如何在超时期限后取消任务等待

Question

我正在使用此方法以编程方式实例化Web浏览器，导航到URL并在文档完成时返回结果。

如果文件加载时间超过5秒，我怎么能够停止Task并让GetFinalUrl()返回null？

我见过许多使用TaskFactory的例子，但我无法将它应用于此代码。

 private Uri GetFinalUrl(PortalMerchant portalMerchant)
    {
        SetBrowserFeatureControl();
        Uri finalUri = null;
        if (string.IsNullOrEmpty(portalMerchant.Url))
        {
            return null;
        }
        Uri trackingUrl = new Uri(portalMerchant.Url);
        var task = MessageLoopWorker.Run(DoWorkAsync, trackingUrl);
        task.Wait();
        if (!String.IsNullOrEmpty(task.Result.ToString()))
        {
            return new Uri(task.Result.ToString());
        }
        else
        {
            throw new Exception("Parsing Failed");
        }
    }

// by Noseratio - http://stackoverflow.com/users/1768303/noseratio    

static async Task<object> DoWorkAsync(object[] args)
{
    _threadCount++;
    Console.WriteLine("Thread count:" + _threadCount);
    Uri retVal = null;
    var wb = new WebBrowser();
    wb.ScriptErrorsSuppressed = true;

    TaskCompletionSource<bool> tcs = null;
    WebBrowserDocumentCompletedEventHandler documentCompletedHandler = (s, e) => tcs.TrySetResult(true);

    foreach (var url in args)
    {
        tcs = new TaskCompletionSource<bool>();
        wb.DocumentCompleted += documentCompletedHandler;
        try
        {
            wb.Navigate(url.ToString());
            await tcs.Task;
        }
        finally
        {
            wb.DocumentCompleted -= documentCompletedHandler;
        }

        retVal = wb.Url;
        wb.Dispose();
        return retVal;
    }
    return null;
}

public static class MessageLoopWorker
{
    #region Public static methods

    public static async Task<object> Run(Func<object[], Task<object>> worker, params object[] args)
    {
        var tcs = new TaskCompletionSource<object>();

        var thread = new Thread(() =>
        {
            EventHandler idleHandler = null;

            idleHandler = async (s, e) =>
            {
                // handle Application.Idle just once
                Application.Idle -= idleHandler;

                // return to the message loop
                await Task.Yield();

                // and continue asynchronously
                // propogate the result or exception
                try
                {
                    var result = await worker(args);
                    tcs.SetResult(result);
                }
                catch (Exception ex)
                {
                    tcs.SetException(ex);
                }

                // signal to exit the message loop
                // Application.Run will exit at this point
                Application.ExitThread();
            };

            // handle Application.Idle just once
            // to make sure we're inside the message loop
            // and SynchronizationContext has been correctly installed
            Application.Idle += idleHandler;
            Application.Run();
        });

        // set STA model for the new thread
        thread.SetApartmentState(ApartmentState.STA);

        // start the thread and await for the task
        thread.Start();
        try
        {
            return await tcs.Task;
        }
        finally
        {
            thread.Join();
        }
    }
    #endregion
}

Answer 1

更新：基于qazxsw poi的控制台网络刮刀的最新版本可以是qazxsw poi。

更新：WebBrowser用于多个并行下载。

您是否有任何机会在控制台应用程序中执行此操作的示例？另外我不认为webBrowser可以是一个类变量，因为我在每个文件中并行运行整个事物，迭代数千个URL

下面是一个或多或少通用的基于found on Github的web scrapper的实现，它作为控制台应用程序。这是我之前与Adding a pool of WebBrowser objects相关的一些努力的合并，包括问题中引用的代码：

• WebBrowser
• WebBrowser
• Capturing an image of the web page with opacity
• Loading a page with dynamic AJAX content
• Creating an STA message loop thread for WebBrowser
• Loading a set of URLs, one after another

