getResolutions(projection) {
    if (!this.useResolutions_) {
      return null;
    }
    let resolutions = super.getResolutions();
    if (!resolutions) {
      for (let i = 0, ii = this.layers_.length; i < ii; ++i) {
        const source = this.layers_[i].getSource();
        resolutions = source.getResolutions(projection);
        if (resolutions) {
          break;
        }
      }
    }
    return resolutions;
  }
@param {import("../proj/Projection").default} [projection] Projection.
@return {Array<number>|null} Resolutions.
@override