User:Mantaroh/Scroll-driven animations

** Note : This Document is WIP(Doesn't translate yet.) **

Scroll-driven animations.

For previous discussion, see Extended Timeline.

Introduction

Dean proposed the features of animation-trigger / animation-timebases / animation-behavior.
And, Brian/Botond proposed the Extended Timeline.
These ideas are similar to each idea. Especially, an amount of animation changes related to a number of scroll changes. We can create architect which using this common idea.

Interface of script proposal

Same as DocumentTimeline, we should set ScrollTimeline to timeline property of animation.
Perhaps, WebIDL of ScrollTimeline is same to previous API proposal.

[Constructor ((Window or Element) scrollSource, Orientation orientation, optional double maxTime)
interface ScrollTimeline : AnimationTimeline {
  attribute Orientation orientation;
  attribute double maxTime;
};

// Is there an existing enum somewhere we can use for this?
enum Orientation { "vertical", "horizontal" };

We will specify the third parameter when iterations of animation is 'Infinite'. And update the range of ScrollTimeline based on the first maching conditions:

- If ScrollTimeline doesn't have any related animation, maxRange should be 0ms.
- If ScrollTimeline has related animation, maxRnage should be max value in animations. 
  (We should ignore the infinity duration.)
- If all of related animation's duration is infinity, maxRange should be max value in animation's duration.

  e.g. we have two animations.
   target1.animation = "anim1 10s linear infinite";
   target2.animation = "anim2 20s linear infinite";

  Then we should be max value of ScrollTimeline is 20s.

Interface of CSS proposal

The CSS interface is same to Dean's proposal. There are three feature in his poroposal, but 'animation-behavior' is not related with timeline. So in this document, we explain 'animation-timeline'(Original is 'animation-timebase') and 'animation-trigger' interface only.

animation-trigger:

- The scroll container should notify the scroll value to the ScrollTimeline.
- The difference point is that animation target element and scroll container object is same.

animation-timeline:

- The feature is same to ScrollTimeline.
- The difference point is that animation target element and scroll container object is same.

[Memo]

- 通知するスクロール量の取得元の役割を明記する。
- スクロールで範囲に入ったときの方向について記述する。
  例：scrollMax -> scrollMin 方向へスクロールしたとき、アニメーションの playbackRate は負数で遷移する。
- スクロールの指定が逆( scroll(300px, 100px)とか)になってる時の挙動も書いてた方がいいかも。scroll-snap で定義されていればそっち参照。

Usage of this API

1) Duration of animation is finite.

 var scrollTimeline = new ScrollTimeline(div, "vertical");
 var animation1 = new Animation(new KeyframeEffect( { marginLeft: ['0px', '200px'] }, 10000), null);
 var animation2 = new Animation(new KeyframeEffect( { marginLeft: ['100px', '300px'] }, 20000), null);
 animation1.timeline = scrollTimeline;
 animation2.timeline = scrollTimeline;

Demonstration:
 http://output.jsbin.com/wiwode

2) Duration of animation is infinite.

 var scrollTimeline = new ScrollTimeline(div, "horizontal", "20s");  // scroll range is mapped 0s - 20s.
 var animation1 = new Animation(new KeyframeEffect( { marginLeft: ['0px', '200px'] }, 10 * 1000), null);
 var animation2 = new Animation(new KeyframeEffect( { marginLeft: ['100px', '300px'] }, {duration:20 * 1000, iterations:Infinity}), null);
 animation1.timeline = scrollTimeline;
 animation2.timeline = scrollTimeline;

Implement overview

We need to implement several features as follow.
 - Notify the scroll value to ScrollTimeline from scroll containers.
 - ScrollTimeline should implement AnimationTimeline interface and RefreshObserver interface.
 - ScrollTimeline need to remain the scroll value which notified from scroll container.
 - ScrollTimeline need to override AnimationTimeline::GetCurrentTime in order to notify current progress of aninmation to own animations.

[Memo]

- e10s の時どうする? 
 -> window から通知されるスクロール量をコンテンツ側の ScrollTimeline へ通知する必要がある

Problem of implementation.

1) The concept of time value
We managed time value in Timeline, However ScrollTimeline doesn't have the concept of time. Because start position of ScrollTimeline is corresponding the start of animation. So ScrollTimeline doesn't have timve value but have relative position of animation. In order to implement these, ScrollTimeline should have the start time of animation.

2) Supporting multi ScrollTimeline
The animation have same timeline object each other.

3) End delay
ScrollTimeline has limited range. i.e. timeline is finite range.