ol.source.Tile sources use a function of this type to get the url
that provides a tile for a given tile coordinate.
This function takes an ol.TileCoord for the tile coordinate, a
{number} representing the pixel ratio and an ol.proj.Projection for
the projection as arguments and returns a {string} representing the tile
URL, or undefined if no tile should be requested for the passed tile
coordinate.
ol.source.Tile sources use a function of this type to get the url that provides a tile for a given tile coordinate.
This function takes an ol.TileCoord for the tile coordinate, a
{number}representing the pixel ratio and an ol.proj.Projection for the projection as arguments and returns a{string}representing the tile URL, or undefined if no tile should be requested for the passed tile coordinate.