React-Motion

import {Motion, spring} from 'react-motion';
// In your render...
<Motion defaultStyle={{x: 0}} style={{x: spring(10)}}>
  {value => <div>{value.x}</div>}
</Motion>

Animate a counter from 0 to 10. For more advanced usage, see below.

Install

• Npm: npm install --save react-motion

• Bower: do not install with bower install react-motion, it won't work. Use bower install --save https://unpkg.com/react-motion/bower.zip. Or in bower.json:

{
  "dependencies": {
    "react-motion": "https://unpkg.com/react-motion/bower.zip"
  }
}

then include as

<script src="bower_components/react-motion/build/react-motion.js"></script>

• 1998 Script Tag:

<script src="https://unpkg.com/react-motion/build/react-motion.js"></script>
(Module exposed as `ReactMotion`)

Works with React-Native v0.18+.

Demos

• Simple Transition
• Chat Heads
• Draggable Balls
• TodoMVC List Transition
• Photo Gallery
• Spring Parameters Chooser
• Water Ripples
• Draggable List

Check the wiki for more!

Try the Demos Locally

git clone https://github.com/chenglou/react-motion.git
cd react-motion
npm install

• With hot reloading (slow, development version): run npm start.
• Without hot reloading (faster, production version): run npm run build-demos and open the static demos/demo_name/index.html file directly. Don't forget to use production mode when testing your animation's performance!

To build the repo yourself: npm run prepublish.

What does this library try to solve?

My React-Europe talk

For 95% of use-cases of animating components, we don't have to resort to using hard-coded easing curves and duration. Set up a stiffness and damping for your UI element, and let the magic of physics take care of the rest. This way, you don't have to worry about petty situations such as interrupted animation behavior. It also greatly simplifies the API.

This library also provides an alternative, more powerful API for React's TransitionGroup.

API

Exports:

• spring
• Motion
• StaggeredMotion
• TransitionMotion
• presets

Here's the well-annotated public Flow type definition file (you don't have to use Flow with React-motion, but the types help document the API below).

P.S. using TypeScript? Here are the React-motion TypeScript definitions!

---

Helpers

- spring: (val: number, config?: SpringHelperConfig) => OpaqueConfig

Used in conjunction with the components below. Specifies the how to animate to the destination value, e.g. spring(10, {stiffness: 120, damping: 17}) means "animate to value 10, with a spring of stiffness 120 and damping 17".

• val: the value.

• config: optional, for further adjustments. Possible fields:

  • stiffness: optional, defaults to 170.
  • damping: optional, defaults to 26.
  • precision: optional, defaults to 0.01. Specifies both the rounding of the interpolated value and the speed (internal).

  It's normal not to feel how stiffness and damping affect your spring; use Spring Parameters Chooser to get a feeling. Usually, you'd just use the list of tasteful stiffness/damping presets below.

- Presets for {stiffness, damping}

Commonly used spring configurations used like so: spring(10, presets.wobbly) or spring(20, {...presets.gentle, precision: 0.1}). See here.

---

<Motion />

Usage

<Motion defaultStyle={{x: 0}} style={{x: spring(10)}}>
  {interpolatingStyle => <div style={interpolatingStyle} />}
</Motion>

Props

- style: Style

Required. The Style type is an object that maps to either a number or an OpaqueConfig returned by spring() above. Must keep the same keys throughout component's existence. The meaning of the values:

• an OpaqueConfig returned from spring(x): interpolate to x.
• a number x: jump to x, do not interpolate.

- defaultStyle?: PlainStyle

Optional. The PlainStyle type maps to numbers. Defaults to an object with the same keys as style above, whose values are the initial numbers you're interpolating on. Note that during subsequent renders, this prop is ignored. The values will interpolate from the current ones to the destination ones (specified by style).

- children: (interpolatedStyle: PlainStyle) => ReactElement

Required function.

