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<!--
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-flex-layout/iron-flex-layout.html">

<!--
`paper-header-panel` contains a header section and a content panel section.

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

Using layout classes, you can make the `paper-header-panel` fill the screen

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

Special support is provided for scrolling modes when one uses a paper-toolbar or equivalent for the
header section.

Example:

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

If you want to use other than `paper-toolbar` for the header, add `paper-header` class to that
element.

Example:

    <paper-header-panel>
      <div class="paper-header">Header</div>
      <div>Content goes here...</div>
    </paper-header-panel>

To have the content fit to the main area, use the `fit` class.

    <paper-header-panel>
      <div class="paper-header">standard</div>
      <div class="fit">content fits 100% below the header</div>
    </paper-header-panel>

Modes

Controls header and scrolling behavior. Options are `standard`, `seamed`, `waterfall`, `waterfall-tall`, `scroll` and
`cover`. Default is `standard`.

Mode | Description
----------------|-------------
`standard` | The header is a step above the panel. The header will consume the panel at the point of entry, preventing it from passing through to the opposite side.
`seamed` | The header is presented as seamed with the panel.
`waterfall` | Similar to standard mode, but header is initially presented as seamed with panel, but then separates to form the step.
`waterfall-tall` | The header is initially taller (`tall` class is added to the header). As the user scrolls, the header separates (forming an edge) while condensing (`tall` class is removed from the header).
`scroll` | The header keeps its seam with the panel, and is pushed off screen.
`cover` | The panel covers the whole `paper-header-panel` including the header. This allows user to style the panel in such a way that the panel is partially covering the header.

Example:

    <paper-header-panel mode="waterfall">
      <div class="paper-header">standard</div>
      <div class="content fit">content fits 100% below the header</div>
    </paper-header-panel>


Styling header panel:

To change the shadow that shows up underneath the header:

    paper-header-panel {
      --paper-header-panel-shadow: {
          height: 6px;
          bottom: -6px;
          box-shadow: inset 0px 5px 6px -3px rgba(0, 0, 0, 0.4);
      };
    }

To change the panel container in different modes:

    paper-slider {
      --paper-header-panel-standard-container: {
        border: 1px solid gray;
      };

      --paper-header-panel-seamed-container: {
        border: 1px solid gray;
      };

      --paper-header-panel-waterfall-container: {
        border: 1px solid gray;
      };

      --paper-header-panel-waterfall-tall-container: {
        border: 1px solid gray;
      };

      --paper-header-panel-scroll-container: {
        border: 1px solid gray;
      };

      --paper-header-panel-cover-container: {
        border: 1px solid gray;
      };
    }

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

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

  <link rel="import" type="css" href="paper-header-panel.css">

  <template>
    <content id="headerContent" select="paper-toolbar, .paper-header"></content>

    <div id="mainPanel">
      <div id="mainContainer" class$="[[_computeMainContainerClass(mode)]]">
        <content id="mainContent" select="*"></content>
      </div>
      <div id="dropShadow"></div>
    </div>
  </template>

</dom-module>

<script>

  (function() {

    'use strict';

    var SHADOW_WHEN_SCROLLING = 1;
    var SHADOW_ALWAYS = 2;


    var MODE_CONFIGS = {

      outerScroll: {
        'scroll': true
      },

      shadowMode: {
        'standard': SHADOW_ALWAYS,
        'waterfall': SHADOW_WHEN_SCROLLING,
        'waterfall-tall': SHADOW_WHEN_SCROLLING
      },

      tallMode: {
        'waterfall-tall': true
      }
    };

    Polymer({

      is: 'paper-header-panel',

      /**
       * Fired when the content has been scrolled.  `event.detail.target` returns
       * the scrollable element which you can use to access scroll info such as
       * `scrollTop`.
       *
       *     <paper-header-panel on-content-scroll="scrollHandler">
       *       ...
       *     </paper-header-panel>
       *
       *
       *     scrollHandler: function(event) {
       *       var scroller = event.detail.target;
       *       console.log(scroller.scrollTop);
       *     }
       *
       * @event content-scroll
       */

