extend(feature) {
    const geometry = feature.getGeometry();
    const lineString = geometry;
    this.sketchFeature_ = feature;
    this.sketchCoords_ = lineString.getCoordinates();
    const last = this.sketchCoords_[this.sketchCoords_.length - 1];
    this.finishCoordinate_ = last.slice();
    this.sketchCoords_.push(last.slice());
    this.sketchPoint_ = new Feature(new Point(last));
    this.updateSketchFeatures_();
    this.dispatchEvent(
      new DrawEvent(DrawEventType.DRAWSTART, this.sketchFeature_),
    );
  }
Initiate draw mode by starting from an existing geometry which will
receive new additional points. This only works on features with
`LineString` geometries, where the interaction will extend lines by adding
points to the end of the coordinates array.
This will change the original feature, instead of drawing a copy.

The function will dispatch a `drawstart` event.

@param {!Feature<LineString>} feature Feature to be extended.
@api