• interpolatedStyle: the interpolated style object passed back to you. E.g. if you gave style={{x: spring(10), y: spring(20)}}, you'll receive as interpolatedStyle, at a certain time, {x: 5.2, y: 12.1}, which you can then apply on your div or something else.

• Return: must return one React element to render.

- onRest?: () => void

Optional. The callback that fires when the animation comes to a rest.

---

<StaggeredMotion />

Animates a collection of (fixed length) items whose values depend on each other, creating a natural, springy, "staggering" effect like so. This is preferred over hard-coding a delay for an array of Motions to achieve a similar (but less natural-looking) effect.

Usage

<StaggeredMotion
  defaultStyles={[{h: 0}, {h: 0}, {h: 0}]}
  styles={prevInterpolatedStyles => prevInterpolatedStyles.map((_, i) => {
    return i === 0
      ? {h: spring(100)}
      : {h: spring(prevInterpolatedStyles[i - 1].h)}
  })}>
  {interpolatingStyles =>
    <div>
      {interpolatingStyles.map((style, i) =>
        <div key={i} style={{border: '1px solid', height: style.h}} />)
      }
    </div>
  }
</StaggeredMotion>

Aka "the current spring's destination value is the interpolating value of the previous spring". Imagine a spring dragging another. Physics, it works!

Props

- styles: (previousInterpolatedStyles: ?Array<PlainStyle>) => Array<Style>

Required function. Don't forget the "s"!

• previousInterpolatedStyles: the previously interpolating (array of) styles (undefined at first render, unless defaultStyles is provided).

• Return: must return an array of Styles containing the destination values, e.g. [{x: spring(10)}, {x: spring(20)}].

- defaultStyles?: Array<PlainStyle>

Optional. Similar to Motion's defaultStyle, but an array of them.

- children: (interpolatedStyles: Array<PlainStyle>) => ReactElement

Required function. Similar to Motion's children, but accepts the array of interpolated styles instead, e.g. [{x: 5}, {x: 6.4}, {x: 8.1}]

(No onRest for StaggeredMotion because we haven't found a good semantics for it yet. Voice your support in the issues section.)

---

<TransitionMotion />

Helps you to do mounting and unmounting animation.

Usage

You have items a, b, c, with their respective style configuration, given to TransitionMotion's styles. In its children function, you're passed the three interpolated styles as params; you map over them and produce three components. All is good.

During next render, you give only a and b, indicating that you want c gone, but that you'd like to animate it reaching value 0, before killing it for good.

Fortunately, TransitionMotion has kept c around and still passes it into the children function param. So when you're mapping over these three interpolated styles, you're still producing three components. It'll keep interpolating, while checking c's current value at every frame. Once c reaches the specified 0, TransitionMotion will remove it for good (from the interpolated styles passed to your children function).

This time, when mapping through the two remaining interpolated styles, you'll produce only two components. c is gone for real.

import createReactClass from 'create-react-class';

const Demo = createReactClass({
  getInitialState() {
    return {
      items: [{key: 'a', size: 10}, {key: 'b', size: 20}, {key: 'c', size: 30}],
    };
  },
  componentDidMount() {
    this.setState({
      items: [{key: 'a', size: 10}, {key: 'b', size: 20}], // remove c.
    });
  },
  willLeave() {
    // triggered when c's gone. Keeping c until its width/height reach 0.
    return {width: spring(0), height: spring(0)};
  },
  render() {
    return (
      <TransitionMotion
        willLeave={this.willLeave}
        styles={this.state.items.map(item => ({
          key: item.key,
          style: {width: item.size, height: item.size},
        }))}>
        {interpolatedStyles =>
          // first render: a, b, c. Second: still a, b, c! Only last one's a, b.
          <div>
            {interpolatedStyles.map(config => {
              return <div key={config.key} style={{...config.style, border: '1px solid'}} />
            })}
          </div>
        }
      </TransitionMotion>
    );
  },
});

Props

First, two type definitions to ease the comprehension.

