11 Dec 17

Introducing Popmotion 8

Popmotion 8 is a functional, reactive animation library.

It’s the result of over three months work, the result of wanting to add a humble pipe method and instead tumbling down the rabbit hole.

It introduces new animations: decay, everyFrame, keyframes and timeline. It makes input dragging a breeze with our revamped pointer and multitouch actions.

Crucially, there’s a new streamlined, reactive API.

This new API reflects Popmotion’s gradual shift towards functional programming and adopts it as a core part of the design philosophy.

The result is a small, flexible and composable library that I hope you’ll find fun to use. Let’s take a look at what’s new.



decay models a form of exponential deceleration to create motion like momentum scrolling on smartphones.

Just provide it velocity and from properties:

  velocity: 1000,
  from: 0
}).start((v) => ...)

It also exposes a modifyTarget option that provides a functional API for adding features like snap-to-grid.


everyFrame, as you might imagine, fires once per frame. It provides the amount of time since it started:

everyFrame().start((timeSinceStart) => ...);

It’s the fundamental driver of every Popmotion animation. We’ve exposed it because we reckon this kind low-level action will be useful in animations, like porting After Effects expressions:


keyframes transitions through a number of states over a set duration of time.

Its API is inspired by Apple’s CAKeyframeAnimation, which makes it trivial to resize the overall animation:

  values: [
    { x: 0, y: 0 },
    { x: 100, y: 0 },
    { x: 100, y: 100 }
  times: [0, 0.3, 1],
  duration: 1000


timeline is used to orchestrate more complicated patterns of tweens.

It supports absolute and relative timestamps, as well as parallel and staggered motion. The output action has all the same playback methods as tween, making it fully scrubbable.

In most animation libraries, the timeline function is a bit of a black box that we chuck setters or selectors into.

As Popmotion is a reactive library, we label each segment with a track property, and that then the latest state gets output as an object with those labels:

  { track: 'shade', from: 0, to: 1 },
    track: 'modal',
    from: { y: -100, opacity: 0 },
    to: { y: 0, opacity: 1 }
]).start(({ shade, modal }) => ...)

This means we can pass the output of timeline through the chainable methods pipe, while and filter (more on those later).

As timelines become more complicated, maintaining the link between labels and setters can become increasingly difficult, but the trade-off is a timeline that is immutable, composable, pure, and testable.

We’re also experimenting with functions that can take a timeline definition and automatically generate the output reaction.



The pointer action has been in Popmotion since before it was called Popmotion. But, its API has always been confused making simple tasks like dragging more convoluted than they needed to be.

In Popmotion 7, pointer output its absolute position and it was up to a second action, trackOffset, to get the movement of pointer relative to another point (say, a DOM element).

Popmotion 8 scraps the trackOffset action entirely. Now, pointer will output its absolute position when used like this:

pointer().start(({ x, y }) => ...)

(Also notice how we no longer need to provide pointer with a MouseEvent or TouchEvent.)

To output that pointer’s movement applied to another point, we simply need to provide that point as our initial argument:

pointer({ x: 0, y: 0 }).start(({ x, y }) => ...)


Thanks to the efforts of Mars, Popmotion has had multitouch support since version 6.

There are two major changes coming in 8. The first and most apparent is we’ve changed the name of the action from touches to multitouch to highlight the second change:

It no longer outputs just an array of touches. touches is joined by scale and rotate properties:

multitouch().start(({ touches, scale, rotate }) => ...)

Like pointer, multitouch accepts scale and rotate arguments and, if defined, will output the change in those properties as applied to the given values.

Reactive API

At the heart of all these new features is a change in the core building block of all animations, the action.

The action function is used to create streams of reactive values. Think of it as an animation-focused, tiny alternative to Rx Observables.

It looks like this:

action(({ update, complete, error }) => {})

For a practical example of how action works, let’s define an function called just. It’ll return an action that, when started, will fire update with the provided value and then complete:

const just = (v) => action(({ update, complete }) => {

just(2).start(console.log); // 2

Every time we start an action, its initialisation function runs anew, creating a new instance of the action. Because all animations are actions, we can define an animation once:

const moveRight = tween({ to: 300 });

And use it multiple times:

moveRight.start(console.log); // 0 - 300
moveRight.start(console.log); // 0 - 300

Actions offer a number of chainable methods (currently filter, pipe and while, with an API for adding more on the way). Each returns a brand new version of the action with the added functionality:

const double = v => v * 2;
const px = v => v + 'px';

const justTwo = just(2);

justTwo.start(console.log); // 2

  .pipe(double, px)
  .start(console.log); // '4px'

In the last 6 months Popmotion has spun out Framesync and Stylefire as standalone libraries.

It’s helped me take greater care and consideration over where to draw the lines between the role and responsibilities of various parts of the library and enables people to use the isolated functionality in their own code or libraries.

I can imagine a near-future where actions are spun out as a tiny reactive library, where people can start dabbling with reactive programming outside of animation without the full payload of something the size of Rx.

And the rest…

File size and individual imports

Popmotion has always tried to respect your bytes. One of the reasons I wrote it in the first place was a dissatisfaction with the size of existing libraries in comparison to the benefits they provided.

Popmotion 8 is a little bigger than 7 (11.5kb vs 10kb). Though, as such a radical rewrite with so many new features, I think there are efficiencies to be made over the coming months.

In the meantime, everything in Popmotion is now available as an individual import. Which means, if you only want to use (for instance) spring, you can import and use that for roughly 2kb.

Colours and multi-prop animations

In Popmotion 7, we exposed the ability to animate colours with colorTween. Multi-property animations could be composed with the composite and parallel compositors.

This has all been streamlined in 8. Every animation can now animate colors, objects, arrays (and objects and arrays of colors!).

It’s as simple as this:

  from: { x: 0, color: '#f00' },
  to: { x: 100, color: '#fff' }
}).start(({ x, color }) => ...)


That’s most of what’s new in Popmotion 8. Existing users should check out our upgrade guide to handle breaking changes.

After three years of development I’m finally happy with the API. I think the reactive model works incredibly well for neatly and declaratively handling streams of values and fits perfectly with the functional approach I was already moving towards.

With these solid foundations in place, the next logical step feels like exploring a way of describing the properties of UI elements and having motion and interactions derive naturally from that. Like a physically-based rendering for motion.

Once more down the rabbit hole.

Get started with Popmotion