小编典典

如何在超时后取消任务等待

c#

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

如果文档加载时间超过5秒,我如何能够停止TaskGetFinalUrl()返回null

我已经看到了许多使用a的示例,TaskFactory但是我无法将其应用于此代码。

 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
}

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2020-05-19

共1个答案

小编典典

已更新
WebBrowser可以在Github找到基于的控制台Web
scraper 的最新版本。

已更新
添加WebBrowser对象池以进行多个并行下载。

您是否有示例说明如何在控制台应用程序中执行此操作?另外,我也不认为webBrowser可以是一个类变量,因为我要为每个变量并行运行整个过程,从而迭代数千个URL

以下是或多或少 WebBrowser 基于通用 Web的scraper的实现
,可作为控制台应用程序使用。这是我以前WebBrowser相关工作的合并,包括问题中引用的代码:

几点:

  • 可重用的MessageLoopApartment类用于使用其自己的消息泵来启动和运行WinForms STA线程。可以从 控制台应用程序 使用它,如下所示。此类公开了TPL任务计划程序(FromCurrentSynchronizationContext)和一组Task.Factory.StartNew包装程序以使用此任务计划程序。

  • 这是在单独的STA线程上async/await运行WebBrowser导航任务的绝佳工具。这样,WebBrowser就可以在该线程上创建,导航和销毁对象。虽然,MessageLoopApartment不是WebBrowser专门捆绑的。

  • 使用浏览器功能控件启用HTML5渲染很重要,因为WebBrowser默认情况下,对象在IE7仿真模式下运行。这就是SetFeatureBrowserEmulation下面的内容。

  • 不一定总是可以确定网页何时以100%的概率完成渲染。一些页面非常复杂,并使用连续的AJAX更新。但是,通过DocumentCompleted首先处理事件,然后轮询页面的当前HTML快照以查找更改并检查WebBrowser.IsBusy属性,我们可以非常接近。这就是NavigateAsync下面的内容。

  • 如果页面渲染永无休止(请注意CancellationTokenSourceCreateLinkedTokenSource),则上述内容中会出现超时逻辑。

    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();
        }
    }
    

    }

2020-05-19