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<!--
@license
Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
-->

<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="../iron-resizable-behavior/iron-resizable-behavior.html">

<!--
`paper-scroll-header-panel` contains a header section and a content section.  The
header is initially on the top part of the view but it scrolls away with the
rest of the scrollable content.  Upon scrolling slightly up at any point, the
header scrolls back into view.  This saves screen space and allows users to
access important controls by easily moving them back to the view.

__Important:__ The `paper-scroll-header-panel` will not display if its parent does not have a height.

Using [layout classes](https://www.polymer-project.org/1.0/docs/migration.html#layout-attributes) or custom properties, you can easily make the `paper-scroll-header-panel` fill the screen

    <body class="fullbleed layout vertical">
      <paper-scroll-header-panel class="flex">
        <paper-toolbar>
          <div>Hello World!</div>
        </paper-toolbar>
      </paper-scroll-header-panel>
    </body>

or, if you would prefer to do it in CSS, just give `html`, `body`, and `paper-scroll-header-panel` a height of 100%:

    html, body {
      height: 100%;
      margin: 0;
    }
    paper-scroll-header-panel {
      height: 100%;
    }

`paper-scroll-header-panel` works well with `paper-toolbar` but can use any element
that represents a header by adding a `paper-header` class to it.

    <paper-scroll-header-panel>
      <paper-toolbar>Header</paper-toolbar>
      <div>Content goes here...</div>
    </paper-scroll-header-panel>

Styling scroll-header-panel:

To change background for toolbar when it is at its full size:

    paper-scroll-header-panel {
      --paper-scroll-header-panel-full-header: {
        background-color: red;
      };
    }

To change the background for toolbar when it is condensed:

    paper-scroll-header-panel {
      --paper-scroll-header-panel-condensed-header: {
        background-color: #f4b400;
      };
    }

@group Paper Element
@element paper-scroll-header-panel
@demo demo/index.html
@hero hero.svg
-->

<dom-module id="paper-scroll-header-panel">

  <style>
    :host {
      display: block;
      position: relative;
      overflow: hidden;
    }

    #mainContainer {
      position: absolute;
      top: 0;
      right: 0;
      bottom: 0;
      left: 0;

      -moz-box-sizing: border-box;
      box-sizing: border-box;

      -webkit-overflow-scrolling: touch;

      overflow-x: hidden;
      overflow-y: auto;
    }

    #headerContainer {
      position: absolute;
      top: 0;
      right: 0;
      left: 0;
    }

    .bg-container {
      position: absolute;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
      overflow: hidden;
    }

    #headerBg {
      @apply(--paper-scroll-header-panel-full-header);
    }

    #condensedHeaderBg {
      @apply(--paper-scroll-header-panel-condensed-header);
    }

    #headerBg, #condensedHeaderBg {
      position: absolute;
      top: 0;
      left: 0;
      width: 100%;
      height: 100%;
      background-repeat: no-repeat;
      background-size: cover;
      background-position: center center;
    }

    #condensedHeaderBg {
      opacity: 0;
    }
  </style>
  <template>
    <div id="mainContainer">
      <content id="mainContent" select=":not(paper-toolbar):not(.paper-header)"></content>
    </div>
    <div id="headerContainer">
      <div class="bg-container">
        <div id="condensedHeaderBg"></div>
        <div id="headerBg"></div>
      </div>
      <content id="headerContent" select="paper-toolbar, .paper-header"></content>
    </div>
  </template>
</dom-module>

<script>
(function() {

  'use strict';

  Polymer({

    /**
     * Fired when the content has been scrolled.
     *
     * @event content-scroll
     */

    /**
     * Fired when the header is transformed.
     *
     * @event paper-header-transform
     */

    is: 'paper-scroll-header-panel',

    behaviors: [
      Polymer.IronResizableBehavior
    ],

    properties: {

      /**
       * If true, the header's height will condense to `condensedHeaderHeight`
       * as the user scrolls down from the top of the content area.
       */
      condenses: {
        type: Boolean,
        value: false
      },

      /**
       * If true, no cross-fade transition from one background to another.
       */
      noDissolve: {
        type: Boolean,
        value: false
      },

      /**
       * If true, the header doesn't slide back in when scrolling back up.
       */
      noReveal: {
        type: Boolean,
        value: false
      },

      /**
       * If true, the header is fixed to the top and never moves away.
       */
      fixed: {
        type: Boolean,
        value: false
      },

      /**
       * If true, the condensed header is always shown and does not move away.
       */
      keepCondensedHeader: {
        type: Boolean,
        value: false
      },

      /**
       * The height of the header when it is at its full size.
       *
       * By default, the height will be measured when it is ready.  If the height
       * changes later the user needs to either set this value to reflect the
       * new height or invoke `measureHeaderHeight()`.
       */
      headerHeight: {
        type: Number,
        value: 0
      },

