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.

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
{
    /// 

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

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

            Task thisTask = null;
            CancellationTokenSource thisCts = null;

            async Task 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> outerTask;

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

                thisCts = CancellationTokenSource.CreateLinkedTokenSource(token);

                outerTask = new Task>(
                    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;
        }
    }

    /// 

    /// ViewModel
    /// 
    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;
    }

    /// 

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

        AsyncOp _asyncOp = new AsyncOp();

        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 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 progress = new Progress(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:

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).