Using Rx to Geocode an address in Bing Maps

Question

I am learning to use the Rx extensions for a Silverlight 4 app I am working on. I created a sample app to nail down the process and I cannot get it to return anything. Here is the main code:

    private IObservable<Location> GetGPSCoordinates(string Address1)
    {
        var gsc = new GeocodeServiceClient("BasicHttpBinding_IGeocodeService") as IGeocodeService;

        Location returnLocation = new Location();
        GeocodeResponse gcResp = new GeocodeResponse();

        GeocodeRequest gcr = new GeocodeRequest();
        gcr.Credentials = new Credentials();
        gcr.Credentials.ApplicationId = APP_ID2;
        gcr.Query = Address1;

        var myFunc = Observable.FromAsyncPattern<GeocodeRequest, GeocodeResponse>(gsc.BeginGeocode, gsc.EndGeocode);
        gcResp = myFunc(gcr) as GeocodeResponse;

        if (gcResp.Results.Count > 0 && gcResp.Results[0].Locations.Count > 0)
        {
            returnLocation = gcResp.Results[0].Locations[0];
        }
        return returnLocation as IObservable<Location>;
    }

gcResp comes back as null. Any thoughts or suggestions would be greatly appreciated.

Answer 1

The observable source you are subscribing to is asynchronous, so you can't access the result immediately after subscribing. You need to access the result in the subscription.

Better yet, don't subscribe at all and simply compose the response:

private IObservable<Location> GetGPSCoordinates(string Address1)
{
    IGeocodeService gsc = 
        new GeocodeServiceClient("BasicHttpBinding_IGeocodeService");

    Location returnLocation = new Location();
    GeocodeResponse gcResp  = new GeocodeResponse();

    GeocodeRequest gcr = new GeocodeRequest();
    gcr.Credentials = new Credentials();
    gcr.Credentials.ApplicationId = APP_ID2;
    gcr.Query = Address1;

    var factory = Observable.FromAsyncPattern<GeocodeRequest, GeocodeResponse>(
        gsc.BeginGeocode, gsc.EndGeocode);

    return factory(gcr)
        .Where(response => response.Results.Count > 0 && 
                           response.Results[0].Locations.Count > 0)
        .Select(response => response.Results[0].Locations[0]);
}

If you only need the first valid value (the location of the address is unlikely to change), then add a .Take(1) between the Where and Select.

Edit: If you want to specifically handle the address not being found, you can either return results and have the consumer deal with it or you can return an Exception and provide an OnError handler when subscribing. If you're thinking of doing the latter, you would use SelectMany:

return factory(gcr)
    .SelectMany(response => (response.Results.Count > 0 && 
        response.Results[0].Locations.Count > 0)
        ? Observable.Return(response.Results[0].Locations[0])
        : Observable.Throw<Location>(new AddressNotFoundException())
    );

Answer 2

If you expand out the type of myFunc you'll see that it is Func<GeocodeRequest, IObservable<GeocodeResponse>>.

Func<GeocodeRequest, IObservable<GeocodeResponse>> myFunc =
    Observable.FromAsyncPattern<GeocodeRequest, GeocodeResponse>
        (gsc.BeginGeocode, gsc.EndGeocode);

So when you call myFunc(gcr) you have an IObservable<GeocodeResponse> and not a GeocodeResponse. Your code myFunc(gcr) as GeocodeResponse returns null because the cast is invalid.

What you need to do is either get the last value of the observable or just do a subscribe. Calling .Last() will block. If you call .Subscribe(...) your response will come thru on the call back thread.

Try this:

gcResp = myFunc(gcr).Last();

Let me know how you go.

Answer 3

Richard (and others),

So I have the code returning the location and I have the calling code subscribing. Here is (hopefully) the final issue. When I call GetGPSCoordinates, the next statement gets executed immediately without waiting for the subscribe to finish. Here's an example in a button OnClick event handler.

Location newLoc = new Location();

GetGPSCoordinates(this.Input.Text).ObserveOnDispatcher().Subscribe(x =>
            {
             if (x.Results.Count > 0 && x.Results[0].Locations.Count > 0)
                  {
                     newLoc = x.Results[0].Locations[0];
                     Output.Text = "Latitude: " + newLoc.Latitude.ToString() +
  ", Longtude: " + newLoc.Longitude.ToString();
                  }
             else
                  {
                    Output.Text = "Invalid address";
                  }
});
            Output.Text = " Outside of subscribe --- Latitude: " + newLoc.Latitude.ToString() +
  ", Longtude: " + newLoc.Longitude.ToString();

The Output.Text assignment that takes place outside of Subscribe executes before the Subscribe has finished and displays zeros and then the one inside the subscribe displays the new location info.

The purpose of this process is to get location info that will then be saved in a database record and I am processing multiple addresses sequentially in a Foreach loop. I chose Rx Extensions as a solution to avoid the problem of the async callback as a coding trap. But it seems I have exchanged one trap for another.

Thoughts, comments, suggestions?