tileUrlFunction(tileCoord, pixelRatio, projection) {
    return undefined;
  }
@param {import("../tilecoord.js").TileCoord} tileCoord Tile coordinate.
@param {number} pixelRatio Pixel ratio.
@param {import("../proj/Projection.js").default} projection Projection.
@return {string|undefined} Tile URL.