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[css-animations-2] Specify the animation-trigger property #10128

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[css-animations-2] Specify the animation-trigger property #10128

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133f6e0
[css-animations-2] Specify the animation-trigger property
ydaniv Mar 25, 2024
f3a49ba
Fixed syntax of single-animation-trigger + some link fixes
ydaniv Mar 25, 2024
cbd4f07
More fixes to links and defs
ydaniv Mar 27, 2024
9f6464a
More fixes to links and defs
ydaniv Mar 28, 2024
912426a
Move all definitions to web-animations-1;
ydaniv Apr 17, 2024
9ae1a13
Fixed idle state definition for animation trigger state
ydaniv Apr 18, 2024
f1d2e89
Moved all new definitions from web-animations-1 to web-animations-2
ydaniv May 11, 2024
e55e476
Added a description to each property of animation-trigger;
ydaniv Jun 2, 2024
a0e90fe
Fixed some syntax errors
ydaniv Jun 6, 2024
361f5d9
Added an issue about timeline becoming idle and did-trigger resetting…
ydaniv Jun 6, 2024
efa38c7
Define animation trigger active interval
ydaniv Jun 6, 2024
085618e
Merge from main
ydaniv Jun 6, 2024
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169 changes: 169 additions & 0 deletions css-animations-2/Overview.bs
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Expand Up @@ -68,6 +68,9 @@ spec:web-animations-1; type:dfn;
text:start delay
text:target element
text:unresolved
spec:web-animations-2; type:dfn;
text:default range
text:exit range
</pre>

<h2 id="delta">Delta specification</h2>
Expand Down Expand Up @@ -690,7 +693,173 @@ The 'animation' shorthand property syntax is as follows:

<span class=prod><dfn>&lt;single-animation></dfn> = <<'animation-duration'>> || <<easing-function>> || <<'animation-delay'>> || <<single-animation-iteration-count>> || <<single-animation-direction>> || <<single-animation-fill-mode>> || <<single-animation-play-state>> || [ none | <<keyframes-name>> ] || <<single-animation-timeline>></span>

<h2 id="animation-triggers">
Declaring Animation Triggers</h2>

Animation Triggers are defined using the 'animation-trigger-*' properties.
These list-valued properties,
which are all [=longhands=] of the 'animation-trigger' [=shorthand=],
form a [=coordinating list property group=]
with 'animation-name' as the [=coordinating list base property=]
and each item in the [=coordinated value list=]
defining the properties of a single animation trigger.
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If this spec doesn't explain how they work but only mentions the relevant properties, it should point to the section in the web-animations-2 spec which explains what animation triggers are and how they work. E.g. this could be as simple as having "animation triggers" link to the section in web animations, similar to how the animation composition property points to the web-animations definition of what a composite operation is.

That said, it's not uncommon to have a plain English explanation for what it means with the details and algorithms left to web-animations. E.g. to take a random property the definition of animation-fill-mode explains what it does without going into the specific algorithmic details that are in the web-animations spec.

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I like both. I guess I'll refactor the text a bit and add a link.

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done


## The 'animation-trigger-type' property ## {#animation-trigger-type}

The 'animation-trigger-type' property specifies the [=animation trigger type=]
of the [=animation trigger=].

<pre class='propdef'>
Name: animation-trigger-type
Value: <<single-animation-trigger-type>>#
Initial: once
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item a keyword as specified
Animation type: not animatable
Canonical order: per grammar
</pre>

<span class=prod><dfn>&lt;single-animation-trigger-type></dfn> = once | repeat | alternate | state</span>
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Can you add a description of what each property value means. Also we probably need a none value on this property to represent the default case right?

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I was pondering all the none/normal/auto options, and I think we currently don't need them, since the initial value is once, with a timeline value of auto, which I think is exactly what we have today.
We could also add an issue on that, in case it needs an approval from the group.

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The definition of the values is done here.
Should I add here another description? More user/less implementor facing?

