启动多个任务并在其完成时进行处理(C# 和 Visual Basic)

使用 Task.WhenAny,可以同时启动多个任务和依次对它们进行处理,并按开始的顺序完成而不是处理它们。

下面的示例使用查询创建任务的集合。每个任务下载一个指定网站的内容。在 while 循环的每次迭代,等待调用 WhenAny 返回在完成它首先下载任务的集合的任务。任务从集合中移除并处理。直到集合的循环不包含没有其他任务。

说明说明

若要运行此示例,需要 Visual Studio 2012 中,Visual Studio express 2012 中的 windows 桌面或在您的计算机上安装 .NET framework 4.5。

下载示例

可以下载完整的 windows 演示基础 (WPF) 项从 Async 示例:优化应用程序 然后按照这些步骤。

  1. 对要下载的文件,然后启动 Visual Studio 2012。

  2. 在菜单栏上,依次选择**“文件”“打开”“项目/解决方案”**。

  3. 打开项目 对话框中,打开包含示例代码。解压缩,然后打开 AsyncFineTuningCS 或 AsyncFineTuningVB 的解决方案的文件夹 (.sln) 文件。

  4. 解决方案资源管理器,打开 ProcessTasksAsTheyFinish 项目的快捷菜单,然后选择 设为启动项目

  5. 选择 F5 键运行项目。

    选择密钥 Ctrl+F5 运行项目,但不对其进行调试。

  6. 运行项目多次验证下载的长度不以相同顺序总是出现。

如果不希望下载该项目,可以查看 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件本主题末尾的。

生成示例

此示例添加到 在完成一个任务后取消剩余任务(C# 和 Visual Basic) 中开发的代码并使用相同的 UI。

若要生成此示例,分步,后面部分的“下载示例的”命令,但是,选择 CancelAfterOneTask 作为 启动项目。添加本主题中的更改到该项目的 AccessTheWebAsync 方法。更改标记为星号。

CancelAfterOneTask 项目还包括,那么,当执行,创建任务的集合的查询。每个调用在下面的代码 ProcessURLAsync 返回 TResult 是整数的 Task<TResult>

Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
    From url In urlList Select ProcessURLAsync(url, client, ct)
IEnumerable<Task<int>> downloadTasksQuery =
    from url in urlList select ProcessURL(url, client, ct);

在项目的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件中,对 AccessTheWebAsync 方法进行以下更改。

  • 通过应用 Enumerable.ToList<TSource> 执行查询而不是 ToArray<TSource>

    Dim downloadTasks As List(Of Task(Of Integer)) = downloadTasksQuery.ToList()
    
    List<Task<int>> downloadTasks = downloadTasksQuery.ToList();
    
  • 添加到集合执行每个任务的以下步骤的 while 循环。

    1. 等待调用 WhenAny 标识集合中的第一个任务完成其下载。

      Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)
      
      Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);
      
    2. 从集合中移除该任务。

      downloadTasks.Remove(firstFinishedTask)
      
      downloadTasks.Remove(firstFinishedTask);
      
    3. 等待 firstFinishedTask,供 ProcessURLAsync的调用返回。firstFinishedTask 变量是 TReturn 是整数的 Task<TResult>。任务已完成,但是,您等待它检索该下载网站的长度,如下例所示。

      Dim length = Await firstFinishedTask
      resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)
      
      int length = await firstFinishedTask;
      resultsTextBox.Text += String.Format("\r\nLength of the download:  {0}", length);
      

您应运行项目多次验证下载的长度不以相同顺序总是出现。

警告说明警告

在循环中使用 WhenAny,如该示例所述,解决涉及少量的任务的问题。但是,因此,如果需要处理,则大量任务其他方法更为有效的。有关更多信息和示例,请参见 处理任务作为完

完整的示例

下面的代码是 MainWindow.xaml.vb 或 MainWindow.xaml.cs 文件的全文该示例的。星号指出在本示例中添加了元素。

通知您必须添加 System.Net.Http的引用。

可以下载项目 Async 示例:优化应用程序

' Add an Imports directive and a reference for System.Net.Http.
Imports System.Net.Http

' Add the following Imports directive for System.Threading.
Imports System.Threading

Class MainWindow

    ' Declare a System.Threading.CancellationTokenSource.
    Dim cts As CancellationTokenSource


    Private Async Sub startButton_Click(sender As Object, e As RoutedEventArgs)

        ' Instantiate the CancellationTokenSource.
        cts = New CancellationTokenSource()

        resultsTextBox.Clear()

        Try
            Await AccessTheWebAsync(cts.Token)
            resultsTextBox.Text &= vbCrLf & "Downloads complete."

