Question

I have a silverlight 5 app that depends on several asynchronous calls to web services to populate the attributes of newly created graphics. I am trying to find a way to handle those asynchronous calls synchronously. I have tried the suggestions listed in this article and this one. i have tried the many suggestions regarding the Dispatcher object. None have worked well, so I am clearly missing something…

Here is what I have:

public partial class MainPage : UserControl {

 AutoResetEvent waitHandle = new AutoResetEvent(false);

 private void AssignNewAttributeValuesToSplitPolygons(List<Graphic> splitGraphics)
 {
   for (int i = 0; i < splitGraphics.Count; i++) 
   {
       Graphic g = splitGraphics[i];

       Thread lookupThread1 = new Thread(new ParameterizedThreadStart(SetStateCountyUtm));
       lookupThread1.Start(g);
       waitHandle.WaitOne();

       Thread lookupThread2 = new Thread(new ParameterizedThreadStart(SetCongressionalDistrict));
       lookupThread1.Start(g);
       waitHandle.WaitOne();
 }

 private void SetStateCountyUtm(object  graphic)
 {
    this.Dispatcher.BeginInvoke(delegate() {
            WrapperSetStateCountyUtm((Graphic)graphic);
     });
 }

 private void WrapperSetStateCountyUtm(Graphic graphic)
    {
        GISQueryEngine gisQEngine = new GISQueryEngine();
        gisQEngine.StateCountyUtmLookupCompletedEvent += new GISQueryEngine.StateCountyUtmLookupEventHandler(gisQEngine_StateCountyUtmLookupCompletedEvent);
        gisQEngine.PerformStateCountyUtmQuery(graphic.Geometry, graphic.Attributes["clu_number"].ToString());
    }

 void gisQEngine_StateCountyUtmLookupCompletedEvent(object sender, StateCountyUtmLookupCompleted stateCountyUtmLookupEventArgs)
 {
     string fred = stateCountyUtmLookupEventArgs.
     waitHandle.Set();
 }     

}

public class GISQueryEngine
{
  public void PerformStateCountyUtmQuery(Geometry inSpatialQueryGeometry, string cluNumber)
    {
        QueryTask queryTask = new QueryTask(stateandCountyServiceURL);
        queryTask.ExecuteCompleted += new EventHandler<QueryEventArgs>(queryTask_StateCountyLookupExecuteCompleted);
        queryTask.Failed += new EventHandler<TaskFailedEventArgs>(queryTask_StateCountyLookupFailed);
        Query spatialQueryParam = new ESRI.ArcGIS.Client.Tasks.Query();
        spatialQueryParam.OutFields.AddRange(new string[] { "*" });
        spatialQueryParam.ReturnGeometry = false;

        spatialQueryParam.Geometry = inSpatialQueryGeometry;
        spatialQueryParam.SpatialRelationship = SpatialRelationship.esriSpatialRelIntersects;
        spatialQueryParam.OutSpatialReference = inSpatialQueryGeometry.SpatialReference;

        queryTask.ExecuteAsync(spatialQueryParam, cluNumber);
    }

  //and a whole bunch of other stuff i can add if needed
}

If I leave the ‘waitHandle.WaitOne()’ method uncommented, no code beyond that method is ever called, at least that I can see with the step through debugger. The application just hangs.

If I comment out the ‘waitHandle.WaitOne()’, everything runs just fine – except asynchronously. In other words, when the app reads the Attribute values of the new graphics, those values may or may not be set depending on how quickly the asynch methods return.

Thanks for any help.

Answer 1

It’s going to be rather difficult to work through a problem like this as there are a few issues you’ll need to address. SL is by nature asynch so forcing it to try and work synchronously is usually a very bad idea. You shouldn’t do it unless it’s absolutely necessary.

Is there a reason that you cannot wait for an async. callback? From what I see you appear to be making two calls for every state that is being rendered. I’m guessing the concern is that one call must complete before the second is made? In scenarios like this, I would kick off the first async call, and in it’s response kick off the second call passing along the result you’ll want to use from the first call. The second call response updates the provided references.

However, in cases where you’ve got a significant number of states to update, this results in a rather chatty, and difficult to debug set of calls. I’d really be looking at creating a service call that can accept a set of state references and pass back a data structure set for the values to be updated all in one hit. (or at least grouping them up to one call per state if the batch will be too time consuming and you want to render/interact with visual elements as they load up.)