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Animation Worklet Design Principles and Goals

for rich interactive effects on the Web Platform

Problem and Motivation

Fact: Fact: It is difficult to build smooth rich interactive effects on the web.

Why do we care? Silky smooth rich interactive user interfaces are now a basic user expectation on modern computing platforms. For web platform to remain competitive it should be capable of high fidelity, smooth, responsive UI.

Where is the difficulty? Three key aspects of rich interactive effects are: smoothness, responsiveness to input (a.k.a. R & A of RAILS model) and their rich interaction model.

The two main methods for creating animations on the web fall short in at least one of these aspects:

  • CSS (Web) Animations: Aimed at supporting stateless declarative time-driven effects. The expressiveness is sufficient for common time-based effects. The resulting animation can be smooth 1. However it's unclear how this model can be scaled to handle the multi-dimensional inputs, conditional values, and statefulness required by the use cases below.
  • requestAnimationFrame: Aimed at creating scripted animation effects. It can support rich interaction models but it is difficult to make smooth or responsive. This expressiveness of Javascript coupled with access to all input methods, application state and dom makes this API capable of building rich interactive effects. However these can only run on main thread alongside all other scripts2 which severely hampers their responsiveness and smoothness. Chrome studies have shown that script is the main culprit to user responsiveness issues.

Animation Worklet aims to help bridge the gap between these two.

Animation Worklet Vision

Animation Worklet aims to rectify this shortcomings by enabling animations that can be:

  • rich (imperative, stateful)
  • fast-by-default (isolated from main thread)
  • respond to rich input e.g., touch, gesture, scroll.

Animation Worklet is a primitive in the extensible web spirit. It exposes browser's fast path to applications in a way that it was never before and reduces browser magic.

Examples of rich interactive effects that are (or will be made) possible with Animation Worklet:

  • Scroll driven effects:
  • Gesture driven effects:
    • Image manipulator that scales, rotates etc.
    • Swipe to dismiss.
    • Drag-N-Drop.
    • Tiled panning e.g., Google maps.
  • Stateful script driven effects:
  • Animated scroll offsets:
    • Having multiple scrollers scroll in sync e.g. diff viewer keeping old/new in sync when you scroll either (demo)
    • Custom smooth scroll animations (e.g., physic based fling curves)

First Principle - Richness

Animation Worklet enables developers to create custom animations by providing an animate function that runs inside animation worklet global scope. The animation logic can take advantage of the full expressive power of JavaScript, maintain local state, modify and coordinated across many elements. This new extension point in browser animation system enables richer effects that go well beyond what can be achieved today with Web Animations and closer to what is possible with requestAnimationFrame.

Further explorations in this direction: Allow richer access to scrolling machinery (e.g., scroll customization), custom paint worklets, and outputs beyond existing KeyframeEffect interface.

Second Principle - Performance

Animation Worklet is designed to be thread agnostic. In particular, it can run off main thread keeping its performance isolated from main thread (also reducing main thread load).

Animation Worklet API encourages the developers to isolate their critical UI work inside limited worklet scope with well-specified input and output. This allows user-agent to make much better scheduling decision about this work and in particular, be much better at maintaining a strict frame-budget to successfully run these animations on its fast path. Presently the above performance guarantees are only accessible to a limited set of declarative time-based effects.

Further explorations in this direction: Introduce more sophisticated per-animation scheduling where a slow animation may run at slower frame-rate without affecting other well-behaved animations, experiment with translating animation code to native code or even GL shaders moving the computation to GPU for even stronger performance guarantees!

Third Principle - Interactivity

Animation Worklet is designed to enable support for animations whose input goes beyond just time, a single-dimensional variable.

Web animation timing model is stateless and driven by a single dimensional variable, time.

Although this model works well for declarative time-based animation, it falls short when it comes to interactive input-driven effects that are inherently stateful. While it is possible to map simple forms of input (e.g., single dimensional scroll) into time, it is much more difficult (almost impossible) to do so for multi-dimensional stateful input such as multi-touch and gesture input.

Animation Worklet has the necessary expressive power and richness to easily accommodate the full richness of multi-dimensional input such as touch, gesture, scroll etc. For example it is trivial to react to scroll phase change, pointer state change, addition/removal of new pointer or state, calculate pointer velocity, acceleration and other computed values inside an animation worklet.

Further explorations in this direction: Expose pointer and gesture as input to animation (current proposal)



  • 1: If authors limit themselves to cheap-to-update properties. In Chrome these are composited properties e.g., transform, opacity, filter but other engines may have a slightly difference subset.