[web-animations-2] Add range offset APIs

[fantasai]

See #7589 and #7637.

[fantasai]

First question to @flackr: Did we decide to do #7589 (comment) ?

[flackr]

First question to @flackr: Did we decide to do #7589 (comment) ?

Yes, I think we're good with this. Note that this type will be for keyframe offsets. The delay and endDelay will remain as is (not referring to ranges) and we'll add rangeStart and rangeEnd to KeyframeAnimationOptions.

[flackr]

Yes, I think we're good with this. Note that this type will be for keyframe offsets. The delay and endDelay will remain as is (not referring to ranges) and we'll add rangeStart and rangeEnd to KeyframeAnimationOptions.

I'm somewhat confused by whether CSSNumericValue and TimelineRangeOffset are indeed distinguishable types. I think it may be okay as interface is distinguishable from interface as long as no platform object implements both right?

[tabatkins]

TimelineRangeOffset is a dictionary, so it can't be "implemented by" anything, it's just a pattern used to destructure an input argument (or construct a plain JS object matching a pattern, for output). So that avoids the one possible case where an interface object like CSSNumericValue isn't distinguishable. And if you really meant CSSNumberish, the numeric types are also distinguishable from dictionaries so we're good.

[flackr]

Thanks for confirming @tabatkins. I had a feeling that most interface types were fancy destructuring but then I was worried CSSNumericValue might just be fancy destructuring too and create possible ambiguity.

[tabatkins]

Interface types are specifically not fancy destructuring, those actually check that it's the correct object type - vended by the correct window object, even! Dictionary is just similar to sequence<> or record<> in that it's fancy destructuring.

[fantasai]

Alright, this is ready for review. :) I tried my best to guess what the right thing to do was for #7637 ; not sure if I got all the details right.

[birtles]

I don't follow what this sentence means (or really this paragraph). What does interpreted mean?

[fantasai]

It means follow the definitions in https://drafts.csswg.org/scroll-animations-1/#animation-range-start for turning the rangeName and offset into a point on the timeline. I didn't want to inline those instructions, because they're changing as we adjust the syntax, defaulting rules, and capabilities of animation-range.

[birtles]

For better or worse this spec is pretty procedural, especially from "Programming interface" onwards, so that you can mostly just convert spec prose into implementation code. I'm just wondering what an implementer is supposed to do with "interpreted as".

From the point of view of this interface, as long as rangeName and rangeOffset get set to the right values, I think we're done. How those values are applied goes somewhere else (ideally somewhere in Web Animations which might then refer to definitions in scroll animations).

[fantasai]

How those values are applied goes somewhere else (ideally somewhere in Web Animations which might then refer to definitions in scroll animations).

Well, yeah, but that needs to be defined somewhere, and we don't have the conceptual infrastructure yet to reference it here. I can move the interpretation out of the definition here and put it in a separate paragraph, but I can't update Web Animations 2 to define the attachment range concepts without a significant amount of additional editing work in sections that have not yet been ported over.

[birtles]

Thanks, yeah, I think we should keep the description here just to how the value gets set. If we don't yet have anywhere to park the description of how those values are applied, we could add it here as a note of some sort indicating that it eventually needs to live somewhere else.