DEPRECATED

Use the @anypoint-web-components/awc module instead.

This is a port of iron-scroll-threshold that works with LitElement and plain WC.

arc-scroll-threshold is a utility element that listens for scroll events from a scrollable region and dispatches 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 are 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

In a Polymer template

static get template() {
  return html`
  <arc-scroll-threshold on-lower-threshold="loadMoreData">
    <template is="dom-repeat" items="[[items]]">
      ...
    </template>
  </arc-scroll-threshold>`;
}

async loadMoreData() {
  // loadData() loads some API data from somewhere
  const data = await loadData();
  // Assuming `items` contains list of rendered items
  this.items = [...this.items, ...data];
}

In a LitElement template

import { LitElement, html } from 'lit-element';
import '@advanced-rest-client/arc-scroll-threshold/arc-scroll-threshold.js';

class SampleElement extends LitElement {
  render() {
    const { items } = this;
    return html`
    <arc-scroll-threshold @lower-threshold="${this.loadMoreData}">
    ${items.map((item) => html`...`)}
    </arc-scroll-threshold>`;
  }

  async loadMoreData() {
    // loadData() loads some API data from somewhere
    const data = await this.loadData();
    // Assuming `items` contains list of rendered items
    this.items = [...this.items, ...data];
  }
}
customElements.define('sample-element', SampleElement);

Imperative use

<script type="module" src="@advanced-rest-client/arc-scroll-threshold/arc-scroll-threshold.js"></script>
<arc-scroll-threshold></arc-scroll-threshold>
<section id="list"></section>
<script>
{
  async function loadMore() {
    const data = await loadData();
    const fragment = document.createDocumentFragment();
    data.forEach((item) => {
      fragment.appendChild(createListItemElement(item))
    })
    document.getElementById('#list').appendChild(fragment)
  }
  document.querySelector('arc-scroll-threshold').onlowerthreshold = () => {
    loadMore();
  };
}
</script>

Development

git clone https://github.com/@advanced-rest-client/arc-scroll-threshold
cd arc-scroll-threshold
npm install

Running the demo locally

npm start

Running the tests

npm test