Question

My WPF application with the MVVM pattern shall basically perform the following:

• Button view binds to a command in the view model. –> Check!
• Command in view model asynchronously queries a webservice for a list of CProject objects for putting it into a ProjectList property. –> Check!

This looks like this…

Command in the view model:

proxy = new SomeService();
proxy.GetProjectList(GetProjectListCallback, username, password);

Callback in the view model:

private void GetProjectListCallback(object sender, GetProjectListCompletedEventArgs e) {
  this.ProjectList = e.Result;
}

SomeService implements an interface ISomeService.

public void GetProjectList(EventHandler<GetProjectListCompletedEventArgs> callback, string username, string password) {
  service.GetProjectListCompleted += callback;
  service.GetProjectListAsync(username, password);
}

So far this works fine. However I feel that I would like to move this callback to the service itself so that the view model only calls something like:

proxy = new SomeService();
this.ProjectList = proxy.GetProjectList(username, password);

But when moving the callback to the service how could it return e.Result to the calling view model? Or would using a Task a better idea?

Answer 1

The easiest way is to re-create your WCF service proxy with VS2012. This will change your asynchronous method signatures to be something like:

Task<MyProjectList> GetProjectListAsync(string username, string password);

and your command becomes:

proxy = new SomeService();
this.ProjectList = await proxy.GetProjectListAsync(username, password);

---

If you don’t want to re-create your WCF service proxy (it will update all your method signatures), then you can wrap the Begin*/End* methods as such:

public static Task<MyProjectList> GetProjectListTaskAsync(this SomeService @this, string username, string password)
{
  return Task<MyProjectList>.Factory.FromAsync(@this.BeginGetProjectList, @this.EndProjectList, username, password, null);
}

I have a full example of this kind of wrapping on my blog.

or the existing *Async/*Completed members as such:

public static Task<MyProjectList> GetProjectListTaskAsync(this SomeService @this, string username, string password)
{
  var tcs = new TaskCompletionSource<MyProjectList>();
  EventHandler<GetProjectListCompletedEventArgs> callback = null;
  callback = args =>
  {
    @this.GetProjectListCompleted -= callback;
    if (args.Cancelled) tcs.TrySetCanceled();
    else if (args.Error != null) tcs.TrySetException(args.Error);
    else tcs.TrySetResult(args.Result);
  };
  @this.GetProjectListCompleted += callback;
  @this.GetProjectListAsync(username, password);
}