      /**
       * The height of the header when it is condensed.
       *
       * By default, `condensedHeaderHeight` is 1/3 of `headerHeight` unless
       * this is specified.
       */
      condensedHeaderHeight: {
        type: Number,
        value: 0
      },

      /**
       * By default, the top part of the header stays when the header is being
       * condensed.  Set this to true if you want the top part of the header
       * to be scrolled away.
       */
      scrollAwayTopbar: {
        type: Boolean,
        value: false
      },

      /**
       * The state of the header. The initial value is `HEADER_STATE_EXPANDED`.
       * Depending on the configuration and the `scrollTop` value,
       * the header state could change to
       * `HEADER_STATE_HIDDEN`, `HEADER_STATE_CONDENSED` and `HEADER_STATE_INTERPOLATED`
       */
      headerState: {
        type: Number,
        readOnly: true,
        value: 0
      },

      _prevScrollTop: {
        type: Number,
        value: 0
      },

      _y: {
        type: Number,
        value: 0
      },

      /** @type {number|null} */
      _defaultCondsensedHeaderHeight: {
        type: Number,
        value: 0
      }
    },

    observers: [
      '_setup(headerHeight, condensedHeaderHeight, fixed)',
      '_condensedHeaderHeightChanged(condensedHeaderHeight)',
      '_headerHeightChanged(headerHeight, condensedHeaderHeight)',
      '_condensesChanged(condenses)',
    ],

    listeners: {
      'iron-resize': 'measureHeaderHeight'
    },

    ready: function() {
      this.async(this.measureHeaderHeight, 5);
      this._scrollHandler = this._scroll.bind(this);
      this.scroller.addEventListener('scroll', this._scrollHandler);
    },

    /**
     * The header's initial state
     *
     * @property HEADER_STATE_EXPANDED
     * @type number
     */
    HEADER_STATE_EXPANDED: 0,

    /**
     * The header's state when it's hidden.
     *
     * @property HEADER_STATE_HIDDEN
     * @type number
     */
    HEADER_STATE_HIDDEN: 1,

    /**
     * The header's state when it's condensed.
     *
     * @property HEADER_STATE_CONDENSED
     * @type number
     */
    HEADER_STATE_CONDENSED: 2,

    /**
     * The header's state when its progress is somewhere between
     * the `hidden` and `condensed` state.
     *
     * @property HEADER_STATE_INTERPOLATED
     * @type number
     */
    HEADER_STATE_INTERPOLATED: 3,

    /**
     * Returns the header element.
     *
     * @property header
     * @type Object
     */
    get header() {
      return Polymer.dom(this.$.headerContent).getDistributedNodes()[0];
    },

    /**
     * Returns the content element.
     *
     * @property content
     * @type Object
     */
    get content() {
      return Polymer.dom(this.$.mainContent).getDistributedNodes()[0];
    },

    /**
     * Returns the scrollable element.
     *
     * @property scroller
     * @type Object
     */
    get scroller() {
      return this.$.mainContainer;
    },

    get _headerMaxDelta() {
      return this.keepCondensedHeader ? this._headerMargin : this.headerHeight;
    },

    get _headerMargin() {
      return this.headerHeight - this.condensedHeaderHeight;
    },

    _y: 0,

    _prevScrollTop: 0,

    /**
     * Invoke this to tell `paper-scroll-header-panel` to re-measure the header's
     * height.
     *
     * @method measureHeaderHeight
     */
    measureHeaderHeight: function() {
      var header = this.header;
      if (header && header.offsetHeight) {
        this.headerHeight = header.offsetHeight;
      }
    },

    /**
     * Scroll to a specific y coordinate.
     *
     * @method scroll
     * @param {number} top The coordinate to scroll to, along the y-axis.
     * @param {boolean} smooth true if the scroll position should be smoothly adjusted.
     */
    scroll: function(top, smooth) {
      // the scroll event will trigger _updateScrollState directly,
      // However, _updateScrollState relies on the previous `scrollTop` to update the states.
      // Calling _updateScrollState will ensure that the states are synced correctly.

      if (smooth) {
        // TODO(blasten): use CSS scroll-behavior once it ships in Chrome.
        var easingFn = function easeOutQuad(t, b, c, d) {
          t /= d;
          return -c * t*(t-2) + b;
        };
        var animationId = Math.random();
        var duration = 200;
        var startTime = Date.now();
        var currentScrollTop = this.scroller.scrollTop;
        var deltaScrollTop = top - currentScrollTop;

        this._currentAnimationId = animationId;

        (function updateFrame() {
          var now = Date.now();
          var elapsedTime = now - startTime;

          if (elapsedTime > duration) {
            this.scroller.scrollTop = top;
            this._updateScrollState(top);

          } else if (this._currentAnimationId === animationId) {
            this.scroller.scrollTop = easingFn(elapsedTime, currentScrollTop, deltaScrollTop, duration);
            requestAnimationFrame(updateFrame.bind(this));
          }

        }).call(this);

      } else {
        this.scroller.scrollTop = top;
        this._updateScrollState(top);
      }
    },

