openlayers/examples/vector-label-decluttering.js

goog.require('ol.Map');
goog.require('ol.View');
goog.require('ol.extent');
goog.require('ol.format.GeoJSON');
goog.require('ol.layer.Vector');
goog.require('ol.source.Vector');
goog.require('ol.style.Fill');
goog.require('ol.style.Stroke');
goog.require('ol.style.Style');
goog.require('ol.style.Text');

var map = new ol.Map({
  target: 'map',
  view: new ol.View({
    center: [0, 0],
    zoom: 1
  })
});

var labelStyle = new ol.style.Style({
  geometry: function(feature) {
    var geometry = feature.getGeometry();
    if (geometry.getType() == 'MultiPolygon') {
      // Only render label for the widest polygon of a multipolygon
      var polygons = geometry.getPolygons();
      var widest = 0;
      for (var i = 0, ii = polygons.length; i < ii; ++i) {
        var polygon = polygons[i];
        var width = ol.extent.getWidth(polygon.getExtent());
        if (width > widest) {
          widest = width;
          geometry = polygon;
        }
      }
    }
    return geometry;
  },
  text: new ol.style.Text({
    font: '12px Calibri,sans-serif',
    overflow: true,
    fill: new ol.style.Fill({
      color: '#000'
    }),
    stroke: new ol.style.Stroke({
      color: '#fff',
      width: 3
    })
  })
});
var countryStyle = new ol.style.Style({
  fill: new ol.style.Fill({
    color: 'rgba(255, 255, 255, 0.6)'
  }),
  stroke: new ol.style.Stroke({
    color: '#319FD3',
    width: 1
  })
});
var style = [countryStyle, labelStyle];

var vectorLayer = new ol.layer.Vector({
  source: new ol.source.Vector({
    url: 'data/geojson/countries.geojson',
    format: new ol.format.GeoJSON()
  }),
  style: function(feature) {
    labelStyle.getText().setText(feature.get('name'));
    return style;
  },
  declutter: true
});

map.addLayer(vectorLayer);