Cancel thread and restart it

Question

When user resizes window some long text should be updated, but if the thread is already running it should be stopped and started over with new width parameter.

int myWidth;
private CancellationTokenSource tokenSource2 = new CancellationTokenSource();
private CancellationToken ct = new CancellationToken();

void container_Loaded(object sender, RoutedEventArgs e)
{
  ct = tokenSource2.Token;
  MyFunction();
}

        void container_SizeChanged(object sender, SizeChangedEventArgs e)
        {
          if (tokenSource2.Token.IsCancellationRequested)
            MyFunction();
          else
            tokenSource2.Cancel();
        }

        void MyFunction()            
        {
           myWidth = GetWidth();
           Task.Factory.StartNew(() =>
           {  
              string s;    
              for (int i=0;i<1000,i++){
                  s=s+Functionx(myWidth);
                  ct.ThrowIfCancellationRequested();
              }
              this.Dispatcher.BeginInvoke(new Action(() => { 
                   ShowText(s); 
              }));
           },tokenSource2.Token)
           .ContinueWith(t => {
              if (t.IsCanceled)
              {
                tokenSource2 = new CancellationTokenSource(); //reset token
                MyFunction(); //restart
              };
           });
        }

What now is happening is when I resize window I see text iteratively updating next several seconds as if old threads were not canceled. What am I doing wrong?

Answer 1

I don't think using global variables is a good idea in this case. Here's how I would do it by adding cancellation logic to my AsyncOp class from a related question. This code also implements the IProgress pattern and throttles the ViewModel updates.

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;
using System.Windows;

namespace Wpf_21611292
{
    /// <summary>
    /// Cancel and restarts an asynchronous operation
    /// </summary>
    public class AsyncOp<T>
    {
        readonly object _lock = new object();
        Task<T> _pendingTask = null;
        CancellationTokenSource _pendingCts = null;

        public Task<T> CurrentTask
        {
            get { lock (_lock) return _pendingTask; }
        }

        public bool IsPending
        {
            get { lock (_lock) return _pendingTask != null && !_pendingTask.IsCompleted; }
        }

        public bool IsCancellationRequested
        {
            get { lock (_lock) return _pendingCts != null && _pendingCts.IsCancellationRequested; }
        }

        public void Cancel()
        {
            lock (_lock)
            {
                if (_pendingTask != null && !_pendingTask.IsCompleted && !_pendingCts.IsCancellationRequested)
                    _pendingCts.Cancel();
            }
        }

        public Task<T> Run(
            Func<CancellationToken, Task<T>> routine,
            CancellationToken token = default,
            bool startAsync = false,
            bool continueAsync = false,
            TaskScheduler taskScheduler = null)
        {
            Task<T> previousTask = null;
            CancellationTokenSource previousCts = null;

            Task<T> thisTask = null;
            CancellationTokenSource thisCts = null;

            async Task<T> routineWrapper()
            {
                // await the old task
                if (previousTask != null)
                {
                    if (!previousTask.IsCompleted && !previousCts.IsCancellationRequested)
                    {
                        previousCts.Cancel();
                    }
                    try
                    {
                        await previousTask;
                    }
                    catch (Exception ex)
                    {
                        if (!(previousTask.IsCanceled || ex is OperationCanceledException))
                            throw;
                    }
                }

                // run and await this task
                return await routine(thisCts.Token);
            };

            Task<Task<T>> outerTask;

            lock (_lock)
            {
                previousTask = _pendingTask;
                previousCts = _pendingCts;

                thisCts = CancellationTokenSource.CreateLinkedTokenSource(token);

                outerTask = new Task<Task<T>>(
                    routineWrapper,
                    thisCts.Token,
                    continueAsync ?
                        TaskCreationOptions.RunContinuationsAsynchronously :
                        TaskCreationOptions.None);

                thisTask = outerTask.Unwrap();