      properties: {

        /**
         * Controls header and scrolling behavior. Options are
         * `standard`, `seamed`, `waterfall`, `waterfall-tall`, `scroll` and
         * `cover`. Default is `standard`.
         *
         * `standard`: The header is a step above the panel. The header will consume the
         * panel at the point of entry, preventing it from passing through to the
         * opposite side.
         *
         * `seamed`: The header is presented as seamed with the panel.
         *
         * `waterfall`: Similar to standard mode, but header is initially presented as
         * seamed with panel, but then separates to form the step.
         *
         * `waterfall-tall`: The header is initially taller (`tall` class is added to
         * the header).  As the user scrolls, the header separates (forming an edge)
         * while condensing (`tall` class is removed from the header).
         *
         * `scroll`: The header keeps its seam with the panel, and is pushed off screen.
         *
         * `cover`: The panel covers the whole `paper-header-panel` including the
         * header. This allows user to style the panel in such a way that the panel is
         * partially covering the header.
         *
         *     <paper-header-panel mode="cover">
         *       <paper-toolbar class="tall">
         *         <core-icon-button icon="menu"></core-icon-button>
         *       </paper-toolbar>
         *       <div class="content"></div>
         *     </paper-header-panel>
         */
        mode: {
          type: String,
          value: 'standard',
          observer: '_modeChanged',
          reflectToAttribute: true
        },

        /**
         * If true, the drop-shadow is always shown no matter what mode is set to.
         */
        shadow: {
          type: Boolean,
          value: false
        },

        /**
         * The class used in waterfall-tall mode.  Change this if the header
         * accepts a different class for toggling height, e.g. "medium-tall"
         */
        tallClass: {
          type: String,
          value: 'tall'
        },

        /**
         * If true, the scroller is at the top
         */
        atTop: {
          type: Boolean,
          value: true,
          readOnly: true
        }
      },

      observers: [
        '_computeDropShadowHidden(atTop, mode, shadow)'
      ],

      ready: function() {
        this.scrollHandler = this._scroll.bind(this);
        this._addListener();

        // Run `scroll` logic once to initialze class names, etc.
        this._keepScrollingState();
      },

      detached: function() {
        this._removeListener();
      },

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

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

      /**
       * Returns true if the scroller has a visible shadow.
       *
       * @property visibleShadow
       * @type Boolean
       */
      get visibleShadow() {
        return this.$.dropShadow.classList.contains('has-shadow');
      },

      _computeDropShadowHidden: function(atTop, mode, shadow) {

        var shadowMode = MODE_CONFIGS.shadowMode[mode];

        if (this.shadow) {
          this.toggleClass('has-shadow', true, this.$.dropShadow);

        } else if (shadowMode === SHADOW_ALWAYS) {
          this.toggleClass('has-shadow', true, this.$.dropShadow);

        } else if (shadowMode === SHADOW_WHEN_SCROLLING && !atTop) {
          this.toggleClass('has-shadow', true, this.$.dropShadow);

        } else {
          this.toggleClass('has-shadow', false, this.$.dropShadow);

        }
      },

      _computeMainContainerClass: function(mode) {
        // TODO:  It will be useful to have a utility for classes
        // e.g. Polymer.Utils.classes({ foo: true });

        var classes = {};

        classes['flex'] = mode !== 'cover';

        return Object.keys(classes).filter(
          function(className) {
            return classes[className];
          }).join(' ');
      },

      _addListener: function() {
        this.scroller.addEventListener('scroll', this.scrollHandler, false);
      },

      _removeListener: function() {
        this.scroller.removeEventListener('scroll', this.scrollHandler);
      },

      _modeChanged: function(newMode, oldMode) {
        var configs = MODE_CONFIGS;
        var header = this.header;
        var animateDuration = 200;

        if (header) {
          // in tallMode it may add tallClass to the header; so do the cleanup
          // when mode is changed from tallMode to not tallMode
          if (configs.tallMode[oldMode] && !configs.tallMode[newMode]) {
            header.classList.remove(this.tallClass);
            this.async(function() {
              header.classList.remove('animate');
            }, animateDuration);
          } else {
            header.classList.toggle('animate', configs.tallMode[newMode]);
          }
        }
        this._keepScrollingState();
      },

      _keepScrollingState: function() {
        var main = this.scroller;
        var header = this.header;

        this._setAtTop(main.scrollTop === 0);

        if (header && this.tallClass && MODE_CONFIGS.tallMode[this.mode]) {
          this.toggleClass(this.tallClass, this.atTop ||
              header.classList.contains(this.tallClass) &&
              main.scrollHeight < this.offsetHeight, header);
        }
      },

      _scroll: function() {
        this._keepScrollingState();
        this.fire('content-scroll', {target: this.scroller}, {bubbles: false});
      },

      _getScrollerForMode: function(mode) {
        return MODE_CONFIGS.outerScroll[mode] ?
            this : this.$.mainContainer;
      }

    });

  })();

</script>