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I was pondering all the none/normal/auto options, and I think we currently don't need them, since the initial value is once, with a timeline value of auto, which I think is exactly what we have today.

Yeah, I like having the default values basically explain the triggering behavior we get today and adding the ability to modify it. It makes it feel like less of an added on property and more just part of the standard animation infrastructure.

We could also add an issue on that, in case it needs an approval from the group.

I think this can be part of the general review when it's all written up.

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Can you add a description of what each property value means.

Done


## The 'animation-trigger-timeline' property ## {#animation-trigger-timeline}

The 'animation-trigger-timeline' property specifies the <a>timeline</a>
of the animation’s associated [=animation trigger=].

<pre class='propdef'>
Name: animation-trigger-timeline
Value: <<single-animation-timeline>>#
Initial: auto
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item either
the keyword ''single-animation-timeline/none'',
the keyword ''single-animation-timeline/auto'',
a case-sensitive [=css identifier=],
a computed ''scroll()'' function,
or
a computed ''view()'' function
Canonical order: per grammar
Animation Type: not animatable
</pre>

Issue: Probably a trigger with timeline "none" is under-specified here. Need to clarify what it means.
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I wonder if the default timeline is the document timeline set up in a way such that the "trigger" is always active explaining the normal behavior of animations? You could imagine then using this as a way to coordinate starting an animation after some length of time, e.g. animation-trigger-range: 10s. Admittedly this has some overlap with animation-delay, but unlike animation delay could use absolute times since page load or other events I suppose?

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Well, I remember we considered this in a previous issue specifically on syncing animations, but we said there that this will be the stuff that will eventually be done via GroupEffects. That said, if you see some way of changing the definition here and keeping this more open for later extensions I'm all for it.

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but unlike animation delay could use absolute times since page load or other events I suppose?

I feel like we'll still the pseudo-selector you suggested in #8175, for example, to defer until DOM and assets are loaded. This is pretty common.

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I think we should remove the none state, I'm not sure what it even means and the initial value is auto so it's not clear what you'd use none for. With the initial value auto, as you've said it has the same meaning as values of animation-timeline and as such would map to the document timeline. I think we should try to go with this model as it explains the existing behavior.

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OK, but none here is part of the single-animation-timeline syntax. Should we remove it just here? Or entirely?


## The 'animation-trigger-range' property ## {#animation-trigger-range}

The 'animation-trigger-range' property is a [=shorthand property|shorthand=]
that sets 'animation-trigger-range-start' and 'animation-trigger-range-end'
together in a single declaration,
specifying the [=animation trigger=]’s associated [=default range=].

<pre class="propdef shorthand">
Name: animation-trigger-range
Value: [ <<'animation-range-start'>> <<'animation-range-end'>>? ]#
</pre>

## The 'animation-trigger-range-start' property ## {#animation-trigger-range-start}

<pre class="propdef">
Name: animation-trigger-range-start
Value: [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]#
Initial: normal
Applies to: all elements
Inherited: no
Percentages: relative to the specified [=named timeline range=] if one was specified, else to the entire timeline
Computed value: list, each item either the keyword ''animation-trigger-range-start/normal'' or a timeline range and progress percentage
Animation type: not animatable
</pre>

The 'animation-trigger-range-start' property specifies the start of the [=animation trigger=]’s
associated [=default range=].

## The 'animation-trigger-range-end' property ## {#animation-trigger-range-end}

<pre class="propdef">
Name: animation-trigger-range-end
Value: [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]#
Initial: normal
Applies to: all elements
Inherited: no
Percentages: relative to the specified [=named timeline range=] if one was specified, else to the entire timeline
Computed value: list, each item either the keyword ''animation-trigger-range-end/normal'' or a timeline range and progress percentage
Animation type: not animatable
</pre>

The 'animation-trigger-range-end' property specifies the end of the [=animation trigger=]’s
associated [=default range=].