        Catch ex As OperationCanceledException
            resultsTextBox.Text &= vbCrLf & "Downloads canceled." & vbCrLf

        Catch ex As Exception
            resultsTextBox.Text &= vbCrLf & "Downloads failed." & vbCrLf
        End Try

        ' Set the CancellationTokenSource to Nothing when the download is complete.
        cts = Nothing
    End Sub


    ' You can still include a Cancel button if you want to.
    Private Sub cancelButton_Click(sender As Object, e As RoutedEventArgs)

        If cts IsNot Nothing Then
            cts.Cancel()
        End If
    End Sub


    ' Provide a parameter for the CancellationToken.
    ' Change the return type to Task because the method has no return statement.
    Async Function AccessTheWebAsync(ct As CancellationToken) As Task

        Dim client As HttpClient = New HttpClient()

        ' Call SetUpURLList to make a list of web addresses.
        Dim urlList As List(Of String) = SetUpURLList()

        ' ***Create a query that, when executed, returns a collection of tasks.
        Dim downloadTasksQuery As IEnumerable(Of Task(Of Integer)) =
            From url In urlList Select ProcessURLAsync(url, client, ct)

        ' ***Use ToList to execute the query and start the download tasks. 
        Dim downloadTasks As List(Of Task(Of Integer)) = downloadTasksQuery.ToList()

        ' ***Add a loop to process the tasks one at a time until none remain.
        While downloadTasks.Count > 0
            ' ***Identify the first task that completes.
            Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

            ' ***Remove the selected task from the list so that you don't
            ' process it more than once.
            downloadTasks.Remove(firstFinishedTask)

            ' ***Await the first completed task and display the results.
            Dim length = Await firstFinishedTask
            resultsTextBox.Text &= String.Format(vbCrLf & "Length of the downloaded website:  {0}" & vbCrLf, length)
        End While

    End Function


    ' Bundle the processing steps for a website into one async method.
    Async Function ProcessURLAsync(url As String, client As HttpClient, ct As CancellationToken) As Task(Of Integer)

        ' GetAsync returns a Task(Of HttpResponseMessage). 
        Dim response As HttpResponseMessage = Await client.GetAsync(url, ct)

        ' Retrieve the website contents from the HttpResponseMessage.
        Dim urlContents As Byte() = Await response.Content.ReadAsByteArrayAsync()

        Return urlContents.Length
    End Function


    ' Add a method that creates a list of web addresses.
    Private Function SetUpURLList() As List(Of String)

        Dim urls = New List(Of String) From
            {
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/en-us/library/hh290138.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290140.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            }
        Return urls
    End Function

End Class


' Sample output:

' Length of the download:  226093
' Length of the download:  412588
' Length of the download:  175490
' Length of the download:  204890
' Length of the download:  158855
' Length of the download:  145790
' Length of the download:  44908
' Downloads complete.
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 ProcessTasksAsTheyFinish
{
    public partial class MainWindow : Window
    {
        // Declare a System.Threading.CancellationTokenSource.
        CancellationTokenSource cts;

        public MainWindow()
        {
            InitializeComponent();
        }

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Clear();

            // Instantiate the CancellationTokenSource.
            cts = new CancellationTokenSource();

            try
            {
                await AccessTheWebAsync(cts.Token);
                resultsTextBox.Text += "\r\nDownloads complete.";
            }
            catch (OperationCanceledException)
            {
                resultsTextBox.Text += "\r\nDownloads canceled.\r\n";
            }
            catch (Exception)
            {
                resultsTextBox.Text += "\r\nDownloads failed.\r\n";
            }

            cts = null;
        }


        private void cancelButton_Click(object sender, RoutedEventArgs e)
        {
            if (cts != null)
            {
                cts.Cancel();
            }
        }


        async Task AccessTheWebAsync(CancellationToken ct)
        {
            HttpClient client = new HttpClient();

            // Make a list of web addresses.
            List<string> urlList = SetUpURLList();

            // ***Create a query that, when executed, returns a collection of tasks.
            IEnumerable<Task<int>> downloadTasksQuery =
                from url in urlList select ProcessURL(url, client, ct);

            // ***Use ToList to execute the query and start the tasks. 
            List<Task<int>> downloadTasks = downloadTasksQuery.ToList();

            // ***Add a loop to process the tasks one at a time until none remain.
            while (downloadTasks.Count > 0)
            {
                    // Identify the first task that completes.
                    Task<int> firstFinishedTask = await Task.WhenAny(downloadTasks);

                    // ***Remove the selected task from the list so that you don't
                    // process it more than once.
                    downloadTasks.Remove(firstFinishedTask);

                    // Await the completed task.
                    int length = await firstFinishedTask;
                    resultsTextBox.Text += String.Format("\r\nLength of the download:  {0}", length);
            }
        }


        private List<string> SetUpURLList()
        {
            List<string> urls = new List<string> 
            { 
                "https://msdn.microsoft.com",
                "https://msdn.microsoft.com/library/windows/apps/br211380.aspx",
                "https://msdn.microsoft.com/en-us/library/hh290136.aspx",
                "https://msdn.microsoft.com/en-us/library/dd470362.aspx",
                "https://msdn.microsoft.com/en-us/library/aa578028.aspx",
                "https://msdn.microsoft.com/en-us/library/ms404677.aspx",
                "https://msdn.microsoft.com/en-us/library/ff730837.aspx"
            };
            return urls;
        }


        async Task<int> ProcessURL(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;
        }
    }
}

// Sample Output:

// Length of the download:  226093
// Length of the download:  412588
// Length of the download:  175490
// Length of the download:  204890
// Length of the download:  158855
// Length of the download:  145790
// Length of the download:  44908
// Downloads complete.

请参见

参考

WhenAny

概念

微调异步应用程序(C# 和 Visual Basic)

使用 Async 和 Await 的异步编程(C# 和 Visual Basic)

其他资源

Async 示例:优化应用程序