Source: bemap-ol/bemap-map-ol-circle.js

/**
 * Add a bemap.Circle to the layer
 * @public
 * @param {bemap.Circle} bemap.Circle.
 * @param {object} options
 * @return {bemap.OlMap}
 */
bemap.OlMap.prototype.addCircle = function(circle, options) {
  if (circle && bemap.inheritsof(circle, bemap.Circle)) {
    circle.native = new ol.Feature({
      geometry: new ol.geom.Polygon.circular(new ol.Sphere(6378137), circle.getCoordinate().getLonLatArray(), circle.getRadius() * ol.proj.METERS_PER_UNIT.m, 128),
    });

    if (circle.map === null) {
      circle.map = this;
    }

    if (circle.id) {
      circle.native.setId(circle.id);
    }

    bemap.OlMap.prototype._addOwnToProperties(circle);

    circle.native.getGeometry().transform(bemap.Map.PROJ.EPSG_WGS84, this.native.getView().getProjection());

    if (circle.style) {
      if (circle.style.native === null) {
        this.buildCircleStyle(circle.style);
      }
      circle.native.setStyle(circle.style.native);
    }

    var opts = options || {};
    var l = null;

    if (opts.layer && bemap.inheritsof(opts.layer, bemap.VectorLayer)) {
      l = opts.layer;
    } else {
      l = this.getLayerByName(bemap.Map.DEFAULT_LAYER.CIRCLE);
      if (l === null) {
        l = new bemap.VectorLayer({
          name: bemap.Map.DEFAULT_LAYER.CIRCLE
        });
        this.addLayer(l);
      }
    }

    if (circle.layer === null) {
      circle.layer = l;
    }

    l.native.getSource().addFeature(circle.native);
  }
  return this;
};

/**
 * Set the coordinates of the circle.
 * @protected
 * @param {bemap.Circle} circle the circle object to remove.
 * @return {bemap.OlMap} this
 */
bemap.OlMap.prototype.setCoordinateCircle = function(circle) {
  var c = circle.getCenter();
  circle.native.getGeometry().setCoordinates(this._fromLonLat(c.getLon(), c.getLat()));
  return this;
};

/**
 * Set the radius of the circle.
 * @protected
 * @param {bemap.Circle} circle the circle object to remove.
 * @return {bemap.OlMap} this
 */
bemap.OlMap.prototype.setRadiusCircle = function(circle) {
  var c = circle.getCenter();
  circle.native.getGeometry().setCoordinates(this._fromLonLat(c.getLon(), c.getLat()));
  return this;
};

/**
 * Remove a circle from his layer.
 * @public
 * @param {bemap.Circle} circle the circle object to remove.
 */
bemap.OlMap.prototype.removeCircle = function(circle) {
  if (circle && circle.layer && circle.layer.native && bemap.inheritsof(circle, bemap.Circle)) {
    circle.layer.native.getSource().removeFeature(circle.native);
    circle.layer = null;
    circle.map = null;
  }
  return this;
};

/**
 * Set the listner when an specified eventType occur on bemap.Circle.
 * @public
 * @param {bemap.Circle} circle
 * @param {bemap.Map.EventType} eventType Event type.
 * @param {function} callback Function will be called when the specified eventType is occur.
 * @param {object} options options.
 * @return {bemap.Listener} this.
 */
bemap.OlMap.prototype.onCircle = function(circle, eventType, callback, options) {
  return this._onFeature(circle, eventType, callback, options, {
    singleFeature: true
  });
};

/**
 * Define the draggable capability for bemap.Circle.
 * @protected
 * @param {bemap.Circle} circle bemap object.
 * @param {function} callback Function will be called when the specified eventType is occur.
 * @param {object} options Options.
 * @param {bemap.Layer} options.layerFilter set the bemap layer used as filter.
 * @return {bemap.Listener} bemap.listener.
 */
bemap.OlMap.prototype.draggableCircle = function(circle, callback, options) {
  return this._draggableFeature(circle, callback, options, {
    singleFeature: true
  });
};

/**
 * Define the draggable capability for all bemap.Circle.
 * @protected
 * @param {function} callback Function will be called when the specified eventType is occur.
 * @param {object} options Options.
 * @param {bemap.Layer} options.layerFilter set the bemap layer used as filter.
 * @return {bemap.Listener} bemap.listener.
 */
bemap.OlMap.prototype.draggableCircles = function(callback, options) {
  return this._draggableFeature(null, callback, options, {
    circles: true
  });
};

/**
 * Update the coordinate from the native map browser.
 * @public
 */
bemap.OlMap.prototype.updateCircleCenter = function(circle) {
  var cir = circle.native.getGeometry().getExtent();
  var oo = ol.extent.getCenter(cir);
  var circ = ol.proj.transform(oo, this.native.getView().getProjection(), bemap.Map.PROJ.EPSG_WGS84);

  var coord = circle.coordinate;

  if (coord && coord !== null && bemap.inheritsof(coord, bemap.Coordinate)) {
    coord.setLon(circ[0]);
    coord.setLat(circ[1]);
  } else {
    circle.coordinate = new bemap.Coordinate(circ[0], circ[1]);
  }

  if (circle.coordinate.length > cir.length) {
    for (var j = cir.length; j < circle.coordinate.length; j++) {
      circle.coordinate.pop();
    }
  }
};