## The 'animation-trigger-exit-range' property ## {#animation-trigger-exit-range}

<pre class="propdef shorthand">
Name: animation-trigger-exit-range
Value: [ <<'animation-trigger-exit-range-start'>> <<'animation-trigger-exit-range-end'>>? ]#
</pre>

The 'animation-trigger-exit-range' property is a [=shorthand property|shorthand=]
that sets 'animation-trigger-exit-range-start' and 'animation-trigger-exit-range-end'
together in a single declaration,
specifying the [=animation trigger=]’s associated [=exit range=].

## The 'animation-trigger-exit-range-start' property ## {#animation-trigger-exit-range-start}

<pre class="propdef">
Name: animation-trigger-exit-range-start
Value: [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]#
Initial: normal
Applies to: all elements
Inherited: no
Percentages: relative to the specified [=named timeline range=] if one was specified, else to the entire timeline
Computed value: list, each item either the keyword ''animation-trigger-exit-range-start/normal'' or a timeline range and progress percentage
Animation type: not animatable
</pre>

The 'animation-trigger-exit-range-start' property specifies the start of the [=animation trigger=]’s
associated [=exit range=].

## The 'animation-trigger-exit-range-end' property ## {#animation-trigger-exit-range-end}

<pre class="propdef">
Name: animation-trigger-exit-range-end
Value: [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]#
Initial: normal
Applies to: all elements
Inherited: no
Percentages: relative to the specified [=named timeline range=] if one was specified, else to the entire timeline
Computed value: list, each item either the keyword ''animation-trigger-exit-range-end/normal'' or a timeline range and progress percentage
Animation type: not animatable
</pre>

The 'animation-trigger-exit-range-end' property specifies the end of the [=animation trigger=]’s
associated [=exit range=].

## The 'animation-trigger' property ## {#animation-trigger}
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The 'animation-trigger' property is a [=shorthand property|shorthand=]
that sets 'animation-trigger-type', 'animation-trigger-timeline',
'animation-trigger-range-start', 'animation-trigger-range-end',
'animation-trigger-exit-range-start', and 'animation-trigger-exit-range-end'
together in a single declaration,
specifying the [=animation trigger=] for an animation.

<pre class='propdef'>
Name: animation-trigger
Value: <<single-animation-trigger>>#
Initial: see individual properties
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: see individual properties
Canonical order: per grammar
Animation Type: not animatable
</pre>

<pre class=prod>
<dfn>&lt;single-animation-trigger></dfn> = <<single-animation-trigger-type>> || [ none | auto | [ [ <<dashed-ident>> | <<scroll()>> | <<view()>> ] [ normal | <<length-percentage>> | <<timeline-range-name>> <<length-percentage>>? ]{0,4} ] ]
</pre>

# Animation Events # {#events}

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3 changes: 3 additions & 0 deletions web-animations-1/Overview.bs
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Expand Up @@ -772,6 +772,9 @@ Animation Frames {#animation-frame-loop}
* Updating the [=animation/current time=] of any [=animations=]
[=associated with a timeline|associated with=] the timeline.

* Updating the [=animation trigger state=] of any [=animation trigger=]
[=associated with a timeline|associated with=] the timeline.
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This can move to web-animations-2 right? Also I think this needs to happen before updating the times of animations so that we can set their start time and know to make their effects active if they've been triggered.

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Well the animation-frame-loop definition exists only here, and I had to only add this one bullet. So what would be the proper way to add it to web-animations-2? Should I add just this delta somehow? Another way?

Regarding order, yes, I'll pull it up to happen before updating animations.

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Yeah, we'd add a delta for this section to the ewb-animations-2 spec.

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done


* Running the [=update an animation's finished state=] procedure
for any animations whose [=animation/current time=] has been updated.

