_updateCache() {
    // create array data from maps.
    this._cache = {
      nodes: Object.values(this._nodeMap),
      edges: Object.values(this._edgeMap),
    };
    this._lastCacheUpdate = this._lastUpdate;
  }
/**
   * update local data cache and _lastCacheUpdate.
   */
update local data cache and _lastCacheUpdate.
update local data cache and _lastCacheUpdate .
_updateCache() {
    this._cache = {
      nodes: Object.values(this._nodeMap),
      edges: Object.values(this._edgeMap),
    };
    this._lastCacheUpdate = this._lastUpdate;
  }
_updateCache ( ) { this . _cache = { nodes : Object . values ( this . _nodeMap ) , edges : Object . values ( this . _edgeMap ) , } ; this . _lastCacheUpdate = this . _lastUpdate ; }
update local data cache and _lastCacheUpdate.
update local data cache and _lastCacheUpdate .
// create array data from maps.