async 및 await를 사용한 비동기 프로그래밍 - 4 ( Task.WhenAny )

https://docs.microsoft.com/ko-kr/dotnet/csharp/programming-guide/concepts/async/cancel-remaining-async-tasks-after-one-is-complete

비동기 작업 하나가 완료되면 남은 비동기 작업 취소(C#)

비동기 작업 하나가 완료되면 남은 비동기 작업 취소

 

1.중요부분

IEnumerable<Task<int>> downloadTasksQuery = from url in urlList select ProcessURLAsync(url, client, ct);

Task<int>[] downloadTasks = downloadTasksQuery.ToArray();

Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

Task.WhenAny 는 배열에 하나라도 완료되면 종료되고 결과가 리턴된다.

 

 

2. 전체코드

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.  
using System.Threading;  
  
namespace CancelAfterOneTask  
{  
    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);  
                resultsTextBox.Text += "\r\nDownload complete.";  
            }  
            catch (OperationCanceledException)  
            {  
                resultsTextBox.Text += "\r\nDownload canceled.";  
            }  
            catch (Exception)  
            {  
                resultsTextBox.Text += "\r\nDownload failed.";  
            }  
  
            // Set the CancellationTokenSource to null when the download is complete.  
            cts = null;  
        }  
  
        // You can still include a Cancel button if you want to.  
        private void cancelButton_Click(object sender, RoutedEventArgs e)  
        {  
            if (cts != null)  
            {  
                cts.Cancel();  
            }  
        }  
  
        // Provide a parameter for the CancellationToken.  
        async Task AccessTheWebAsync(CancellationToken ct)  
        {  
            HttpClient client = new HttpClient();  
  
            // Call SetUpURLList to make a list of web addresses.  
            List<string> urlList = SetUpURLList();  
  
            // ***Comment out or delete the loop.  
            //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";
            //}  
  
            // ***Create a query that, when executed, returns a collection of tasks.  
            IEnumerable<Task<int>> downloadTasksQuery =  
                from url in urlList select ProcessURLAsync(url, client, ct);  
  
            // ***Use ToArray to execute the query and start the download tasks.   
            Task<int>[] downloadTasks = downloadTasksQuery.ToArray();  
  
            // ***Call WhenAny and then await the result. The task that finishes   
            // first is assigned to firstFinishedTask.  
            Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);  
  
            // ***Cancel the rest of the downloads. You just want the first one.  
            cts.Cancel();  
  
            // ***Await the first completed task and display the results.   
            // Run the program several times to demonstrate that different  
            // websites can finish first.  
            var length = await firstFinishedTask;  
            resultsTextBox.Text += $"\r\nLength of the downloaded website:  {length}\r\n";
        }  
  
        // ***Bundle the processing steps for a website into one async method.  
        async Task<int> ProcessURLAsync(string url, HttpClient client, CancellationToken ct)  
        {  
            // GetAsync returns a Task<HttpResponseMessage>.   
            HttpResponseMessage response = await client.GetAsync(url, ct);  
  
            // Retrieve the website contents from the HttpResponseMessage.  
            byte[] urlContents = await response.Content.ReadAsByteArrayAsync();  
  
            return urlContents.Length;  
        }  
  
        // 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;  
        }  
    }  
    // Sample output:  
  
    // Length of the downloaded website:  158856  
  
    // Download complete.  
}