ol.source.Vector sources use a function of this type to get the url
to load features from.
This function takes an ol.Extent representing the area to be loaded,
a {number} representing the resolution (map units per pixel) and an
ol.proj.Projection for the projection as arguments and returns a
{string} representing the URL.
ol.source.Vector sources use a function of this type to get the url to load features from.
This function takes an ol.Extent representing the area to be loaded, a
{number}representing the resolution (map units per pixel) and an ol.proj.Projection for the projection as arguments and returns a{string}representing the URL.