c# - 如何在超时后取消任务等待

Question

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

如果文档加载时间超过 5 秒，我将如何停止 Task 并让 GetFinalUrl() 返回 null ？

我见过许多使用 TaskFactory 的示例，但我无法将其应用到这段代码中。

 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

已更新:最新版本的基于WebBrowser的控制台网络scraper可以是found on Github .

更新:Adding a pool of WebBrowser objects用于多个并行下载。

Do you have an example of how to do this in a console app by anychance? Also I don't think webBrowser can be a class variable becauseI am running the whole thing in a parallell for each, iteratingthousands of URLs

下面是或多或少通用的 **WebBrowser-based web scraper ** 的实现，它作为控制台应用程序工作。它整合了我之前与 WebBrowser 相关的一些工作，包括问题中引用的代码:

• 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

• Printing a set of URLs with WebBrowser

• Web page UI automation

几点:

• 可重用 MessageLoopApartment 类用于启动和运行带有自己的消息泵的 WinForms STA 线程。它可以从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 (FromCurrentSynchronizationContext) 和一组 Task.Factory.StartNew 包装器以使用此任务计划程序。

• 这使得 async/await 成为在单独的 STA 线程上运行 WebBrowser 导航任务的绝佳工具。这样，一个 WebBrowser 对象就在该线程上被创建、导航和销毁。虽然，MessageLoopApartment 并未专门绑定(bind)到 WebBrowser。

• 使用 Browser FeatureControl 启用 HTML5 渲染很重要，否则 WebBrowser 对象默认在 IE7 仿真模式下运行。这就是 SetFeatureBrowserEmulation 下面所做的。

• 可能无法始终以 100% 的概率确定网页何时完成呈现。一些页面非常复杂并且使用连续的 AJAX 更新。然而我们通过首先处理 DocumentCompleted 事件，然后轮询页面的当前 HTML 快照以了解更改并检查 WebBrowser.IsBusy 属性，可以非常接近。这就是 NavigateAsync 下面所做的。

• 超时逻辑存在于上述之上，以防页面呈现永无止境(注意 CancellationTokenSource 和 CreateLinkedTokenSource)。

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 ScrapeSitesAsync(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 = ScrapeSitesAsync(
                    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();
        }
    }
}