Box Selection

No countries selected

Using a DragBox interaction to select features.

This example shows how to use a DragBox interaction to select features. Selected features are added to the feature overlay of a select interaction (ol/interaction/Select) for highlighting.

Use Ctrl+Drag (Command+Drag on Mac) to draw boxes.

main.js

import 'ol/ol.css';
import GeoJSON from 'ol/format/GeoJSON';
import Map from 'ol/Map';
import View from 'ol/View';
import {DragBox, Select} from 'ol/interaction';
import {OSM, Vector as VectorSource} from 'ol/source';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';
import {platformModifierKeyOnly} from 'ol/events/condition';

const vectorSource = new VectorSource({
  url: 'data/geojson/countries.geojson',
  format: new GeoJSON(),
});

const map = new Map({
  layers: [
    new TileLayer({
      source: new OSM(),
    }),
    new VectorLayer({
      source: vectorSource,
    }),
  ],
  target: 'map',
  view: new View({
    center: [0, 0],
    zoom: 2,
    constrainRotation: 16,
  }),
});

// a normal select interaction to handle click
const select = new Select();
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 () {
  // 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;
  const candidateFeatures = oblique ? [] : selectedFeatures;
  const extent = dragBox.getGeometry().getExtent();
  vectorSource.forEachFeatureIntersectingExtent(extent, function (feature) {
    candidateFeatures.push(feature);
  });

  // 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.rotate(-rotation, anchor);
    const extent = geometry.getExtent();
    candidateFeatures.forEach(function (feature) {
      const geometry = feature.getGeometry().clone();
      geometry.rotate(-rotation, anchor);
      if (geometry.intersectsExtent(extent)) {
        selectedFeatures.push(feature);
      }
    });
  }
});

// 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(function (feature) {
    return feature.get('name');
  });
  if (names.length > 0) {
    infoBox.innerHTML = names.join(', ');
  } else {
    infoBox.innerHTML = 'No countries selected';
  }
});

index.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Box Selection</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;
      }
      .ol-dragbox {
        background-color: rgba(255,255,255,0.4);
        border-color: rgba(100,150,0,1);
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <div id="info">No countries selected</div>
    <script src="main.js"></script>
  </body>
</html>

package.json

{
  "name": "box-selection",
  "dependencies": {
    "ol": "6.6.1"
  },
  "devDependencies": {
    "parcel": "^2.0.0-beta.1"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --public-url . index.html"
  }
}