The function to be called when request the data.

The type of the wijmaps layer. Possible values are: <ul> <li>vector: The vector layer. Used to draw a shape or placemark. </li> <li>items: The items layer. Draw user defined items in the map. </li> <li>virtual: The virual layer. Similar to items layer. Can draw different items in diffrent zoom.</li> <li>custom: The user customized layer. </li> </ul>