GUI froze while Dispatcher.BeginInvoke or Task.StartNew

Question

NOTE: Depends on this quetion

I have a view-model like this:

public class ViewModel {
    private readonly IPersonService _personService;
    private readonly ObservableCollection<SearchPersonModel> _foundedList;
    private readonly DispatcherTimer _timer;
    private readonly Dispatcher _dispatcher;
    private CancellationTokenSource _tokenSource;

    public SearchPatientViewModel(IPersonService personService) {
        _personService = personService;
        _foundedList = new ObservableCollection<SearchPersonModel>();
        _dispatcher = (/*CurrentApplication*/).Dispatcher;
        _timer = new DispatcherTimer(
            TimeSpan.FromMilliseconds(1000),
            DispatcherPriority.Normal,
            TimerCallBack,
            _dispatcher);
        _tokenSource = new CancellationTokenSource();
    }

    public string Term {
        get { return _term; }
        set {
            // implementing INotifyPropertyChanged
            if(_term== value)
                return;
            _term= value;
            OnPropertyChanged(() => Term);
            tokenSource.Cancel(); // canceling prev search query
            _timer.Stop(); // stop the timer to reset it
            // start it again to do a search query if user change not the term for 1000ms
            _timer.Start(); 
        }
    }

    private void TimerCallBack(object sender, EventArgs e) {
        _timer.Stop();
        _tokenSource = new CancellationTokenSource();
        var task = Task<IEnumerable<SearchPersonModel>>.Factory
            .StartNew(Search, _tokenSource.Token);
        _dispatcher.BeginInvoke((Action)(() => {
            _foundedList.Clear();
            foreach(var item in task.Result)
                _foundedList.Add(item);
        }), DispatcherPriority.Background);
    }

    private IEnumerable<SearchPersonModel> Search() {
        return _personService.DoSearch(this.Term);
    }

}

and in the IPersonService implementation I do this:

public class PersonService : IPersonService {
    public IEnumerable<SearchPersonModel> DoSearch(string term){
        System.Threading.Thread.Sleep(10000);
        return some-search-result;
    }
}

However, I expect that while search query is executing, GUI be free. But it froze! Have you any idea where is my mistake?

Answer 1

The problem is that evaluating task.Result will block until the query has completed.

The simplest option is probably to make the Search method perform the _dispatcher.BeginInvoke call at the end instead.

Another option - which will become easier with C# 5 - would be to add a continuation to the task, so that when it's completed you can update the UI. At the moment you'd use Task.ContinueWith; with C# 5 you'd use async and await.