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Re: Request for Comments: Proposal for Touch-Based Animation Scrubbing

From: Rik Cabanier <cabanier@gmail.com>
Date: Wed, 28 Nov 2012 20:23:03 -0800
Message-ID: <CAGN7qDB7Wyj8RcuDSF5L9Gs2nfOKm22UXEesj=u5e1d421_C3g@mail.gmail.com>
To: "Tab Atkins Jr." <jackalmage@gmail.com>
Cc: www-style list <www-style@w3.org>
Can you explain how your image carousel works?
Specifically, how are the animations on the images triggered and
synchronized by the timeline?

*<div class=’carousel’>

.carousel {
display: flexbox;
width: 600px;
timeline-gesture: “carousel” scroll-horizontal;
timeline-momentum: momentum;
timeline-length: 200px infinite;
.carousel > img {
width: 200px;
animation: carousel-translate 6s linear infinite;
animation-timeline: “carousel”;
@keyframes carousel-translate {
0% { transform: translateX(0); }
50% { transform: translateX(600px); }
50.01% { transform: translateX(-600px); }
100% { transform: translateX(0); }

On Wed, Nov 28, 2012 at 5:36 PM, Tab Atkins Jr. <jackalmage@gmail.com>wrote:

> I've been shopping this idea around in hallway conversation at the
> last two meetings, and it seems that other browsers don't think I'm
> crazy, so I've got a new proposal to put forth to all of you, for an
> extension to the way animations work.
> (This proposal is also located at
> https://docs.google.com/document/d/1vRUo_g1il-evZs975eNzGPOuJS7H5UBxs-iZmXHux48/edit
> if you want it in a more readable form, with worked-out examples.  No
> need for a Google account to view.  If the proposal evolves during
> discussion, that doc will also contain the most up-to-date version of
> the proposal.)
> Warning, this email is long.  If you want the gist, just read the
> intro or Problem section, and then the suggested new properties.  To
> save on further length, I've removed the worked-out example code from
> this email - you can find it at the Google Doc above.
> Touch-Based Animation Scrubbing
> ===============================
> Or, Scrolling Through Time
> This document introduces a set of CSS properties allowing authors to
> declaratively bind CSS animations to touch and scroll gestures,
> achieving a variety of common and useful effects.  This approach
> should allow a common set of interaction designs to be handled very
> easily, where today they require non-trivial JS, and should allow them
> to be done purely on the compositor thread, for minimum jank.  Even
> for animations that cannot be done purely on the compositor, these
> properties allow for the possibility of heavier optimization, as they
> avoid hanging the animation’s progress on the JS thread.
> The Problem
> -----------
> We can do CSS animations faster than JS animations, because there’s no
> need to invoke JS.  For a certain subset of CSS animations (ones that
> only animate a subset of “compositor-friendly” properties), we can do
> them *much* faster, and completely off the main thread, so JS-caused
> jank is completely eliminated.
> However, CSS animations are more-or-less static.  You define them
> up-front with their triggers, and they simply happen, with no ability
> to control them beyond pausing/removing them.  As soon as you want to
> do something animation-like that is tied to user interaction, you’re
> screwed - you’re forced back into JS animations (properties updated in
> a rAF loop), which incurs an inherent and unavoidable chunk of delay,
> and is subject to UI thread jank.  This makes it difficult to create
> good touch-based interactions, particularly on mobile where JS-jank is
> much more common.
> Prior Art
> ---------
> touch-based image carousels. Some of these are a finite stream, others
> cycle back to the beginning.  Sometimes these want to “drift” into
> showing the current image well.
> the Android pull-down notifications drawer
> the clank tab-list, where you can shift the tab list up and down,
> revealing or hiding individual tabs, or flick tabs sideways, causing
> them to curve downward and fade away
> “zoom-based” UIs, where scrolling down instead zooms the view:
> http://2011.beercamp.com/
> scroll-based animations, where pictures move around, reveal
> themselves, and otherwise change based on your current scroll
> position: http://www.milwaukeepolicenews.com/ and many, many others
> Twitter’s “pull-to-refresh” action on their mobile app, where
> attempting to overscroll past the top reveals some elements and
> triggers an XHR.
> Additions to Animations
> -----------------------
> animation-timeline: global | <string>;
> Add a new ‘animation-timeline’ property.  It accepts “global”, giving
> the current normal behavior of animations, or a <string> indicating a
> named timeline the animation will tie itself to.
> The concept of named timelines is a fairly general one.  This proposal
> will control the timeline declaratively, but there’s obvious
> possibilities of, for example, creating the timeline as a manipulable
> JS object, where scrubbing can be done directly in JS.
> New Scrubbing Properties
> ------------------------
> These names are very much tentative.  As well, it may end up being
> easier to build this functionality directly into scrolling/overflow
> behavior, as it hijacks scrolling for the element, rather than
> reinventing several concepts on our own.
> This is also a first draft of these properties, so the particular way
> I’m laying out the syntax shouldn’t be considered too important right
> now.  The general idea is what should be critiqued.
> timeline-gesture: [ <gesture> && <string> ]#
> timeline-length:  [
>                     <time> ||
>                     [ <length> | <percentage> ] ||
>                     [ <integer> | infinite ]
>                   ]#
> timeline-momentum: [ none | momentum ]#;
> timeline-notches: [ none | [ force | bias ] [ <time> | <percentage> ]+ ]#
> <gesture> = scroll-up/down/left/right, scroll-vertical/horizontal,
> overscroll-up/down/left/right, pinch-in/out, more?
> ‘timeline-gesture’ provides a list of timeline names and the gestures
> that manipulate them.  The same timeline name can be provided for
> multiple gestures in this property, or across multiple elements - it
> just means that the same timeline can be scrubbed in multiple ways.
> The scroll-up/down/left/right gestures can’t scrub a timeline into
