private CLLocationManager locationManager;
public override void ViewDidLoad ()
{
  base.ViewDidLoad ();
  
  // Perform any additional setup after loading the view, typically from a nib.
  this.locationManager = new CLLocationManager();
  this.locationManager.LocationsUpdated += LocationManager_LocationsUpdated;
  this.btnStart.TouchUpInside += (s, e) => {
    this.lblOutput.Text = "Starting monitoring significant location changes...";
    this.locationManager.StartMonitoringSignificantLocationChanges();
  } ;
  this.btnStop.TouchUpInside += (s, e) => {
    this.locationManager.StopMonitoringSignificantLocationChanges...