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Example of a Layer swipe map.

Example of a Layer swipe map.

main.js
import 'ol/ol.css';
import Map from 'ol/Map';
import OSM from 'ol/source/OSM';
import TileLayer from 'ol/layer/Tile';
import View from 'ol/View';
import XYZ from 'ol/source/XYZ';
import {getRenderPixel} from 'ol/render';

const osm = new TileLayer({
  source: new OSM(),
});

const key = 'Get your own API key at https://www.maptiler.com/cloud/';
const attributions =
  '<a href="https://www.maptiler.com/copyright/" target="_blank">&copy; MapTiler</a> ' +
  '<a href="https://www.openstreetmap.org/copyright" target="_blank">&copy; OpenStreetMap contributors</a>';

const aerial = new TileLayer({
  source: new XYZ({
    attributions: attributions,
    url: 'https://api.maptiler.com/tiles/satellite/{z}/{x}/{y}.jpg?key=' + key,
    maxZoom: 20,
  }),
});

const map = new Map({
  layers: [osm, aerial],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
  }),
});

const swipe = document.getElementById('swipe');

aerial.on('prerender', function (event) {
  const ctx = event.context;
  const mapSize = map.getSize();
  const width = mapSize[0] * (swipe.value / 100);
  const tl = getRenderPixel(event, [width, 0]);
  const tr = getRenderPixel(event, [mapSize[0], 0]);
  const bl = getRenderPixel(event, [width, mapSize[1]]);
  const br = getRenderPixel(event, mapSize);

  ctx.save();
  ctx.beginPath();
  ctx.moveTo(tl[0], tl[1]);
  ctx.lineTo(bl[0], bl[1]);
  ctx.lineTo(br[0], br[1]);
  ctx.lineTo(tr[0], tr[1]);
  ctx.closePath();
  ctx.clip();
});

aerial.on('postrender', function (event) {
  const ctx = event.context;
  ctx.restore();
});

const listener = function () {
  map.render();
};
swipe.addEventListener('input', listener);
swipe.addEventListener('change', listener);
index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Layer Swipe</title>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://unpkg.com/elm-pep"></script>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v3/polyfill.min.js?features=fetch,requestAnimationFrame,Element.prototype.classList,URL,TextDecoder,Number.isInteger"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <input id="swipe" type="range" style="width: 100%">
    <script src="main.js"></script>
  </body>
</html>
package.json
{
  "name": "layer-swipe",
  "dependencies": {
    "ol": "6.9.0"
  },
  "devDependencies": {
    "parcel": "^2.0.0-beta.1"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --public-url . index.html"
  }
}