   /**
     * Condense the header.
     *
     * @method condense
     * @param {boolean} smooth true if the scroll position should be smoothly adjusted.
     */
    condense: function(smooth) {
      if (this.condenses && !this.fixed && !this.noReveal) {
        switch (this.headerState) {
          case this.HEADER_STATE_HIDDEN:
            this.scroll(this.scroller.scrollTop - (this._headerMaxDelta - this._headerMargin), smooth);
          break;
          case this.HEADER_STATE_EXPANDED:
          case this.HEADER_STATE_INTERPOLATED:
            this.scroll(this._headerMargin, smooth);
          break;
        }
      }
    },

    /**
     * Scroll to the top of the content.
     *
     * @method scrollToTop
     * @param {boolean} smooth true if the scroll position should be smoothly adjusted.
     */
    scrollToTop: function(smooth) {
      this.scroll(0, smooth);
    },

    _headerHeightChanged: function(headerHeight) {
      if (this._defaultCondsensedHeaderHeight !== null) {
        this._defaultCondsensedHeaderHeight = Math.round(headerHeight * 1/3);
        this.condensedHeaderHeight = this._defaultCondsensedHeaderHeight;
      }
    },

    _condensedHeaderHeightChanged: function(condensedHeaderHeight) {
      if (condensedHeaderHeight) {
        // a user custom value
        if (this._defaultCondsensedHeaderHeight != condensedHeaderHeight) {
          // disable the default value
          this._defaultCondsensedHeaderHeight = null;
        }
      }
    },

    _condensesChanged: function() {
      this._updateScrollState(this.scroller.scrollTop);
      this._condenseHeader(null);
    },

    _setup: function() {
      var s = this.scroller.style;

      s.paddingTop = this.fixed ? '' : this.headerHeight + 'px';
      s.top = this.fixed ? this.headerHeight + 'px' : '';

      if (this.fixed) {
        this._setHeaderState(this.HEADER_STATE_EXPANDED);
        this._transformHeader(null);
      } else {
        switch (this.headerState) {
          case this.HEADER_STATE_HIDDEN:
            this._transformHeader(this._headerMaxDelta);
            break;
          case this.HEADER_STATE_CONDENSED:
            this._transformHeader(this._headerMargin);
            break;
        }
      }
    },

    _transformHeader: function(y) {
      this._translateY(this.$.headerContainer, -y);

      if (this.condenses) {
        this._condenseHeader(y);
      }

      this.fire('paper-header-transform',
        { y: y,
          height: this.headerHeight,
          condensedHeight: this.condensedHeaderHeight
        }
      );
    },

    _condenseHeader: function(y) {
      var reset = (y === null);

      // adjust top bar in paper-header so the top bar stays at the top
      if (!this.scrollAwayTopbar && this.header && this.header.$ && this.header.$.topBar) {
        this._translateY(this.header.$.topBar,
            reset ? null : Math.min(y, this._headerMargin));
      }
      // transition header bg
      if (!this.noDissolve) {
        this.$.headerBg.style.opacity = reset ? '' :
            ( (this._headerMargin - y) / this._headerMargin);
      }
      // adjust header bg so it stays at the center
      this._translateY(this.$.headerBg, reset ? null : y / 2);
      // transition condensed header bg
      if (!this.noDissolve) {
        this.$.condensedHeaderBg.style.opacity = reset ? '' :
            (y / this._headerMargin);

        // adjust condensed header bg so it stays at the center
        this._translateY(this.$.condensedHeaderBg, reset ? null : y / 2);
      }
    },

    _translateY: function(node, y) {
      this.transform((y === null) ? '' : 'translate3d(0, ' + y + 'px, 0)', node);
    },

    /** @param {Event=} event */
    _scroll: function(event) {
      if (this.header) {
        this._updateScrollState(this.scroller.scrollTop);

        this.fire('content-scroll', {
          target: this.scroller
        },
        {
          cancelable: false
        });
      }
    },

    _updateScrollState: function(scrollTop) {
      var deltaScrollTop = scrollTop - this._prevScrollTop;
      var y = Math.max(0, (this.noReveal) ? scrollTop : this._y + deltaScrollTop);

      if (y > this._headerMaxDelta) {
        y = this._headerMaxDelta;
        this._setHeaderState(this.HEADER_STATE_HIDDEN);

      } else if (this.condenses && this._prevScrollTop >= scrollTop && scrollTop >= this._headerMargin) {
        y = Math.max(y, this._headerMargin);
        this._setHeaderState(this.HEADER_STATE_CONDENSED);

      } else if (y === 0) {
        this._setHeaderState(this.HEADER_STATE_EXPANDED);

      } else {
        this._setHeaderState(this.HEADER_STATE_INTERPOLATED);
      }

      if (!this.fixed && y !== this._y) {
        this._transformHeader(y);
      }

      this._prevScrollTop = Math.max(scrollTop, 0);
      this._y = y;
    }
  });

})();

</script>