@Override
    @Nullable
    public Viewport getViewport() {
        final BoundingBox bb = mMap == null ? null : mMap.getBoundingBox(0);
        if (bb == null) {
            return null;
        }
        return Viewport.forE6(bb.minLatitudeE6, bb.minLongitudeE6, bb.maxLatitudeE6, bb.maxLongitudeE6);
    }
call this instead of VTM.layers().add so that we can keep track of layers added by the tile provider
main/src/main/java/cgeo/geocaching/unifiedmap/mapsforgevtm/MapsforgeVtmFragment.java