Xamarin.Forms DependencyService difficulty getting

问题:

So after finally getting my head around Xamarin.Forms DependencyService I have nearly got it returning the device's current location.

my interface

public interface ICurrentLocation
{
    MyLocation SetCurrentLocation();
}

MyLocation

public class MyLocation
{
    public double Latitude {get; set;}
    public double Longitude{get; set;}
}

the line that calls it

MyLocation location = DependencyService.Get<ICurrentLocation>().SetCurrentLocation();

and in the CurrentLocation class in the Android project that implements the Geolocation class of Xamarin.Mobile

[assembly: Dependency(typeof(CurrentLocation))]
namespace MyCockburn.Droid
{

    public class CurrentLocation : Activity, ICurrentLocation

        Geolocator locator;
        Position position = new Position();
        MyLocation location;

        public MyLocation SetCurrentLocation()
        {
            GetPosition();
            location = new MyLocation()
            {
                Latitude = position.Latitude,
                Longitude = position.Longitude
            };


            return location;
       }

       async void GetPosition()
       {
           try
           {
                 locator = new Geolocator(this) { DesiredAccuracy = 50 };
                 if (locator.IsListening != true)
                       locator.StartListening(minTime: 1000, minDistance: 0);

                 position = await locator.GetPositionAsync(timeout: 20000);

           }
           catch (Exception e)
           {
               Log.Debug("GeolocatorError", e.ToString());
           }
       }
   }

my problem seems to be that is returning location before position holds the longitude and latitude

I am hoping my mistake is glaringly obvious

EDIT: the code works if I run it as a normal Android Activity

回答1:

I would do a slight modification since best practice is to either do all async or none. When you try to return the result from an async method from a non async method you can run into problems with deadlocks. Also, since you aren't using the await keyword when calling the GetPosition method, you are getting back a Task, but aren't checking when the operation is complete. I suggest slightly modifying your code as such.

public interface ICurrentLocation {
    Task<MyLocation> GetCurrentLocation();
}

public async Task<MyLocation> GetCurrentLocation()
{
    var position = await GetPosition();
    return new MyLocation()
    {
        Latitude = position.Latitude,
        Longitude = position.Longitude
    };
}

async Task<Location> GetPosition()
{
    try
    {
          locator = new Geolocator(this) { DesiredAccuracy = 50 };
          if (locator.IsListening != true)
                locator.StartListening(minTime: 1000, minDistance: 0);

          return await locator.GetPositionAsync(timeout: 20000);

    }
    catch (Exception e)
    {
        Log.Debug("GeolocatorError", e.ToString());
    }
}

回答2:

You aren't waiting for the position function to finish. Many different options and keeping it async is the best one but if you want it synchronous then try this blocking call:

   void GetPosition()
   {
       try
       {
             locator = new Geolocator(this) { DesiredAccuracy = 50 };
             if (locator.IsListening != true)
                   locator.StartListening(minTime: 1000, minDistance: 0);

             position = locator.GetPositionAsync(timeout: 20000).Result;

       }
       catch (Exception e)
       {
           Log.Debug("GeolocatorError", e.ToString());
       }
   }

I also recommend taking a look at Xamarin.Forms.Labs as it already has abstracted GPS service and working sample that is functional for all 3 platforms:

https://github.com/XForms/Xamarin-Forms-Labs

回答3:

Try adding the assembly above the namespace and awaiting the GetPosition method.

Take a look at this image: http://developer.xamarin.com/guides/cross-platform/xamarin-forms/dependency-service/Images/solution.png