text: /*
   * Copyright 2016 Google Inc. All rights reserved.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  'use strict';
  
  var now = require('./now');
  
  function tween(duration, update, done) {
    var cancelled = false;
  
    var startTime = now();
  
    function runUpdate() {
      if(cancelled) { return; }
      var tweenVal = (now() - startTime)/duration;
      if(tweenVal < 1) {
        update(tweenVal);
        requestAnimationFrame(runUpdate);
      }
      else {
        update(1);
        done();
      }
    }
  
    update(0);
    requestAnimationFrame(runUpdate);
  
    return function cancel() {
      cancelled = true;
      done.apply(null, arguments);
    }
  }
  
  module.exports = tween;