Box Selection

DragBox1 feature12 selection2 box3

Using a DragBox interaction to select features.

main.js

import GeoJSON from 'ol/format/GeoJSON.js';
import Map from 'ol/Map.js';
import VectorLayer from 'ol/layer/Vector.js';
import VectorSource from 'ol/source/Vector.js';
import View from 'ol/View.js';
import {DragBox, Select} from 'ol/interaction.js';
import {Fill, Stroke, Style} from 'ol/style.js';
import {getWidth} from 'ol/extent.js';
import {platformModifierKeyOnly} from 'ol/events/condition.js';

const vectorSource = new VectorSource({
  url: 'https://openlayers.org/data/vector/ecoregions.json',
  format: new GeoJSON(),
});

const style = new Style({
  fill: new Fill({
    color: '#eeeeee',
  }),
});

const map = new Map({
  layers: [
    new VectorLayer({
      source: vectorSource,
      background: '#1a2b39',
      style: function (feature) {
        const color = feature.get('COLOR_BIO') || '#eeeeee';
        style.getFill().setColor(color);
        return style;
      },
    }),
  ],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
    constrainRotation: 16,
  }),
});

const selectedStyle = new Style({
  fill: new Fill({
    color: 'rgba(255, 255, 255, 0.6)',
  }),
  stroke: new Stroke({
    color: 'rgba(255, 255, 255, 0.7)',
    width: 2,
  }),
});

// a normal select interaction to handle click
const select = new Select({
  style: function (feature) {
    const color = feature.get('COLOR_BIO') || '#eeeeee';
    selectedStyle.getFill().setColor(color);
    return selectedStyle;
  },
});
map.addInteraction(select);

const selectedFeatures = select.getFeatures();

// a DragBox interaction used to select features by drawing boxes
const dragBox = new DragBox({
  condition: platformModifierKeyOnly,
});

map.addInteraction(dragBox);

dragBox.on('boxend', function () {
  const boxExtent = dragBox.getGeometry().getExtent();

  // if the extent crosses the antimeridian process each world separately
  const worldExtent = map.getView().getProjection().getExtent();
  const worldWidth = getWidth(worldExtent);
  const startWorld = Math.floor((boxExtent[0] - worldExtent[0]) / worldWidth);
  const endWorld = Math.floor((boxExtent[2] - worldExtent[0]) / worldWidth);

  for (let world = startWorld; world <= endWorld; ++world) {
    const left = Math.max(boxExtent[0] - world * worldWidth, worldExtent[0]);
    const right = Math.min(boxExtent[2] - world * worldWidth, worldExtent[2]);
    const extent = [left, boxExtent[1], right, boxExtent[3]];

    const boxFeatures = vectorSource
      .getFeaturesInExtent(extent)
      .filter(
        (feature) =>
          !selectedFeatures.getArray().includes(feature) &&
          feature.getGeometry().intersectsExtent(extent),
      );

    // features that intersect the box geometry are added to the
    // collection of selected features

    // if the view is not obliquely rotated the box geometry and
    // its extent are equalivalent so intersecting features can
    // be added directly to the collection
    const rotation = map.getView().getRotation();
    const oblique = rotation % (Math.PI / 2) !== 0;

    // when the view is obliquely rotated the box extent will
    // exceed its geometry so both the box and the candidate
    // feature geometries are rotated around a common anchor
    // to confirm that, with the box geometry aligned with its
    // extent, the geometries intersect
    if (oblique) {
      const anchor = [0, 0];
      const geometry = dragBox.getGeometry().clone();
      geometry.translate(-world * worldWidth, 0);
      geometry.rotate(-rotation, anchor);
      const extent = geometry.getExtent();
      boxFeatures.forEach(function (feature) {
        const geometry = feature.getGeometry().clone();
        geometry.rotate(-rotation, anchor);
        if (geometry.intersectsExtent(extent)) {
          selectedFeatures.push(feature);
        }
      });
    } else {
      selectedFeatures.extend(boxFeatures);
    }
  }
});

// clear selection when drawing a new box and when clicking on the map
dragBox.on('boxstart', function () {
  selectedFeatures.clear();
});

const infoBox = document.getElementById('info');

selectedFeatures.on(['add', 'remove'], function () {
  const names = selectedFeatures.getArray().map((feature) => {
    return feature.get('ECO_NAME');
  });
  if (names.length > 0) {
    infoBox.innerHTML = names.join(', ');
  } else {
    infoBox.innerHTML = 'None';
  }
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Box Selection</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <div>Selected regions: <span id="info">None</span></div>

    <script type="module" src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "box-selection",
  "dependencies": {
    "ol": "10.3.1"
  },
  "devDependencies": {
    "vite": "^3.2.3"
  },
  "scripts": {
    "start": "vite",
    "build": "vite build"
  }
}