api-console-assets/bower_components/app-layout/app-scroll-effects/effects/blend-background.html

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

<script>
  /**
   * While scrolling down, fade in the rear background layer and fade out the front background
   * layer (opacity interpolated based on scroll position).
   */
  Polymer.AppLayout.registerEffect('blend-background', {
    /** @this Polymer.AppLayout.ElementWithBackground */
    setUp: function setUp() {
      var fx = {};
      fx.backgroundFrontLayer = this._getDOMRef('backgroundFrontLayer');
      fx.backgroundRearLayer = this._getDOMRef('backgroundRearLayer');
      fx.backgroundFrontLayer.style.willChange = 'opacity';
      fx.backgroundFrontLayer.style.transform = 'translateZ(0)';
      fx.backgroundRearLayer.style.willChange = 'opacity';
      fx.backgroundRearLayer.style.transform = 'translateZ(0)';
      fx.backgroundRearLayer.style.opacity = 0;
      this._fxBlendBackground = fx;
    },
    /** @this Polymer.AppLayout.ElementWithBackground */
    run: function run(p, y) {
      var fx = this._fxBlendBackground;
      fx.backgroundFrontLayer.style.opacity = 1 - p;
      fx.backgroundRearLayer.style.opacity = p;
    },
    /** @this Polymer.AppLayout.ElementWithBackground */
    tearDown: function tearDown() {
      delete this._fxBlendBackground;
    }
  });
</script>