polymer_1.8.1/bower_components/app-layout/app-scroll-effects/effects/fade-background.html

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<link rel="import" href="../app-scroll-effects-behavior.html">

<script>
  /**
   * Upon scrolling past a threshold, fade in the rear background layer and fade out the front
   * background layer (opacity CSS transitioned over time).
   *
   *
   */
  Polymer.AppLayout.registerEffect('fade-background', {
    /** @this Polymer.AppLayout.ElementWithBackground */
    setUp: function setUp(config) {
      var fx = {};
      var duration = config.duration || '0.5s';
      fx.backgroundFrontLayer = this._getDOMRef('backgroundFrontLayer');
      fx.backgroundRearLayer = this._getDOMRef('backgroundRearLayer');
      fx.backgroundFrontLayer.style.willChange = 'opacity';
      fx.backgroundFrontLayer.style.webkitTransform = 'translateZ(0)';
      fx.backgroundFrontLayer.style.transitionProperty = 'opacity';
      fx.backgroundFrontLayer.style.transitionDuration = duration;
      fx.backgroundRearLayer.style.willChange = 'opacity';
      fx.backgroundRearLayer.style.webkitTransform = 'translateZ(0)';
      fx.backgroundRearLayer.style.transitionProperty = 'opacity';
      fx.backgroundRearLayer.style.transitionDuration = duration;
      this._fxFadeBackground = fx;
    },
    /** @this Polymer.AppLayout.ElementWithBackground */
    run: function run(p, y) {
      var fx = this._fxFadeBackground;
      if (p >= 1) {
        fx.backgroundFrontLayer.style.opacity = 0;
        fx.backgroundRearLayer.style.opacity = 1;
      } else {
        fx.backgroundFrontLayer.style.opacity = 1;
        fx.backgroundRearLayer.style.opacity = 0;
      }
    },
    /** @this Polymer.AppLayout.ElementWithBackground */
    tearDown: function tearDown() {
      delete this._fxFadeBackground;
    }
  });
</script>