Expand Down
139 changes: 138 additions & 1 deletion web-animations-2/Overview.bs
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Expand Up @@ -85,7 +85,7 @@ Abstract: This specification defines a model for synchronization and
specifications will define declarative means for exposing these
features.
Ignored Vars: auto-rewind, current direction, index, initial progress,
timeline, new timeline, t, going forwards
timeline, new timeline, t, going forwards, did trigger
</pre>
<pre class="anchors">
urlPrefix: https://drafts.csswg.org/web-animations-1/; type: dfn; spec: web-animations-1
Expand Down Expand Up @@ -122,6 +122,7 @@ urlPrefix: https://drafts.csswg.org/web-animations-1/; type: dfn; spec: web-anim
text: play an animation
text: playback direction
text: pause an animation
text: reverse an animation
text: ready
text: set the timeline of an animation
text: simple iteration progress
Expand Down Expand Up @@ -2353,6 +2354,142 @@ Items sorted earlier are executed before those sorted later.

</ins>

Animation Triggers {#triggers}
-----------------

### Overview ### {#triggers-overview}

An <dfn export>animation trigger</dfn> is used to control the playback
of its associated [=animation=] for time-driven animations.
Like animations, [=animation triggers=] are associated with a [=timeline=]
and are attached to an [=animation attachment range|attachment range=].

Issue: Should there be any effect of triggers on scroll-driven animations?

### Animation Trigger State ### {#trigger-state}

An [=animation trigger=] |trigger| with [=animation trigger type=] |type|
and an associated [=animation=] |animation|
has an internal <dfn export lt="animation trigger state">state</dfn> that can be one of the following:

<dl dfn-for="animation trigger state" dfn-type=value>
<dt><dfn>idle</dfn>
<dd>
The [=animation effect=] associated |animation| remains in
its [=animation effect/before phase=] and stays at zero [=animation/start time=].

<dt><dfn>primary</dfn>
<dd>
The ''primary'' effect defined by |type| is applied to |animation|.
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There's a bit of a mismatch between this and the text in css-animations-2 where the text for single-animation-trigger-type specifies when the effect is applied. The text above implies that the effect is applied continually while in the state, but the text in single-animation-trigger-type implies that the effect is applied at a discrete moment in time. If all of the effects are discrete then I think it would be simpler to have the effect explicitly only invoked when the trigger's state changes.


<dt><dfn>inverse</dfn>
<dd>
The ''inverse'' effect defined by |type| is applied to |animation|.

</dl>

Issue: Do we need a formal resolution on the spec of the idle state?

The [=animation trigger state=] |state| of an [=animation trigger=] |trigger|
is controlled by the [=animation attachment range|attachment range=] it is attached to,
given a flag |did trigger|, as follows:
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I think you need to say something about how when the current time of the trigger timeline is changed, we run the following steps. We also need to say how did trigger is stored, it's probably state associated with the animation trigger? E.g. if i assign a new trigger to an animation i probably want to reset the animation trigger.


<dl class=switch>
: If |trigger|’s [=local time=] is [=unresolved=],
::
Then |state| is ''animation trigger state/idle'', and |did trigger| is set to false.
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Wouldn't setting did trigger to false this reset once triggers anytime the timeline goes idle (e.g. anytime a ScrollTimeline's scroller becomes large enough that it doesn't scroll)? I think this is probably unexpected.

Note, it doesn't look like these links, e.g. ''animation trigger state/idle'' have linked to the definitions correctly.

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Interesting! I'm not sure whether this is unexpected. I guess we should open another issue for that.

Regarding the links, I'll check and fix.

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Wouldn't setting did trigger to false this reset once triggers anytime the timeline goes idle (e.g. anytime a ScrollTimeline's scroller becomes large enough that it doesn't scroll)? I think this is probably unexpected.

Gave this another thought. I think this should be expected, but definitely worth opening an issue and discuss this explicitly.

Note, it doesn't look like these links, e.g. ''animation trigger state/idle'' have linked to the definitions correctly.

Right, appears as though these aren't linked correctly, but I used the suggested syntax, so not sure what to do.

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On re-reading this, I realized the did trigger flag actually says nothing about whether it will retrigger once animations, and the text the definition for once says:

The animation is triggered and played once and only once.

