Rebound

Rebound is a simple library that models Spring dynamics for the purpose of driving physical animations.

Origin

Rebound was originally written in Java to provide a lightweight physics system for Home and Chat Heads on Android. It's now been adopted by several other Android applications. This JavaScript port was written to provide a quick way to demonstrate Rebound animations on the web for a conference talk. Since then the JavaScript version has been used to build some really nice interfaces. Check out brandonwalkin.com for an example.

Overview

The Library provides a SpringSystem for maintaining a set of Spring objects and iterating those Springs through a physics solver loop until equilibrium is achieved. The Spring class is the basic animation driver provided by Rebound. By attaching a listener to a Spring, you can observe its motion. The observer function is notified of position changes on the spring as it solves for equilibrium. These position updates can be mapped to an animation range to drive animated property updates on your user interface elements (translation, rotation, scale, etc).

Check out the tests, and examples for more details.

Example

Here's a simple example. Pressing and releasing on the logo below will cause it to scale up and down with a springy animation.

<div style="text-align:center; margin-bottom:50px; margin-top:50px">
  <img
    src="http://facebook.github.io/rebound/images/rebound.png"
    id="logo"
  />
</div>
<script src="../rebound.min.js"></script>
<script>
  function scale(el, val) {
    el.style.mozTransform =
    el.style.msTransform =
    el.style.webkitTransform =
    el.style.transform = 'scale3d(' + val + ', ' + val + ', 1)';
  }
  var el = document.getElementById('logo');

  var springSystem = new rebound.SpringSystem();
  var spring = springSystem.createSpring(50, 3);
  spring.addListener({
    onSpringUpdate: function(spring) {
      var val = spring.getCurrentValue();
      val = rebound.MathUtil.mapValueInRange(val, 0, 1, 1, 0.5);
      scale(el, val);
    }
  });

  el.addEventListener('mousedown', function() {
    spring.setEndValue(1);
  });

  el.addEventListener('mouseout', function() {
    spring.setEndValue(0);
  });

  el.addEventListener('mouseup', function() {
    spring.setEndValue(0);
  });
</script>

Here's how it works.

// Get a reference to the logo element.
var el = document.getElementById('logo');

// create a SpringSystem and a Spring with a bouncy config.
var springSystem = new rebound.SpringSystem();
var spring = springSystem.createSpring(50, 3);

// Add a listener to the spring. Every time the physics
// solver updates the Spring's value onSpringUpdate will
// be called.
spring.addListener({
  onSpringUpdate: function(spring) {
    var val = spring.getCurrentValue();
    val = rebound.MathUtil
                 .mapValueInRange(val, 0, 1, 1, 0.5);
    scale(el, val);
  }
});

// Listen for mouse down/up/out and toggle the
//springs endValue from 0 to 1.
el.addEventListener('mousedown', function() {
  spring.setEndValue(1);
});

el.addEventListener('mouseout', function() {
  spring.setEndValue(0);
});

el.addEventListener('mouseup', function() {
  spring.setEndValue(0);
});

// Helper for scaling an element with css transforms.
function scale(el, val) {
  el.style.mozTransform =
  el.style.msTransform =
  el.style.webkitTransform =
  el.style.transform = 'scale3d(' +
    val + ', ' + val + ', 1)';
}

Code of Conduct