几点：

• 可重用的Printing a set of URLs with WebBrowser类用于启动和运行具有自己的消息泵的WinForms STA线程。它可以在控制台应用程序中使用，如下所示。此类公开TPL任务计划程序（Web page UI automation）和一组MessageLoopApartment包装程序以使用此任务计划程序。
• 这使得FromCurrentSynchronizationContext成为在单独的STA线程上运行Task.Factory.StartNew导航任务的绝佳工具。这样，在该线程上创建，导航和销毁async/await对象。虽然，WebBrowser没有特别与WebBrowser捆绑。
• 使用MessageLoopApartment启用HTML5渲染非常重要，否则WebBrowser obejcts默认在IE7仿真模式下运行。这就是Browser Feature Control在下面做的事情。
• 可能无法始终确定网页何时以100％的概率完成渲染。有些页面非常复杂，并且使用连续的AJAX更新。然而，通过首先处理WebBrowser事件，然后轮询页面的当前HTML快照以进行更改并检查SetFeatureBrowserEmulation属性，我们可以非常接近。这就是DocumentCompleted在下面做的事情。
• 如果页面呈现永无止境（注意WebBrowser.IsBusy和NavigateAsync），则在上面提供超时逻辑。

CancellationTokenSource

Answer 2

我怀疑在另一个线程上运行处理循环不会很好，因为CreateLinkedTokenSource是一个承载ActiveX控件的UI组件。

当您编写

using Microsoft.Win32;
using System;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Console_22239357
{
    class Program
    {
        // by Noseratio - https://stackoverflow.com/a/22262976/1768303

        // main logic
        static async Task ScrapSitesAsync(string[] urls, CancellationToken token)
        {
            using (var apartment = new MessageLoopApartment())
            {
                // create WebBrowser inside MessageLoopApartment
                var webBrowser = apartment.Invoke(() => new WebBrowser());
                try
                {
                    foreach (var url in urls)
                    {
                        Console.WriteLine("URL:\n" + url);

                        // cancel in 30s or when the main token is signalled
                        var navigationCts = CancellationTokenSource.CreateLinkedTokenSource(token);
                        navigationCts.CancelAfter((int)TimeSpan.FromSeconds(30).TotalMilliseconds);
                        var navigationToken = navigationCts.Token;

                        // run the navigation task inside MessageLoopApartment
                        string html = await apartment.Run(() =>
                            webBrowser.NavigateAsync(url, navigationToken), navigationToken);

                        Console.WriteLine("HTML:\n" + html);
                    }
                }
                finally
                {
                    // dispose of WebBrowser inside MessageLoopApartment
                    apartment.Invoke(() => webBrowser.Dispose());
                }
            }
        }

        // entry point
        static void Main(string[] args)
        {
            try
            {
                WebBrowserExt.SetFeatureBrowserEmulation(); // enable HTML5

                var cts = new CancellationTokenSource((int)TimeSpan.FromMinutes(3).TotalMilliseconds);

                var task = ScrapSitesAsync(
                    new[] { "http://example.com", "http://example.org", "http://example.net" },
                    cts.Token);

                task.Wait();

                Console.WriteLine("Press Enter to exit...");
                Console.ReadLine();
            }
            catch (Exception ex)
            {
                while (ex is AggregateException && ex.InnerException != null)
                    ex = ex.InnerException;
                Console.WriteLine(ex.Message);
                Environment.Exit(-1);
            }
        }
    }

    /// <summary>
    /// WebBrowserExt - WebBrowser extensions
    /// by Noseratio - https://stackoverflow.com/a/22262976/1768303
    /// </summary>
    public static class WebBrowserExt
    {
        const int POLL_DELAY = 500;