> negatives, only 0+.  Once you start a scroll-up gesture in one
> direction, the other direction won’t activate until the first scrubs
> all the way back to zero.  That is, if you scroll down 200px, then up
> 300px, the effect is to scrub the scroll-down timeline by 200px, then
> back to 0, then scrub the scroll-up timeline by 100px.
> Scroll-vertical/horizontal can scrub a timeline into negatives.
> The overscroll-* gestures only scrub when you try to scroll “past the
> edge” of a scrollable container.
> The gestures, in general, are meant to be multi-modal.  For example,
> you should be able to trigger the scroll gestures through touch
> scrolling, scrollbar scrolling, mousewheel scrolling, keyboard
> navigation, whatever.
> ‘timeline-length’ sets up the conversion between scrolling distance
> and time, and how long the timeline is.  The length defaults to
> “100%”, which is the width/height of the element, and the time
> defaults to 1s.  The integer provided afterward specifies how many
> multiples of the length can be scrubbed before the timeline stops,
> defaulting to 1.
> ‘timeline-momentum’ controls whether, once you release the touch
> gesture, the timeline continues to drift or not.  Possibly this can be
> extended to specify the momentum curve.
> ‘timeline-notches’ establishes “notches” in the timeline, spots that
> have special behavior when you stop scrubbing.  The behavior of the
> notches is determined by the optional keyword at the start of the
> property: “bias” will adjust the momentum curve, if a scrub would land
> near a notch, to instead land exactly on the notch; “force” will cause
> a timeline to automatically drift back to the nearest notch when you
> stop actively scrubbing.
> We should probably add an animation event that fires when you hit a
> notch.  The notches will often correspond to important actions - for
> example, the 0% and 100% notches in the mobile Chrome tab-fling
> gesture indicate that the tab is closed - and authors will want
> reliable, easy ways to respond to them, since the actual end of the
> action won’t be easily observable (due to momentum and such).
> Known Issues
> ------------
> * Most of the time you’ll want the animation-timing-function to be
> “linear”, so that the animation tracks the finger well, but this isn’t
> the default.  Not much to do about this, I think.
> * Take the “200px down, 300px up” example from the description of
> ‘timeline-gesture’.  What happens if a scroll-down gesture isn’t
> registered at all?  Does scrolling down do nothing, then all 300px of
> the upward scrolling goes to the scroll-up gesture?  Or do we continue
> to track the total offset?  (I lean toward the former.)
> * If an element and a child have the same (or compatible) gestures
> registered, which wins?  Innermost or outermost?
> * Using 'timeline-length' to specify a conversion ratio between
> distance and time is a hack, but it's the shortest-path way to hook
> this into the current animation syntax.  Are there better things we
> can do?  Perhaps let 'animation-duration' accept a <string> to refer
> to a timeline?  However, we should still allow an animation to only
> occupy a fraction of a timeline, to make the "scrolling page
> animations" thing easier to set up and maintain.  What I currently
> have may be the best solution short of a larger re-architecting of the
> way animations work.
> * Some use-cases are annoyingly manual to set up.  For example, the
> image carousel requires you to know ahead of time the size of each
> item and how many there are, and spread this knowledge across a few
> places.  If the items aren’t all the same size, or if there aren’t
> enough of them to allow you to do a “safe” general wrap-around, you’ll
> have to create individual @keyframes rules for each item.  I’m not
> sure how to fix this without a more specialized solution that caters
> to this specific use-case (which might be appropriate to build on top
> of this).
> * What happens if an element driving a vertical timeline has children
> which can be vertically scrolled?  Do the children win when I start a
> drag/mousewheel in them?  What if I’m using a mousewheel to scroll the
> child and hit its bottom - should the scrolling “spill out” and turn
> into scrubbing on the parent?  Should this be controllable?  What’s
> the interaction of this and the overscroll gestures?
> * We need an animation event that fires when you hit a notch.  The
> notches will often correspond to important actions (examples: the 0%
> and 100% notches in the mobile Chrome tab-fling gesture indicate that
> the tab is closed; the 100% notch of the Twitter "pull to refresh"
> animation should trigger an XHR) and authors will want reliable, easy
> ways to respond to them, since the actual end of the action won’t be
> easily observable (due to momentum and such).
> Alternate Syntax Ideas
> ----------------------
> Right now, if multiple gestures have the same timeline name, it’s
> because they’re referring to the same timeline.  They can all scrub
> the same timeline.  It seems potentially useful to have a “local”
> timeline that only applies to your children, so that you don’t have to
> invent unique names for every instance of a component on your page.
> Maybe create ‘timeline-scope’ for creating a local timeline for your
> subtree?
> timeline-scope: none | <string>#
> Now, if a ‘timeline-gesture’ or ‘animation-timeline’ property refers
> to a named timeline, it walks up the tree looking for the first
> ancestor with a ‘timeline-scope’ property with the given name.  If it
> doesn’t find one, it assumes a global timeline of that name.
> Thoughts?  Does anyone I haven't spoken to about this think it's
> crazy?  Would you like to see working examples (done with a JS shim,
> of course) of the functionality?  We haven't done any impl work yet,
> but implementing this or something that solves the same problem is a
> priority for the Chrome team in the next few months, so feedback
> sooner rather than later would be great.
> (If you're following the Web Animations work being done by some Moz
> and WK engineers, they're aware of this and know how they'll slot it
> into the general model.  It's pretty simple.)
> ~TJ
Received on Thursday, 29 November 2012 04:23:32 UTC

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