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Windows Phone 8 : Building Location Aware Apps - Getting Started with Location (part 1) - Geolocator Class

1/27/2015 3:06:07 AM
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A new geographic location API is included with the Windows Phone 8 SDK. This WinPRT API supersedes the Windows Phone 7 .NET API that relies on the GeoCoordinateWatcher class. Although the .NET API is still supported, for new apps it is recommended that you use the new Geolocator class that is part of the new WinPRT API. The new WinPRT geo location API has many similarities with the .NET API.

The new geographic location API is located in the namespace Windows.Devices.Geolocation.

Geolocator Class

The Geolocator class can be used by your app to monitor the location of the phone. It contains events for monitoring hardware state changes and geographic position changes.

Retrieving the Current Location

Geolocator allows you to track the phone’s location, using its PositionChanged event, and to retrieve the phone’s current location using its GetGeopositionAsync method.

The GetGeopositionAsync method forgoes the need to subscribe to the PositionChanged event when all you need is a single reading. With GetGeopositionAsync you can use the await keyword to conveniently wait for the method to return the Geoposition result, as shown in the following example:

var geolocator = new Geolocator();
Geoposition geoposition = await geolocator.GetGeopositionAsync();
Geocoordinate geocoordinate = geoposition.Coordinate;


Note

When attempting to call the Geolocator object’s GetGeopositionAsync method, if location is disabled in the phone’s settings, an UnauthorizedAccessException is raised.



Note

The Coordinate property of the Geoposition class is of type Windows.Devices.Geolocation.Geocoordinate, which is not to be confused with the System.Device.Location.GeoCoordinate class. Notice the capital “C” in the latter type name.

Other types, such as the Geoposition class, use the same casing pattern to indicate that they belong to the WinPRT API and not the .NET API.


The .NET GeoCoordinate type (and not the WinPRT Geocoordinate type) is used heavily by the phone’s mapping API. Converting between the two types is a matter of copying over the various property values. Listing 1 shows an extension method that is used to convert a WinPRT Geocoordinate into a .NET GeoCoordinate.

LISTING 1. GeocoordinateExtensions.ToGeoCoordinate Method


static class GeocoordinateExtensions
{
    public static GeoCoordinate ToGeoCoordinate(this Geocoordinate geocoordinate)
    {
        return new GeoCoordinate
            (
                geocoordinate.Latitude,
                geocoordinate.Longitude,
                geocoordinate.Altitude ?? Double.NaN,
                geocoordinate.Accuracy,
                geocoordinate.AltitudeAccuracy ?? Double.NaN,
                geocoordinate.Speed ?? Double.NaN,
                geocoordinate.Heading ?? Double.NaN);
    }
}


The Geoposition result of the GetGeopositionAsync method potentially represents a cached reading. Caching the reading saves the geo location infrastructure from having to engage hardware components. GetGeopositionAsync has a method overload that allows you to specify the maximum age of the cached reading and the maximum number of seconds that the OS should wait to get a location result before timing out and raising an exception.


Best Practices

It is recommended that you use the GetGeopositionAsync method when tracking is not required because it results in better battery life and, therefore, a better user experience.


DesiredAccuracy Property

Geolocator allows you to request a desired accuracy level via its DesiredAccuracy property, which can be set to either the default accuracy level, which lets the OS decide the accuracy level, or a high accuracy level, which causes the OS to prefer the accuracy of GPS. To create a Geolocator that uses the default accuracy setting, use the following:

Geolocator defaultAccuracyLocator = new Geolocator();

For high accuracy, set the DesiredAccuracy property like so:

Geolocator highAccuracyLocator
    = new Geolocator {DesiredAccuracy = PositionAccuracy.High};

The PositionAccuracy enum has the following two values:

- DefaultResults in a power optimized (or low power) configuration at the probable cost of decreased accuracy. Moreover, the phone device favors the use of cell tower triangulation and Wi-Fi triangulation above GPS.

- HighFavors GPS when available at the cost of greater power usage and, thus, reduced battery life.


Best Practices

Use the default (power optimized) accuracy setting unless a higher level of accuracy is required. Using lower accuracy minimizes power consumption, thereby increasing battery life.


DesiredAccuracyInMeters Property

In addition to the DesiredAccuracy property, Geolocator includes a DesiredAccuracyInMeters property, which is not present in the older GeoCoordinateWatcher API. The DesiredAccuracyInMeters property is a nullable unsigned integer value that lets the OS choose the mechanism for geo location. Lower values cause the phone to prefer the accuracy of GPS.

