Box Selection

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.

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index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Box Selection</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;
      }
      .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="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 {platformModifierKeyOnly} from 'ol/events/condition';
import GeoJSON from 'ol/format/GeoJSON';
import {DragBox, Select} from 'ol/interaction';
import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';
import {OSM, Vector as VectorSource} from 'ol/source';


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


var 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
var select = new Select();
map.addInteraction(select);

var selectedFeatures = select.getFeatures();

// a DragBox interaction used to select features by drawing boxes
var 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
  var rotation = map.getView().getRotation();
  var oblique = rotation % (Math.PI / 2) !== 0;
  var candidateFeatures = oblique ? [] : selectedFeatures;
  var 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) {
    var anchor = [0, 0];
    var geometry = dragBox.getGeometry().clone();
    geometry.rotate(-rotation, anchor);
    var extent$1 = geometry.getExtent();
    candidateFeatures.forEach(function(feature) {
      var geometry = feature.getGeometry().clone();
      geometry.rotate(-rotation, anchor);
      if (geometry.intersectsExtent(extent$1)) {
        selectedFeatures.push(feature);
      }
    });
  }

});

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

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

selectedFeatures.on(['add', 'remove'], function() {
  var names = selectedFeatures.getArray().map(function(feature) {
    return feature.get('name');
  });
  if (names.length > 0) {
    infoBox.innerHTML = names.join(', ');
  } else {
    infoBox.innerHTML = 'No countries selected';
  }
});