[英]Dispatcher in WPF apps implementing multiple async Tasks
在以下WPF
应用程序的MSDN示例中,该示例演示了多个异步Tasks
的异步执行async/await
实现,显然不使用/不需要Dispatcher
对象,即异步执行的Tasks
似乎可以直接访问UI控件(在这种情况下, resultTextBox
TextBox
控件-参见行resultsTextBox.Text += String.Format("\\r\\nLength of the download: {0}", length);
)。 该应用程序已经过测试,性能符合预期。
但是, 如果此实现是否能够正确处理可能的争用条件 , 则仍然存在问题 ,例如,等待并完成的Task
尝试访问该TextBox
控件,而后者仍在处理先前完成的Task
的更新? 实际上, 是否仍需要WPF Dispatcher
对象来处理async/await
多任务实现中的这种潜在的并发/争用条件问题 (或者可能已经在这种异步/等待编程结构中隐式实现了互锁功能)?
清单1 。 MSDN文章“启动多个异步任务并在完成时处理它们”( https://msdn.microsoft.com/zh-cn/library/jj155756.aspx )
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>
{
"http://msdn.microsoft.com",
"http://msdn.microsoft.com/library/windows/apps/br211380.aspx",
"http://msdn.microsoft.com/en-us/library/hh290136.aspx",
"http://msdn.microsoft.com/en-us/library/dd470362.aspx",
"http://msdn.microsoft.com/en-us/library/aa578028.aspx",
"http://msdn.microsoft.com/en-us/library/ms404677.aspx",
"http://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;
}
}
}
注意 :我要感谢Stephen Cleary的出色回答和颇有见地的解释,并且还想强调他的解决方案中概述的建议改进,即:用紧凑的解决方案WhenAny
使用WhenAny
的原始MSDN示例中的不必要/复杂的代码块封装在一行代码中,即: await Task.WhenAll(downloadTasks);
(顺便说一句,我在许多实际应用中都使用了这种替代方法,特别是在处理带有多个股票网络查询的在线市场数据应用中)。 非常感谢,斯蒂芬!
但是,仍然存在问题,即此实现是否能够正确处理可能的争用条件,例如,等待并完成的Task是否尝试访问该TextBox控件,而后者仍在处理来自先前完成的Task的更新?
没有比赛条件。 UI线程一次只做一件事。
实际上,是否仍需要WPF Dispatcher对象来处理异步/等待多任务实现中的这种潜在的并发/争用条件问题(或者可能已经在这种异步/等待编程结构中隐式实现了互锁功能)?
是的,但是您不必显式使用它。 正如我在async
介绍中所述, await
关键字(默认情况下)将捕获当前上下文并在该上下文中继续执行async
方法。 “上下文”为SynchronizationContext.Current
(如果当前SyncCtx为null
则为TaskScheduler.Current
)。
在这种情况下,它将捕获UI SynchronizationContext
,该UI会在幕后使用WPF Dispatcher来调度UI线程上的其余async
方法。
附带说明一下,我不是“ Task.WhenAny
列表并在列表完成Task.WhenAny
其删除”的Task.WhenAny
。 我发现如果您通过添加DownloadAndUpdateAsync
方法进行重构,则代码将更加简洁:
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> downloadTasksQuery =
from url in urlList select DownloadAndUpdateAsync(url, client, ct);
// ***Use ToList to execute the query and start the tasks.
List<Task> downloadTasks = downloadTasksQuery.ToList();
await Task.WhenAll(downloadTasks);
}
async Task DownloadAndUpdateAsync(string url, HttpClient client, CancellationToken ct)
{
var length = await ProcessURLAsync(url, client, ct);
resultsTextBox.Text += String.Format("\r\nLength of the download: {0}", length);
}
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;
}
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