How does location cache works? - xamarin

Question

I have an application that gets the GPS location of the devices every time the user fills-up the form. My problem is capturing the GPS location takes too long. It takes about 40 seconds to 60 seconds before the GPS has been captured. I am using jamesmontemagno's Geolocator plugin.
GPS Parameters:
Accuracy: 100 meters
Timeout: 1 minute
Here is my code that I am using right now:
var defaultgpsaccuracy = Convert.ToDouble(Preferences.Get("gpsaccuracy", String.Empty, "private_prefs"));
var defaultgpstimeout = Convert.ToDouble(Preferences.Get("gpstimeout", String.Empty, "private_prefs"));
var locator = CrossGeolocator.Current;
locator.DesiredAccuracy = defaultgpsaccuracy;
position = await locator.GetLastKnownLocationAsync();
if (position != null)
{
string location = position.Latitude + "," + position.Longitude;
lblStartLocation.Text = location;
}
else
{
position = await locator.GetPositionAsync(TimeSpan.FromMinutes(defaultgpstimeout), null, false);
string location = position.Latitude + "," + position.Longitude;
lblStartLocation.Text = location;
}
These are my questions:
I used locator.GetLastKnownLocationAsync(); how long before the location cache refreshes?
And does the last known location refreshes when there is a change of location
And does the location refreshes when the devices is outside the accuracy range for example the accuracy is 100 meters does the location cache refresh when the device is outside the 100 meters range of the last know location?

Answer 1

I would highly recommend looking through the documentation for that particular plugin, James Montemagno is a well known and respected developer employed my Microsoft working on the Xamarin framework, so his plugins, extensions and toolkits tend to be pretty highly optimised for use in cross-platform applications.
Looking at the documentation it's clear that trying to get the last 'known' location looks at an internally cached location data set and is not necessarily optimized for near real-time queries. However it can be used to reduce the number of actual location queries you have to do within your app.
The full snippet from the linked documentation is as follows:
public async Task<Position> GetCurrentLocation()
{
public static async Task<Position> GetCurrentPosition()
{
Position position = null;
try
{
var locator = CrossGeolocator.Current;
locator.DesiredAccuracy = 100;
position = await locator.GetLastKnownLocationAsync();
if (position != null)
{
//got a cahched position, so let's use it.
return position;
}
if (!locator.IsGeolocationAvailable || !locator.IsGeolocationEnabled)
{
//not available or enabled
return null;
}
position = await locator.GetPositionAsync(TimeSpan.FromSeconds(20), null, true);
}
catch (Exception ex)
{
Debug.WriteLine("Unable to get location: " + ex);
}
if (position == null)
return null;
var output = string.Format("Time: {0} \nLat: {1} \nLong: {2} \nAltitude: {3} \nAltitude Accuracy: {4} \nAccuracy: {5} \nHeading: {6} \nSpeed: {7}",
position.Timestamp, position.Latitude, position.Longitude,
position.Altitude, position.AltitudeAccuracy, position.Accuracy, position.Heading, position.Speed);
Debug.WriteLine(output);
return position;
}
}
This method is designed to follow a hierarchical patter of location retrieval, it starts with the last 'known' position and queries if necessary, you could also take this a step further and add in a timeout to checking for a cached location if you wanted to.
In regards to how often this data is refreshed, I'd refer you to the documentation section titled 'Background Updates'
James talks about a driving app as an example of how this works. The refresh is handled differently across Android and iOS but here's the snippet regarding Android:
For this you will want to integrate a foreground service that
subscribes to location changes and the user interface binds to. Please
read through the Xamarin.Android Services documentation
In the code example he uses he shows you how to create a 'listener' that will check for changes to location periodically. This might be a better fit for what you're trying to do depending on the purpose of your application.