Windows Phone 8 : Location APIs (part 2) - Accessing Location Information

// Normal Accuracy
var locator = new Geolocator();
locator.DesiredAccuracy = PositionAccuracy.High;

The level of accuracy affects not only how accurate the information is, but also how fast you can get the information back. With the default level of accuracy, the Geolocator class will return after it can get a location instead of waiting for high-accuracy information.

When developing your application, you need to also determine how often you need the geolocation information. Retrieving geolocation information can be a drain on the battery, so deciding whether you need a one-time location or need to constantly monitor the change of location should be part of your application design. Let’s look at ways to do both.

protected async override void OnNavigatedTo(NavigationEventArgs e)
{
  base.OnNavigatedTo(e);

  var locator = new Geolocator();

  Geoposition location = await _locator.GetGeopositionAsync();
}

By using await, the method will wrap the operation in an asynchronous handler so you can simply write the code as if it were asynchronous. If location information is not available, the method will return a null reference. After you have the Geoposition object, you can simply see the location information (or even the civic information like city, state, country, and postal code):

var locator = new Geolocator();

Geoposition location = await _locator.GetGeopositionAsync();

if (location != null)
{
  // Get coordinates
  double latitude = location.Coordinate.Latitude;
  double longitude = location.Coordinate.Longitude;
  double? altitude = location.Coordinate.Altitude;

  // Get civic location information
  string country = location.CivicAddress.Country;
  string city = location.CivicAddress.City;
}

You can also check the status of the location by using the Geolocator class’s LocationStatus property:

var locator = new Geolocator();

Geoposition location = await _locator.GetGeopositionAsync();

if (location != null)
{
  // Get coordinates
  double latitude = location.Coordinate.Latitude;
  double longitude = location.Coordinate.Longitude;
  double? altitude = location.Coordinate.Altitude;

  // Get civic location information
  string country = location.CivicAddress.Country;
  string city = location.CivicAddress.City;
}
else
{
  if (locator.LocationStatus == PositionStatus.Disabled)
  {
    statusMessage.Text = "Location Information has been disabled";
  }
}

The LocationStatus property returns the status of the location information. This can be one of six statuses:

• Ready: Location information is available.

• Initializing: Location information should be available soon. The device is initializing.

• NoData: The device is working correctly, but no data is available. This could be because none of the different location information is available (no GPS, no cell towers, and no Wi-Fi).

• NotInitialized: The location services haven’t been initialized. This usually happens before calling the GetPositionAsync method the first time.

• NotAvailable: The device doesn’t support location services.

• Disabled: Location information was disabled for this application. This is usually if the user was asked and rejected the permission to let your application use location information or the user has disabled location services on the phone.

For this one-time use, the Geoposition result is all you need. The data available from the Geoposition class includes

• Latitude: This is the current latitude of the phone (location on the surface of the Earth).

• Longitude: This is the current longitude of the phone (location on the surface of the Earth).

• IsUnknown: This says whether the coordinate is invalid or unknown.

• Speed: This is an approximate speed of the user in meters per second.

• Course: This is an approximate heading as related to true north (0–360 degrees).

• Altitude: This is the approximate altitude above sea level.

• HorizontalAccuracy: This is the distance of uncertainty in this location information (in meters).

• VerticalAccuracy: This is the distance of uncertainty in this location information (in meters).

Continuously monitoring the geolocation of the phone is similar to a one-time retrieval of location information, but the method structuring the code is a little different. You should start by creating a Geolocator as a class-level variable (to maintain its lifetime) and register for both the StatusChanged and the PositionChanged events from the UI thread (for example, in response to an AppBar button click):

private void startButton_Click(object sender, EventArgs e)
{

  // Make sure we can only call it after
  ((ApplicationBarIconButton)sender).IsEnabled = false;


  // Wire up for events
  _locator.PositionChanged += _locator_PositionChanged;
  _locator.StatusChanged += _locator_StatusChanged;
}

Depending on your particular use-case, you also should specify when to update you with location information. You can set both time and movement thresholds that will dictate how often to update you on location changes:

  // Set Thresholds
  _locator.ReportInterval = 2000;   // 2 seconds
  _locator.MovementThreshold = 100; // 100 meters

  // Wire up for events
  _locator.PositionChanged += _locator_PositionChanged;
  _locator.StatusChanged += _locator_StatusChanged;

The ReportInterval property accepts time in milliseconds between calls. The MovementThreshold specifies how far the user needs to move before you are notified.

In both of the event handlers, you will have to marshal the data to the UI thread (using Dispatcher.BeginInvoke). For example, in the StatusChanged event you will need to tell the user what is happening based on the statuses that are returned, like so:

void _locator_StatusChanged(Geolocator sender,
                            StatusChangedEventArgs args)
{
  Dispatcher.BeginInvoke(() =>
    {
      switch (args.Status)
      {
        // Tell the user the location information is coming
        case PositionStatus.Initializing:
          statusMessage.Text = "Location Initializing";
          break;

        // Alert the user that data isn't available currently
        case PositionStatus.NoData:
          statusMessage.Text =
            "No location available...try again later";
          break;

        // If status is Ready, then PositionChanged fired correctly
        case PositionStatus.Ready:
          statusMessage.Text = "Receiving Location Information";
          break;

        // If Disabled, tell the user
        case PositionStatus.Disabled:
          statusMessage.Text = "Location Information is disabled";
          break;
      }
    });
}

Because these statuses can change although your application is running, you should continue to monitor the PositionStatus values and keep the user apprised of the validity of this information. Note that, unlike the one-time retrieval, we do not get the status information from the watcher when the Ready status fires. This is because the other event (PositionChanged) will be fired as the location changes:

void _locator_PositionChanged(Geolocator sender,
                              PositionChangedEventArgs args)
{
  Dispatcher.BeginInvoke(() =>
    {
      // Just bind the position to the UI so we can data-bind to it
      DataContext = args.Position.Coordinate;
    });
}