in the help file:
QGeoPositionInfo QGeoPositionInfoSource::lastKnownPosition ( bool fromSatellitePositioningMethodsOnly = false ) const [pure virtual]
Returns an update containing the last known position, or a null update if none is available.
If fromSatellitePositioningMethodsOnly is true, this returns the last known position received from a satellite positioning method; if none is available, a null update is returned.
my question is how to determine the return value is availbale. what's "a null update"? sample code is prefered.