[css-animationworklet] Pure/idempotent representation

From @appsforartists on September 27, 2016 18:50

The animators in the README use the class keyword and static properties to construct an animation worklet. For animators that remember state across frames (#4), this seems reasonable. However, many animators will be pure transformations of input to output. For both readability/stylistic purposes and to avoid needlessly allocating memory/CPU cycles, it may be desirable to have an alternate representation for pure animators.

Perhaps:

function animator(root, children, timeline) {
  // compute frame
}

animator.inputProperties = ['--scroll-position'];
animator.outputProperties = ['opacity'];

or

const animator = { 
  inputProperties: [
    '--scroll-position',
  ],
  outputProperties: [
    'opacity',
  ],
  animate(root, children, timeline) {
    // compute frame
  },
}

NOTE: The present API is also mutative - output is expressed by settings properties on the input, not by returning values. Some authors will prefer a truly pure API, but that's more a matter of style than of minimizing resource usage.

Copied from original issue: WICG/animation-worklet#14

Start with the animator examples in the README and the stateful animator context in issue #4. Determine whether the project should specify an alternate pure representation and clarify its mutation or return-value semantics; done means the API direction is resolved and reflected in the relevant specification or documentation.