[web-animations-2] Make TimelineTrigger interface support multiple timelines

[DavMila]

A bit of background due to the out-of-date AnimationTrigger spec: since the AnimationTrigger interface was initially written, we've resolved to have separate CSS property namespaces for different types of animation triggers, i.e. event-trigger, timeline-trigger. Accordingly, the web-animations spec will be updated to have different interfaces, i.e. EventTrigger, TimelineTrigger. TimelineTrigger will likely resemble/replace the current AnimationTrigger interface (which was initially conceived for just timelines).

One of the future extensions to the timeline-trigger property will be support for multiple timelines. This will be important for use cases such as pointer timelines whose use cases are likely to hinge on 2-D pointer intersections, e.g. an author can declare a trigger range as follows: timeline-trigger: --pointer-trigger pointer(x) pointer(y) where pointer(x) and pointer(y) declare pointer-timelines.

Given this, I propose the TimelineTrigger interface (currently in the outdated spec as AnimationTrigger) have its members as lists rather than single items, i.e. the following:

interface TimelineTrigger {
   attribute sequence<AnimationTimeline> timelineList;
   attribute sequence<any> rangeStartList;
   attribute sequence<any> rangeEndList;
   attribute sequence<any> exitRangeStartList;
   attribute sequence<any> exitRangeEndList;
}

cc/ @ydaniv @tabatkins @flackr

[DavMila]

Here is a summary of the change:

The current interface is this:

interface TimelineTrigger {
   attribute AnimationTimeline timeline;
   attribute any rangeStart;
   attribute any rangeEnd;
   attribute any exitRangeStart;
   attribute any exitRangeEnd;
}

I am proposing we change it to the following:

interface TimelineTrigger {
   attribute sequence<AnimationTimeline> timelineList;
   attribute sequence<any> rangeStartList;
   attribute sequence<any> rangeEndList;
   attribute sequence<any> exitRangeStartList;
   attribute sequence<any> exitRangeEndList;
}

Concerning the choice of *List as a way to pluralize the fields, we have some precedents in HTMLElement.relList and Element.classList.

[flackr]

Agree we should support multiple sources since this is a likely extension to support cases like 2d timelines. It feels a bit odd that it is a bunch of coordinated lists. Would it make sense to have a list of trigger conditions or sources in the top level trigger interface? E.g. something like this:

interface TimelineTrigger {
  attribute sequence<TimelineTriggerRange> ranges;
}

interface TimelineTriggerRange {
   attribute AnimationTimeline timeline;
   attribute any rangeStart;
   attribute any rangeEnd;
   attribute any exitRangeStart;
   attribute any exitRangeEnd;
}

Open to bikeshedding on the term ranges.

[ydaniv]

Regarding timeline => timelineList, LGTM!

Regarding listfying each range edge separately, I'm not sure this is the ideal API we want to allow, but not sure if I have a better alternative...

Maybe what @flackr suggested?