• TransitionStyle: an object of the format {key: string, data?: any, style: Style}.

  • key: required. The ID that TransitionMotion uses to track which configuration is which across renders, even when things are reordered. Typically reused as the component key when you map over the interpolated styles.

  • data: optional. Anything you'd like to carry along. This is so that when the previous section example's c disappears, you still get to access c's related data, such as the text to display along with it.

  • style: required. The actual starting style configuration, similar to what you provide for Motion's style. Maps keys to either a number or an OpaqueConfig returned by spring().

• TransitionPlainStyle: similar to above, except the style field's value is of type PlainStyle, aka an object that maps to numbers.

- styles: Array<TransitionStyle> | (previousInterpolatedStyles: ?Array<TransitionPlainStyle>) => Array<TransitionStyle>

Required. Accepts either:

• an array of TransitionStyle configs, e.g. [{key: 'a', style: {x: spring(0)}}, {key: 'b', style: {x: spring(10)}}].

• a function similar to StaggeredMotion, taking the previously interpolating styles (undefined at first call, unless defaultStyles is provided), and returning the previously mentioned array of configs. You can do staggered mounting animation with this.

- defaultStyles?: Array<TransitionPlainStyle>

Optional. Similar to the other components' defaultStyle/defaultStyles.

- children: (interpolatedStyles: Array<TransitionPlainStyle>) => ReactElement

Required function. Similar to other two components' children. Receive back an array similar to what you provided for defaultStyles, only that each style object's number value represent the currently interpolating value.

- willLeave?: (styleThatLeft: TransitionStyle) => ?Style

Optional. Defaults to () => null. The magic sauce property.

• styleThatLeft: the e.g. {key: ..., data: ..., style: ...} object from the styles array, identified by key, that was present during a previous render, and that is now absent, thus triggering the call to willLeave. Note that the style property is exactly what you passed in styles, and is not interpolated. For example, if you passed a spring for x you will receive an object like {x: {stiffness, damping, val, precision}}.

• Return: null to indicate you want the TransitionStyle gone immediately. A Style object to indicate you want to reach transition to the specified value(s) before killing the TransitionStyle.

- didLeave?: (styleThatLeft: {key: string, data?: any}) => void

Optional. Defaults to () => {}.

• styleThatLeft: the {key:..., data:...} that was removed after the finished transition.

- willEnter?: (styleThatEntered: TransitionStyle) => PlainStyle

Optional. Defaults to styleThatEntered => stripStyle(styleThatEntered.style). Where stripStyle turns {x: spring(10), y: spring(20)} into {x: 10, y: 20}.

• styleThatEntered: similar to willLeave's, except the TransitionStyle represents the object whose key value was absent during the last render, and that is now present.

• Return: a defaultStyle-like PlainStyle configuration, e.g. {x: 0, y: 0}, that serves as the starting values of the animation. Under this light, the default provided means "a style config that has the same starting values as the destination values".

Note that willEnter and defaultStyles serve different purposes. willEnter only triggers when a previously inexistent TransitionStyle inside styles comes into the new render.

(No onRest for TransitionMotion because we haven't found a good semantics for it yet. Voice your support in the issues section.)

---

FAQ

• How do I set the duration of my animation?

Hard-coded duration goes against fluid interfaces. If your animation is interrupted mid-way, you'd get a weird completion animation if you hard-coded the time. That being said, in the demo section there's a great Spring Parameters Chooser for you to have a feel of what spring is appropriate, rather than guessing a duration in the dark.

• How do I unmount the TransitionMotion container itself?

You don't. Unless you put it in another TransitionMotion...

• How do I do staggering/chained animation where items animate in one after another?

See StaggeredMotion

• My ref doesn't work in the children function.

React string refs won't work:

<Motion style={...}>{currentValue => <div ref="stuff" />}</Motion>

This is how React works. Here's the callback ref solution.