                _pendingTask = thisTask;
                _pendingCts = thisCts;
            }

            var scheduler = taskScheduler;
            if (scheduler == null)
            {
                scheduler = SynchronizationContext.Current != null ?
                    TaskScheduler.FromCurrentSynchronizationContext() :
                    TaskScheduler.Default;
            }

            if (startAsync)
                outerTask.Start(scheduler);
            else
                outerTask.RunSynchronously(scheduler);

            return thisTask;
        }
    }

    /// <summary>
    /// ViewModel
    /// </summary>
    public class ViewModel : INotifyPropertyChanged
    {
        string _width;

        string _text;

        public string Width
        {
            get
            {
                return _width;
            }
            set
            {
                if (_width != value)
                {
                    _width = value;
                    this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Width)));
                }
            }
        }

        public string Text
        {
            get
            {
                return _text;
            }
            set
            {
                if (_text != value)
                {
                    _text = value;
                    this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(nameof(Text)));
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }

    /// <summary>
    /// MainWindow
    /// </summary>
    public partial class MainWindow : Window
    {
        ViewModel _model = new ViewModel { Text = "Starting..." };

        AsyncOp<DBNull> _asyncOp = new AsyncOp<DBNull>();

        CancellationTokenSource _workCts = new CancellationTokenSource();

        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = _model;

            this.Loaded += MainWindow_Loaded;
            this.SizeChanged += MainWindow_SizeChanged;
        }

        void MainWindow_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            _asyncOp.Run(WorkAsync, _workCts.Token);
        }

        void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            _asyncOp.Run(WorkAsync, _workCts.Token);
        }

        async Task<DBNull> WorkAsync(CancellationToken token)
        {
            const int limit = 200000000;
            var throttle = TimeSpan.FromMilliseconds(200);

            // update ViewModel's Width
            _model.Width = $"Width: {this.Width:#.##}";

            // update ViewModel's Text using IProgress pattern 
            // and throttling updates
            IProgress<int> progress = new Progress<int>(i =>
            {
                _model.Text = $"{(double)i / (limit - 1)* 100:0.}%";
            });

            var stopwatch = new Stopwatch();
            stopwatch.Start();

            // do some CPU-intensive work
            await Task.Run(() =>
            {
                int i;
                for (i = 0; i < limit; i++)
                {
                    if (stopwatch.Elapsed > throttle)
                    {
                        progress.Report(i);
                        stopwatch.Restart();
                    }
                    if (token.IsCancellationRequested)
                        break;
                }
                progress.Report(i);
            }, token);

            return DBNull.Value;
        }
    }
}

XAML:

<Window x:Class="Wpf_21611292.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <TextBox Width="200" Height="30" Text="{Binding Path=Width}"/>
        <TextBox Width="200" Height="30" Text="{Binding Path=Text}"/>
    </StackPanel>
</Window>

It uses async/await, so if you target .NET 4.0, you'd need Microsoft.Bcl.Async and VS2012+. Alternatively, you can convert async/await to ContinueWith, which is a bit tedious, but always possible (that's more or less what the C# 5.0 compiler does behind the scene).

Answer 2

You should use Microsoft's Reactive Framework (aka Rx) - NuGet System.Reactive.Windows.Threading (for WPF) and add using System.Reactive.Linq; - then you can do this:

    public MainWindow()
    {
        InitializeComponent();

        _subscription =
            Observable
                .FromEventPattern<SizeChangedEventHandler, SizeChangedEventArgs>(
                    h => container.SizeChanged += h,
                    h => container.SizeChanged -= h)
                .Select(e => GetWidth())
                .Select(w => Observable.Start(
                        () => String.Concat(Enumerable.Range(0, 1000).Select(n => Functionx(w)))))
                .Switch()
                .ObserveOnDispatcher()
                .Subscribe(t => ShowText(t));
    }

    private IDisposable _subscription = null;

That's all the code needed.

This responds to the SizeChanged event, calls GetWidth and then pushes the Functionx to another thread. It uses Switch() to always switch to the latest SizeChanged and then ignores any in-flight code. It pushes the result to the dispatcher and then calls ShowText.

If you need to close the form or stop the subscription running just call _subscription.Dispose().

Simple.