forEachFeature(callback) {
    if (this.featuresRtree_) {
      return this.featuresRtree_.forEach(callback);
    }
    if (this.featuresCollection_) {
      this.featuresCollection_.forEach(callback);
    }
  }
Iterate through all features on the source, calling the provided callback
with each one.  If the callback returns any "truthy" value, iteration will
stop and the function will return the same value.
Note: this function only iterate through the feature that have a defined geometry.

@param {function(FeatureType): T} callback Called with each feature
    on the source.  Return a truthy value to stop iteration.
@return {T|undefined} The return value from the last call to the callback.
@template T
@api