        // navigate and download 
        public static async Task<string> NavigateAsync(this WebBrowser webBrowser, string url, CancellationToken token)
        {
            // navigate and await DocumentCompleted
            var tcs = new TaskCompletionSource<bool>();
            WebBrowserDocumentCompletedEventHandler handler = (s, arg) =>
                tcs.TrySetResult(true);

            using (token.Register(() => tcs.TrySetCanceled(), useSynchronizationContext: true))
            {
                webBrowser.DocumentCompleted += handler;
                try
                {
                    webBrowser.Navigate(url);
                    await tcs.Task; // wait for DocumentCompleted
                }
                finally
                {
                    webBrowser.DocumentCompleted -= handler;
                }
            }

            // get the root element
            var documentElement = webBrowser.Document.GetElementsByTagName("html")[0];

            // poll the current HTML for changes asynchronosly
            var html = documentElement.OuterHtml;
            while (true)
            {
                // wait asynchronously, this will throw if cancellation requested
                await Task.Delay(POLL_DELAY, token);

                // continue polling if the WebBrowser is still busy
                if (webBrowser.IsBusy)
                    continue;

                var htmlNow = documentElement.OuterHtml;
                if (html == htmlNow)
                    break; // no changes detected, end the poll loop

                html = htmlNow;
            }

            // consider the page fully rendered 
            token.ThrowIfCancellationRequested();
            return html;
        }

        // enable HTML5 (assuming we're running IE10+)
        // more info: https://stackoverflow.com/a/18333982/1768303
        public static void SetFeatureBrowserEmulation()
        {
            if (System.ComponentModel.LicenseManager.UsageMode != System.ComponentModel.LicenseUsageMode.Runtime)
                return;
            var appName = System.IO.Path.GetFileName(System.Diagnostics.Process.GetCurrentProcess().MainModule.FileName);
            Registry.SetValue(@"HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION",
                appName, 10000, RegistryValueKind.DWord);
        }
    }

    /// <summary>
    /// MessageLoopApartment
    /// STA thread with message pump for serial execution of tasks
    /// by Noseratio - https://stackoverflow.com/a/22262976/1768303
    /// </summary>
    public class MessageLoopApartment : IDisposable
    {
        Thread _thread; // the STA thread

        TaskScheduler _taskScheduler; // the STA thread's task scheduler

        public TaskScheduler TaskScheduler { get { return _taskScheduler; } }

        /// <summary>MessageLoopApartment constructor</summary>
        public MessageLoopApartment()
        {
            var tcs = new TaskCompletionSource<TaskScheduler>();

            // start an STA thread and gets a task scheduler
            _thread = new Thread(startArg =>
            {
                EventHandler idleHandler = null;

                idleHandler = (s, e) =>
                {
                    // handle Application.Idle just once
                    Application.Idle -= idleHandler;
                    // return the task scheduler
                    tcs.SetResult(TaskScheduler.FromCurrentSynchronizationContext());
                };

                // handle Application.Idle just once
                // to make sure we're inside the message loop
                // and SynchronizationContext has been correctly installed
                Application.Idle += idleHandler;
                Application.Run();
            });

            _thread.SetApartmentState(ApartmentState.STA);
            _thread.IsBackground = true;
            _thread.Start();
            _taskScheduler = tcs.Task.Result;
        }

        /// <summary>shutdown the STA thread</summary>
        public void Dispose()
        {
            if (_taskScheduler != null)
            {
                var taskScheduler = _taskScheduler;
                _taskScheduler = null;

                // execute Application.ExitThread() on the STA thread
                Task.Factory.StartNew(
                    () => Application.ExitThread(),
                    CancellationToken.None,
                    TaskCreationOptions.None,
                    taskScheduler).Wait();

