async 및 await를 사용한 비동기 프로그래밍 - 2

https://docs.microsoft.com/ko-kr/dotnet/csharp/programming-guide/concepts/async/cancel-an-async-task-or-a-list-of-tasks

비동기 작업 또는 작업 목록 취소(C#)

 

작동중인 async 를 멈추려면

 CancellationTokenSource cts;
 cts.Cancel();

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

// Add a using directive and a reference for System.Net.Http.
using System.Net.Http;

// Add the following using directive for System.Threading.
using System.Threading;

namespace CancelAListOfTasks
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainWindow()
        {
            InitializeComponent();
        }

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            resultsTextBox.Clear();

            try
            {
                await AccessTheWebAsync(cts.Token);
                // ***Small change in the display lines.
                resultsTextBox.Text += "\r\nDownloads complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownloads canceled.";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownloads failed.";
            }

            // Set the CancellationTokenSource to null when the download is complete.
            cts = null;
        }

        // Add an event handler for the Cancel button.
        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }

        // Provide a parameter for the CancellationToken.
        // ***Change the return type to Task because the method has no return statement.
        async Task AccessTheWebAsync(CancellationToken ct)
        {
            // Declare an HttpClient object.
            HttpClient client = new HttpClient();

            // ***Call SetUpURLList to make a list of web addresses.
            List<string> urlList = SetUpURLList();

            // ***Add a loop to process the list of web addresses.
            foreach (var url in urlList)
            {
                // GetAsync returns a Task<HttpResponseMessage>.
                // Argument ct carries the message if the Cancel button is chosen.
                // ***Note that the Cancel button can cancel all remaining downloads.
                HttpResponseMessage response = await client.GetAsync(url, ct);

                // Retrieve the website contents from the HttpResponseMessage.
                byte[] urlContents = await response.Content.ReadAsByteArrayAsync();

                resultsTextBox.Text +=
                    $"\r\nLength of the downloaded string: {urlContents.Length}.\r\n";
            }
        }

        // ***Add a method that creates a list of web addresses.
        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string>
            {
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/library/hh290138.aspx",
                "https://msdn.microsoft.com/library/hh290140.aspx",
                "https://msdn.microsoft.com/library/dd470362.aspx",
                "https://msdn.microsoft.com/library/aa578028.aspx",
                "https://msdn.microsoft.com/library/ms404677.aspx",
                "https://msdn.microsoft.com/library/ff730837.aspx"
            };
            return urls;
        }
    }

    // Output if you do not choose to cancel:

    //Length of the downloaded string: 35939.

    //Length of the downloaded string: 237682.

    //Length of the downloaded string: 128607.

    //Length of the downloaded string: 158124.

    //Length of the downloaded string: 204890.

    //Length of the downloaded string: 175488.

    //Length of the downloaded string: 145790.

    //Downloads complete.

    // Sample output if you choose to cancel:

    //Length of the downloaded string: 35939.

    //Length of the downloaded string: 237682.

    //Length of the downloaded string: 128607.

    //Downloads canceled.
}