jellyfin-web/dashboard-ui/bower_components/iron-scroll-threshold/iron-scroll-threshold.html

<!--
@license
Copyright (c) 2016 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
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-->

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

<!--
`iron-scroll-threshold` is a utility element that listens for `scroll` events from a
scrollable region and fires events to indicate when the scroller has reached a pre-defined
limit, specified in pixels from the upper and lower bounds of the scrollable region.
This element may wrap a scrollable region and will listen for `scroll` events bubbling
through it from its children.  In this case, care should be taken that only one scrollable
region with the same orientation as this element is contained within. Alternatively,
the `scrollTarget` property can be set/bound to a non-child scrollable region, from which
it will listen for events.

Once a threshold has been reached, a `lower-threshold` or `upper-threshold` event will
be fired, at which point the user may perform actions such as lazily-loading more data
to be displayed. After any work is done, the user must then clear the threshold by
calling the `clearTriggers` method on this element, after which it will
begin listening again for the scroll position to reach the threshold again assuming
the content in the scrollable region has grown. If the user no longer wishes to receive
events (e.g. all data has been exhausted), the threshold property in question (e.g.
`lowerThreshold`) may be set to a falsy value to disable events and clear the associated
triggered property.

### Example

```html
<iron-scroll-threshold on-lower-threshold="loadMoreData">
  <div>content</div>
</iron-scroll-threshold>
```

```js
  loadMoreData: function() {
    // load async stuff. e.g. XHR
    asyncStuff(function done() {
      ironScrollTheshold.clearTriggers();
    });
  }
```

### Using dom-repeat

```html
<iron-scroll-threshold on-lower-threshold="loadMoreData">
  <dom-repeat items="[[items]]">
    <template>
      <div>[[index]]</div>
    </template>
  </dom-repeat>
</iron-scroll-threshold>
```

### Using iron-list

```html
<iron-scroll-threshold on-lower-threshold="loadMoreData" id="threshold">
  <iron-list scroll-target="threshold" items="[[items]]">
    <template>
      <div>[[index]]</div>
    </template>
  </iron-list>
</iron-scroll-threshold>
```

@group Iron Element
@element iron-scroll-threshold
@demo demo/scrolling-region.html Scrolling Region
@demo demo/document.html Document Element
-->

<dom-module id="iron-scroll-threshold">
  <template>
    <style>
      :host {
        display: block;
      }
    </style>

    <content></content>

  </template>
</dom-module>

<script>

  Polymer({

    is: 'iron-scroll-threshold',

    properties: {

      /**
       * Distance from the top (or left, for horizontal) bound of the scroller
       * where the "upper trigger" will fire.
       */
      upperThreshold: {
        type: Number,
        value: 100
      },

      /**
       * Distance from the bottom (or right, for horizontal) bound of the scroller
       * where the "lower trigger" will fire.
       */
      lowerThreshold: {
        type: Number,
        value: 100
      },

      /**
       * Read-only value that tracks the triggered state of the upper threshold.
       */
      upperTriggered: {
        type: Boolean,
        value: false,
        notify: true,
        readOnly: true
      },

      /**
       * Read-only value that tracks the triggered state of the lower threshold.
       */
      lowerTriggered: {
        type: Boolean,
        value: false,
        notify: true,
        readOnly: true
      },

      /**
       * True if the orientation of the scroller is horizontal.
       */
      horizontal: {
        type: Boolean,
        value: false
      }
    },

    behaviors: [
      Polymer.IronScrollTargetBehavior
    ],

    observers: [
      '_setOverflow(scrollTarget)',
      '_initCheck(horizontal, isAttached)'
    ],

    get _defaultScrollTarget() {
      return this;
    },

    _setOverflow: function(scrollTarget) {
      this.style.overflow = scrollTarget === this ? 'auto' : '';
    },

    _scrollHandler: function() {
      // throttle the work on the scroll event
      var THROTTLE_THRESHOLD = 200;
      if (!this.isDebouncerActive('_checkTheshold')) {
        this.debounce('_checkTheshold', function() { 
          this.checkScrollThesholds();
        }, THROTTLE_THRESHOLD);
      }
    },

    _initCheck: function(horizontal, isAttached) {
      if (isAttached) {
        this.debounce('_init', function() {
          this.clearTriggers();
          this.checkScrollThesholds();
        });
      }
    },

    /**
     * Checks the scroll thresholds.
     * This method is automatically called by iron-scroll-threshold.
     *
     * @method checkScrollThesholds
     */
    checkScrollThesholds: function() {
      if (!this.scrollTarget || (this.lowerTriggered && this.upperTriggered)) {
        return;
      }
      var upperScrollValue = this.horizontal ? this._scrollLeft : this._scrollTop;
      var lowerScrollValue = this.horizontal ? 
          this.scrollTarget.scrollWidth - this._scrollTargetWidth - this._scrollLeft :
              this.scrollTarget.scrollHeight - this._scrollTargetHeight - this._scrollTop;

      // Detect upper threshold
      if (upperScrollValue <= this.upperThreshold && !this.upperTriggered) {
        this._setUpperTriggered(true);
        this.fire('upper-threshold');
      }
      // Detect lower threshold
      if (lowerScrollValue <= this.lowerThreshold && !this.lowerTriggered) {
        this._setLowerTriggered(true);
        this.fire('lower-threshold');
      }
    },

    /**
     * Clear the upper and lower threshold states.
     *
     * @method clearTriggers
     */
    clearTriggers: function() {
      this._setUpperTriggered(false);
      this._setLowerTriggered(false);
    }

    /**
     * Fires when the lower threshold has been reached.
     *
     * @event lower-threshold
     */

    /**
     * Fires when the upper threshold has been reached.
     *
     * @event upper-threshold
     */

  });

</script>