Image Load Events

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Image Load Events

Example using image load events.

Image sources fire events related to image loading. You can listen for imageloadstart, imageloadend, and imageloaderror type events to monitor image loading progress. This example registers listeners for these events and renders an image loading progress bar at the bottom of the map.

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index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Image Load Events</title>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
      .map {
        background: #E0ECED;
      }
      .wrapper {
        position: relative;
      }
      #progress {
        position: absolute;
        bottom: 0;
        left: 0;
        height: 2px;
        background: rgba(0, 60, 136, 0.4);
        width: 0;
        transition: width 250ms;
      }    </style>
  </head>
  <body>
    <div class="wrapper">
      <div id="map" class="map"></div>
      <div id="progress"></div>
    </div>
    <script src="index.js"></script>
  </body>
</html>

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index.jsimport 'ol/ol.css';
import Map from 'ol/Map';
import View from 'ol/View';
import ImageLayer from 'ol/layer/Image';
import ImageWMS from 'ol/source/ImageWMS';


/**
 * Renders a progress bar.
 * @param {HTMLElement} el The target element.
 * @constructor
 */
function Progress(el) {
  this.el = el;
  this.loading = 0;
  this.loaded = 0;
}


/**
 * Increment the count of loading tiles.
 */
Progress.prototype.addLoading = function() {
  if (this.loading === 0) {
    this.show();
  }
  ++this.loading;
  this.update();
};


/**
 * Increment the count of loaded tiles.
 */
Progress.prototype.addLoaded = function() {
  var this_ = this;
  setTimeout(function() {
    ++this_.loaded;
    this_.update();
  }, 100);
};


/**
 * Update the progress bar.
 */
Progress.prototype.update = function() {
  var width = (this.loaded / this.loading * 100).toFixed(1) + '%';
  this.el.style.width = width;
  if (this.loading === this.loaded) {
    this.loading = 0;
    this.loaded = 0;
    var this_ = this;
    setTimeout(function() {
      this_.hide();
    }, 500);
  }
};


/**
 * Show the progress bar.
 */
Progress.prototype.show = function() {
  this.el.style.visibility = 'visible';
};


/**
 * Hide the progress bar.
 */
Progress.prototype.hide = function() {
  if (this.loading === this.loaded) {
    this.el.style.visibility = 'hidden';
    this.el.style.width = 0;
  }
};

var progress = new Progress(document.getElementById('progress'));

var source = new ImageWMS({
  url: 'https://ahocevar.com/geoserver/wms',
  params: {'LAYERS': 'topp:states'},
  serverType: 'geoserver'
});

source.on('imageloadstart', function() {
  progress.addLoading();
});

source.on('imageloadend', function() {
  progress.addLoaded();
});
source.on('imageloaderror', function() {
  progress.addLoaded();
});

var map = new Map({
  layers: [
    new ImageLayer({source: source})
  ],
  target: 'map',
  view: new View({
    center: [-10997148, 4569099],
    zoom: 4
  })
});

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package.json{
  "name": "image-load-events",
  "dependencies": {
    "ol": "6.1.1"
  },
  "devDependencies": {
    "parcel": "1.11.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --experimental-scope-hoisting --public-url . index.html"
  }
}