啟動多項非同步工作並在它們完成時進行處理 (C# 和 Visual Basic)

發行項
06/10/2015

您可以使用 Task.WhenAny，同時啟動多個工作並在這些工作完成時逐一處理，而非依照它們啟動的順序處理。

下列範例會使用查詢建立工作的集合。每個工作都會下載指定之網站的內容。在每個 while 迴圈反覆項目中，等候的 WhenAny 呼叫會傳回工作集合中先完成下載的工作。該工作會從集合中移除及處理。這迴圈會重複執行直到集合不包含其他工作。

注意事項
若要執行範例，您必須將 Visual Studio 2012、Visual Studio 2013、Visual Studio Express 2012 for Windows Desktop、Visual Studio Express 2013 for Windows 或 .NET Framework 4.5 或 4.5.1 安裝在您的電腦上。

下載範例

您可以從非同步範例：微調應用程式下載完整的 Windows Presentation Foundation (WPF) 專案，然後依照下列步驟執行。

解壓縮您下載的檔案，然後啟動 Visual Studio。
在功能表列上，選擇 [檔案]、[開啟]、[專案/方案]。
在 [開啟專案] 對話方塊中，開啟含有已解壓縮之範例程式碼的資料夾，然後開啟 AsyncFineTuningCS 或 AsyncFineTuningVB 的方案 (.sln) 檔。
在 [方案總管] 中，開啟 [ProcessTasksAsTheyFinish] 專案的捷徑功能表，然後選取 [設定為啟始專案]。
選擇 F5 鍵以執行專案。

選取 Ctrl+F5 鍵執行專案，但不偵錯。
執行專案數次，確認下載的長度不一定以相同順序出現。

如果您不要下載專案，您可以檢視本主題結尾的 MainWindow.xaml.vb 和 MainWindow.xaml.cs 檔案。

建置範例

這個範例會加入至在當其中一項工作完成時，取消剩餘的非同步工作 (C# 和 Visual Basic) 所開發的程式碼並使用相同的 UI。

若要建置範例，請依照＜下載範例＞中的指示逐步執行，不過選取 CancelAfterOneTask 為 [啟始專案]。將本主題中的變更加入至該專案中的 AccessTheWebAsync 方法。變更已使用星號標記。

CancelAfterOneTask 專案已經包含查詢，這個查詢執行時，會建立工作集合。對 ProcessURLAsync 的每個呼叫都會在下列程式碼中傳回 TResult，其中 Task 為整數。

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);

在專案的 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案中，對 AccessTheWebAsync 方法進行下列變更。

藉由套用 Enumerable.ToList``1 而不是 ToArray``1，來執行查詢。

Dim downloadTasks As List(Of Task(Of Integer)) = downloadTasksQuery.ToList()

List<Task<int>> downloadTasks = downloadTasksQuery.ToList();

為集合中的每個工作加入執行下列步驟的 while 迴圈。

等候呼叫 WhenAny 來識別集合中的第一個工作以完成下載。

Dim firstFinishedTask As Task(Of Integer) = Await Task.WhenAny(downloadTasks)

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

從集合中移除該工作。

downloadTasks.Remove(firstFinishedTask)

downloadTasks.Remove(firstFinishedTask);

等候 firstFinishedTask，這是由 ProcessURLAsync 的呼叫傳回。 firstFinishedTask 變數是 Task，其中 TReturn 是整數。工作已經完成，不過，您需要等候它來擷取下載之網站的長度，如下列範例所示。

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);

您應該執行專案幾次來驗證下載的長度不一定以相同順序出現。

警告

您可以在迴圈中使用 WhenAny (如本範例所述)，以解決涉及少量工作的問題。不過，如果您需要處理大量工作，其他方法會更有效率。如需詳細資訊和範例，請參閱當工作完成時處理工作。

完整範例

下列程式碼是範例的完整 MainWindow.xaml.vb 或 MainWindow.xaml.cs 檔案文字。星號標示出已為這個範例加入的項目。

請注意，您必須加入 System.Net.Http 的參考。

您可以從 Async 範例:微調應用程式下載專案。

' 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.

請參閱

共用方式為

啟動多項非同步工作並在它們完成時進行處理 (C# 和 Visual Basic)

下載範例

建置範例

完整範例

請參閱

參考

概念

其他資源

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