                _thread.Join();
                _thread = null;
            }
        }

        /// <summary>Task.Factory.StartNew wrappers</summary>
        public void Invoke(Action action)
        {
            Task.Factory.StartNew(action,
                CancellationToken.None, TaskCreationOptions.None, _taskScheduler).Wait();
        }

        public TResult Invoke<TResult>(Func<TResult> action)
        {
            return Task.Factory.StartNew(action,
                CancellationToken.None, TaskCreationOptions.None, _taskScheduler).Result;
        }

        public Task Run(Action action, CancellationToken token)
        {
            return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
        }

        public Task<TResult> Run<TResult>(Func<TResult> action, CancellationToken token)
        {
            return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler);
        }

        public Task Run(Func<Task> action, CancellationToken token)
        {
            return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
        }

        public Task<TResult> Run<TResult>(Func<Task<TResult>> action, CancellationToken token)
        {
            return Task.Factory.StartNew(action, token, TaskCreationOptions.None, _taskScheduler).Unwrap();
        }
    }
}

时，我建议使用扩展方法来保持代码清洁：

WebBrowser

现在您的代码可以轻松应用超时：

TAP over EAP wrappers

可以这样消费：

public static Task<string> NavigateAsync(this WebBrowser @this, string url)
{
  var tcs = new TaskCompletionSource<string>();
  WebBrowserDocumentCompletedEventHandler subscription = null;
  subscription = (_, args) =>
  {
    @this.DocumentCompleted -= subscription;
    tcs.TrySetResult(args.Url.ToString());
  };
  @this.DocumentCompleted += subscription;
  @this.Navigate(url);
  return tcs.Task;
}

Answer 3

我正试图从Noseratio的解决方案中受益，并遵循Stephen Cleary的建议。

这是我更新的代码，包括来自Stephen的代码，来自Noseratio的关于AJAX技巧的代码。

第一部分：斯蒂芬建议的

async Task<string> GetUrlAsync(string url)
{
  using (var wb = new WebBrowser())
  {
    var navigate = wb.NavigateAsync(url);
    var timeout = Task.Delay(TimeSpan.FromSeconds(5));
    var completed = await Task.WhenAny(navigate, timeout);
    if (completed == navigate)
      return await navigate;
    return null;
  }
}

private async Task<Uri> GetFinalUrlAsync(PortalMerchant portalMerchant)
{
  SetBrowserFeatureControl();
  if (string.IsNullOrEmpty(portalMerchant.Url))
    return null;
  var result = await GetUrlAsync(portalMerchant.Url);
  if (!String.IsNullOrEmpty(result))
    return new Uri(result);
  throw new Exception("Parsing Failed");
}

第二部分：一个新的Task NavigateAsync来运行AJAX的提示（基于Noseratio的代码）

public static Task<string> NavigateAsync(this WebBrowser @this, string url)
{
  var tcs = new TaskCompletionSource<string>();
  WebBrowserDocumentCompletedEventHandler subscription = null;
  subscription = (_, args) =>
  {
    @this.DocumentCompleted -= subscription;
    tcs.TrySetResult(args.Url.ToString());
  };
  @this.DocumentCompleted += subscription;
  @this.Navigate(url);
  return tcs.Task;
}

第三部分：新的Task NavAjaxAsync获取导航和AJAX

public static async Task<string> NavAjaxAsync(this WebBrowser @this)
{
  // get the root element
  var documentElement = @this.Document.GetElementsByTagName("html")[0];

  // poll the current HTML for changes asynchronosly
  var html = documentElement.OuterHtml;

  while (true)
  {
    // wait asynchronously
    await Task.Delay(POLL_DELAY);

    // continue polling if the WebBrowser is still busy
    if (webBrowser.IsBusy)
      continue;

    var htmlNow = documentElement.OuterHtml;
    if (html == htmlNow)
      break; // no changes detected, end the poll loop

    html = htmlNow;
  }

  return @this.Document.Url.ToString();
}

第四部分和最后一部分：来自Stephen的更新的Task NavAndAjaxAsync和Noseratio的AJAX代码

public static async Task NavAndAjaxAsync(this WebBrowser @this, string url)
{
  await @this.NavigateAsync(url);
  await @this.NavAjaxAsync();
}

我想知道这是否是正确的方法。