Setting the DesiredAccuracyInMeters property to a non-null value overrides the DesiredAccuracy property.

MovementThreshold Property

In some environments, signal noise can cause a GPS device to register movement when there is no movement. This can be caused by surface reflection, which occurs when signals bounce off walls or other structures, or by other environmental factors. It is also exacerbated by the absence of a GPS antenna on Windows Phone devices.

The MovementThreshold property of the Geolocator allows you to specify the minimum amount of movement required to register a change in position. When a change is registered, the Geolocator’s PositionChanged event is raised.

MovementThreshold is a double value, which indicates the distance in meters, relative to the last coordinate received. The MovementThreshold property allows position change notifications to be smoothed. If the value is set too low, the PositionChanged event may be raised too frequently, or when there is no actual change in location. Conversely, setting the value too high reduces the ability of the device to provide up-to-date location coordinates. A road navigation app, for example, with a MovementThreshold value set too high, may cause a user to miss a turn. Set too low, the application may provide continuous directions when the phone is, in fact, stationary.


Tip

Setting the Geolocator.MovementThreshold property to at least 20 helps to smooth out the signal, causing the PositionChanged event to be raised only when the location has changed significantly.


Monitoring Position Changes

The Geolocator class exposes the following two events:

- PositionChangedThis event is raised when a change in location is detected.

- StatusChangedThis event is raised when the underlying location service changes state; for example, when the location component is receiving data or when it has been disabled.

PositionChanged Event

The PositionChanged event is raised whenever a significant change is detected in the phone’s location. How large the change needs to be is determined by the Geolocator object’s MovementThreshold property.

The following code fragment demonstrates how to subscribe to the PositionChanged event:

geolocator.PositionChanged += HandlePositionChanged;

When the PositionChanged event is raised, the handler receives a PositionChangedEventArgs object that contains a Geoposition property, as shown:

void HandlePositionChanged(
    Geolocator sender, PositionChangedEventArgs e)
{
    Geoposition geoposition = e.Position;
    Geocoordinate geocoordinate = geoposition.Coordinate;
    CivicAddress civicAddress = geoposition.CivicAddress;
}

The Position property of the PositionChangedEventArgs is a Geoposition instance containing the following two properties:

- Coordinate (of type Geocoordinate)

- CivicAddress (of type CivicAddress

In the context of the geo location API, a position can be thought of as a location at a point in time. Geocoordinate includes a Timestamp property of type DateTimeOffset, which indicates when the reading was taken.


Best Practices

Geolocator leverages phone hardware that consumes power. Subscribe to its PositionChanged event only when it is needed by your app, and unsubscribe as soon as it is no longer needed. This reduces power consumption and increases the battery life of the phone.


StatusChanged Event

The status of the underlying geographic location component is indicated by the Geolocator object’s LocationStatus property, which is of type PositionStatus. PositionStatus is an enum that contains the following values:

- DisabledThe location provider is disabled. No position updates occur.

- InitializingThe location provider is initializing. This status occurs, for example, when the GPS component is obtaining a fix.

- NoDataNo location data is available from any location provider.

- NotAvailableThe Windows Sensor and Location Platform is not available on the device. The minimum Windows Phone hardware requirements preclude this status.

- NotInitializedAn operation to retrieve location has not yet been initialized. LocationStatus will have this value if the application has not yet called GetGeopositionAsync or registered an event handler for the PositionChanged event.

- ReadyA location provider is ready to supply new data. In this state, the Geolocator is able to raise its PositionChanged event.

The location provider transitions between these states, as depicted in Figure 1.

Image

FIGURE 1 Geolocator transitions between PositionStatus values.

When the Geolocator object’s StatusChanged event is raised, the handler receives a StatusChangedEventArgs object, as shown in the following excerpt:

void HandleStatusChanged(Geolocator sender, StatusChangedEventArgs args)
{
    PositionStatus status = args.Status;
    switch (status)
    {
            case PositionStatus.Disabled:
            // ...
            break;
            case PositionStatus.Initializing:
            // ...
            break;
            case PositionStatus.NoData:
            // ...
            break;
            case PositionStatus.NotInitialized:
            // ...
            break;
            case PositionStatus.Ready:
            // ...
            break;
    }
}

 
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