Cancel async task at startup

Question

I have the following method, called several times (e.g. onkeyup of textbox), which filters the items in the list asynchronously.

private async void filterCats(string category,bool deselect)
{

    List<Category> tempList = new List<Category>();
    //Wait for categories
    var tokenSource = new CancellationTokenSource();
    var token = tokenSource.Token;

    //HERE,CANCEL TASK IF ALREADY RUNNING


    tempList= await _filterCats(category,token);
    //Show results
    CAT_lb_Cats.DataSource = tempList;
    CAT_lb_Cats.DisplayMember = "strCategory";
    CAT_lb_Cats.ValueMember = "idCategory";

}

and the next task

private async Task<List<Category>> _filterCats(string category,CancellationToken token)
{
    List<Category> result = await Task.Run(() =>
    {
        return getCatsByStr(category);
    },token);

    return result;
}

and I would like to check if the task is running, and if it is canceled and run with a new value. I know how to cancel a task, but how can I check if it is running?

Answer 1

This is the code I use for this:

if (_tokenSource != null)
{
    _tokenSource.Cancel();
}

_tokenSource = new CancellationTokenSource();

try
{
    await loadPrestatieAsync(_bedrijfid, _projectid, _medewerkerid, _prestatieid, _startDate, _endDate, _tokenSource.Token);
}
catch (OperationCanceledException ex)
{
}

and to call the procedure this is (simplified, of course):

private async Task loadPrestatieAsync(int bedrijfId, int projectid, int medewerkerid, int prestatieid,
        DateTime? startDate, DateTime? endDate, CancellationToken token)
{
    await Task.Delay(100, token).ConfigureAwait(true);
    try{
        //do stuff

        token.ThrowIfCancellationRequested();

    }
    catch (OperationCanceledException ex)
    {
       throw;
    }
    catch (Exception Ex)
    {
        throw;
    }
}

I make a delay of 100 ms, because the same action starts quite quickly and repeatedly, a small delay of 100 ms makes it look like the GUI is more efficient.

Answer 2

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, Rx (Reactive Extensions) , , ..

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, RxAutoComplete, , .

namespace TryOuts
{
    using System;
    using System.Linq;
    using System.Threading.Tasks;
    using System.Windows.Forms;
    using System.Reactive.Linq;
    using System.Threading;

    // Simulated async search service, that can fail.
    public class FakeWordSearchService
    {
        private static Random _rnd = new Random();
        private static string[] _allWords = new[] {
            "gideon", "gabby", "joan", "jessica", "bob", "bill", "sam", "johann"
        };

        public async Task<string[]> Search(string searchTerm, CancellationToken cancelToken)
        {
            await Task.Delay(_rnd.Next(600), cancelToken); // simulate async call.
            if ((_rnd.Next() % 5) == 0) // every 5 times, we will cause a search failure
                throw new Exception(string.Format("Search for '{0}' failed on purpose", searchTerm));
            return _allWords.Where(w => w.StartsWith(searchTerm)).ToArray();
        }
    }

    public static class RxAutoComplete
    {
        // Returns an observable that pushes the 'txt' TextBox text when it has changed.
        static IObservable<string> TextChanged(TextBox txt)
        {
            return from evt in Observable.FromEventPattern<EventHandler, EventArgs>(
                h => txt.TextChanged += h,
                h => txt.TextChanged -= h)
                select ((TextBox)evt.Sender).Text.Trim();
        }

        // Throttles the source.
        static IObservable<string> ThrottleInput(IObservable<string> source, int minTextLength, TimeSpan throttle)
        {
            return source
                .Where(t => t.Length >= minTextLength) // Wait until we have at least 'minTextLength' characters
                .Throttle(throttle)      // We don't start when the user is still typing
                .DistinctUntilChanged(); // We only fire, if after throttling the text is different from before.
        }

        // Provides search results and performs asynchronous, 
        // cancellable search with automatic retries on errors
        static IObservable<string[]> PerformSearch(IObservable<string> source, FakeWordSearchService searchService)
        {
            return from term in source // term from throttled input
                   from result in Observable.FromAsync(async token => await searchService.Search(term, token))
                        .Retry(3)          // Perform up to 3 tries on failure
                        .TakeUntil(source) // Cancel pending request if new search request was made.
                   select result;
        }

        // Putting it all together.
        public static void RunUI()
        {
            // Our simple search GUI.
            var inputTextBox = new TextBox() { Width = 300 };
            var searchResultLB = new ListBox { Top = inputTextBox.Height + 10, Width = inputTextBox.Width };
            var searchStatus = new Label { Top = searchResultLB.Height + 30, Width = inputTextBox.Width };
            var mainForm = new Form { Controls = { inputTextBox, searchResultLB, searchStatus }, Width = inputTextBox.Width + 20 }; 

            // Our UI update handlers.
            var syncContext = SynchronizationContext.Current;
            Action<Action> onUITread = (x) => syncContext.Post(_ => x(), null);
            Action<string> onSearchStarted = t => onUITread(() => searchStatus.Text = (string.Format("searching for '{0}'.", t)));
            Action<string[]> onSearchResult = w => { 
                searchResultLB.Items.Clear(); 
                searchResultLB.Items.AddRange(w);
                searchStatus.Text += string.Format(" {0} maches found.", w.Length > 0 ? w.Length.ToString() : "No");
            };

            // Connecting input to search
            var input = ThrottleInput(TextChanged(inputTextBox), 1, TimeSpan.FromSeconds(0.5)).Do(onSearchStarted);
            var result = PerformSearch(input, new FakeWordSearchService());

            // Running it
            using (result.ObserveOn(syncContext).Subscribe(onSearchResult, ex => Console.WriteLine(ex)))
                Application.Run(mainForm);
        }
    }
}