getRowAndSectionCount(): number {
    return this._cachedRowCount + this.sectionIdentities.length;
  }
Returns the total number of rows in the data source (see `getRowCount` for how this is calculated) plus the number of sections in the data.

If you are specifying the rowIdentities or sectionIdentities, then `getRowAndSectionCount` will return the number of rows & sections in the filtered data source.