Which implies that once animations are not retriggered by this, which I think addresses my primary concern.

I was thinking about whether resizing the scroller / browser such that the area is no longer scrollable should be treated differently than exiting the exit range? My thinking is it shouldn't be any different. You by definition can't be in the exit range. With this in mind I'm not exactly sure what did trigger is trying to accomplish other than perhaps giving different behavior to being outside the active range to start, and going inactive. If this is the case, I think we should just consider the previous state rather than introduce this did trigger variable. E.g. if you're outside the active range and you have no previous state or your previous state was idle, you're still idle.


: Otherwise,
::

<dl class=switch>
: If |trigger| is inside its [=active interval=],
::
Then |state| is ''animation trigger state/primary'', and |did trigger| is set to true.

: Otherwise,
::

<dl class=switch>
: If |did trigger| flag is false,
::
Then |state| is ''animation trigger state/idle''.

: Otherwise,
::
|state| is ''animation trigger state/inverse''.

</dl>

</dl>

</dl>

### Animation Trigger Types ### {#trigger-types}

The behavior an [=animation trigger=] applies to its associated [=animation=]’s
playback is defined by its <dfn export>animation trigger type</dfn>,
which can be one of the following:

<dl dfn-type=value dfn-for='animation trigger type'>
<dt><dfn>once</dfn>
<dd>
The ''primary'' behavior is [=play an animation|triggering=] the associated animation.
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I think this needs to define how the behavior is only applied once. E.g. the suggested triggered state associated with an animation, initially false


<dt><dfn>repeat</dfn>
<dd>
The ''primary'' behavior is [=play an animation|triggering=] the associated animation.
The ''inverse'' behavior is resetting the [=animation trigger state=] back to ''animation trigger state/idle''.

<dt><dfn>alternate</dfn>
<dd>
<dl class=switch>
: if the |did trigger| flag is false,
::
The ''primary'' behavior is [=play an animation|triggering=] the associated animation.

: Otherwise,
::
The ''primary'' behavior is [=reverse an animation|reversing=] the associated animation.

</dl>

The ''inverse'' behavior is [=reverse an animation|reversing=] the associated animation.

<dt><dfn>state</dfn>
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I wonder if something like play-pause would be clearer.

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That's another option. The idea behind state was a reference to play-state.

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Right, I could see play-state as an alternative. State feels too strange, I didn't have any idea what this meant until I read the description :-).

<dd>
The ''primary'' behavior is [=play an animation|triggering or resuming=] the associated animation.
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I think it would be more consistent if state only changes the animation play state, i.e. the animation would be started in a paused state until the active trigger range is entered.

The ''inverse'' behavior is [=pause an animation|pausing=] the associated animation.
</dl>

Issue: Need to bike-shed the name for type "state" type.

### Animation Trigger Ranges ### {#trigger-ranges}

An [=animation trigger=] has at least one [=animation attachment range|range=],
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I think we should define a new term for the trigger interval's active range, as the combination of the two ranges would probably be worth eventually having a diagram to illustrate.

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Sounds good, I'll try adding that.

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done

its <dfn>default range</dfn>, which defines its [=active interval=].
This range defines when the [=animation trigger|trigger=] is in its ''animation trigger state/primary'' state,
as specified in [[#trigger-state]].

An [=animation trigger=] may also have a second [=animation attachment range|range=],
the <dfn>exit range</dfn>. The [=exit range=] is used to replace [=default range=] and extend
the [=active interval=] of an [=animation trigger=] |trigger| as follows:

<dl class=switch>
: If |trigger| is inside its [=active interval=],
::
It’s attached to its [=exit range=].

: Otherwise,
::
It’s attached to its [=default range=].

</dl>

Issue: Should we specify that that exit range’s edges can only be equal to or greater than the default range’s edges?

Issue: Do we need to specify the process for determining the effective attachment range in the HTML Processing Model?

<!-- End of animation model -->

<h2 id="programming-interface">Programming interface</h2>
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