from content:'foo' to content:'bar'". We can merely pick a consistent time
> at which one value replaces the other when you specify these in @keyframes.
>
> Yes, in some cases you want to go through a series of strings that could
> be interpolated e.g. because you just want to animate through a contiguous
> sequence of single-character things. That'd solve your problem, but is it
> something we can always assume any time the start/end values of the content
> property are single characters? If I want to turn bullets in a custom list
> control into checkmarks as part of an animation I probably do not want all
> the intermediate code points in between the bullet and the checkmark to
> show up in between. Or maybe I do want to animate through a small series of
> characters in an icon font but their values are in fact not contiguous.
>
> So the challenge here is defining a default animation behavior that is
> generally useful for a wide subset of the values and use-cases people use
> the content property for. Iterating between Unicode code point value is not
> a good default. Though it'll be nice some of the time it'll likely be
> surprising and unwanted most of the time.
>

The SMIL/SVG <set/> animation element can handle discrete animation with no
problem Any property that can be set should be animatable by discrete
animation, with no interpolation.