c# - 如何调用 WebBrowser Navigate 遍历多个 url？

Question

要收集网页上的信息，我可以使用 WebBrowser.Navigated 事件。

首先，导航到 url:

WebBrowser wbCourseOverview = new WebBrowser();
wbCourseOverview.ScriptErrorsSuppressed = true;
wbCourseOverview.Navigate(url);
wbCourseOverview.Navigated += wbCourseOverview_Navigated;

然后在调用Navigated时处理网页:

void wbCourseOverview_Navigated(object sender, WebBrowserNavigatedEventArgs e)
    {
        //Find the control and invoke "Click" event...
    }

当我尝试遍历 url 字符串数组时，困难的部分就来了。

foreach (var u in courseUrls)
        {
            WebBrowser wbCourseOverview = new WebBrowser();
            wbCourseOverview.ScriptErrorsSuppressed = true;
            wbCourseOverview.Navigate(u);

            wbCourseOverview.Navigated += wbCourseOverview_Navigated;
        }

此处，由于页面加载需要时间，因此永远不会到达 wbCourseOverview_Navigated。

我尝试在 C#5 中使用 async await。任务和基于事件的异步模式 (EAP) 可在 here 中找到.另一个示例可以在 The Task-based Asynchronous Pattern 中找到.

问题是 WebClient 有像 DownloadDataAsync 和 DownloadStringAsync 这样的异步方法。但是 WebBrowser 中没有 NavigateAsync。

有哪位高手可以给我一些建议吗？谢谢。

---

StackOverflow 中有一篇文章 ( here )。但是，有谁知道如何在其答案中实现该 strut 吗？

---

再次更新。

在 another post here in StackOverflow 中建议,

public static Task WhenDocumentCompleted(this WebBrowser browser)
{
    var tcs = new TaskCompletionSource<bool>();
    browser.DocumentCompleted += (s, args) => tcs.SetResult(true);
    return tcs.Task;
}

所以我有:

foreach (var c in courseBriefs)
    {
        wbCourseOverview.Navigate(c.Url);
        await wbCourseOverview.WhenDocumentCompleted();
    }

在我的网络浏览器访问第二个 url 之前，它看起来不错。

An attempt was made to transition a task to a final state when it had already completed.

我知道我一定是在 foreach 循环中犯了一个错误。因为循环到第二轮的时候还没有引发DocumentCompleted事件。在 foreach 循环中编写此 await 的正确方法是什么？

Answer 1

There is a post in StackOverflow (here). But, does anyone know how to implement that strut in its answer?

好的，所以你需要一些带有 awaiter 的代码。我做了两段代码。第一个使用 TPL 的内置等待器:

 public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            ProcessUrlsAsync(new[] { "http://google.com", "http://microsoft.com", "http://yahoo.com" })
                .Start();
        }

        private Task ProcessUrlsAsync(string[] urls)
        {
            return new Task(() =>
            {
                foreach (string url in urls)
                {
                    TaskAwaiter<string> awaiter = ProcessUrlAsync(url);
                    // or the next line, in case we use method *
                    // TaskAwaiter<string> awaiter = ProcessUrlAsync(url).GetAwaiter();                     
                    string result = awaiter.GetResult();

                    MessageBox.Show(result);
                }
            });
        }        

        // Awaiter inside
        private TaskAwaiter<string> ProcessUrlAsync(string url)
        {
            TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>();
            var handler = new WebBrowserDocumentCompletedEventHandler((s, e) =>
            {
                // TODO: put custom processing of document right here
                taskCompletionSource.SetResult(e.Url + ": " + webBrowser1.Document.Title);
            });
            webBrowser1.DocumentCompleted += handler;
            taskCompletionSource.Task.ContinueWith(s => { webBrowser1.DocumentCompleted -= handler; });

            webBrowser1.Navigate(url);
            return taskCompletionSource.Task.GetAwaiter();
        }

        // (*) Task<string> instead of Awaiter
        //private Task<string> ProcessUrlAsync(string url)
        //{
        //    TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>();
        //    var handler = new WebBrowserDocumentCompletedEventHandler((s, e) =>
        //    {
        //        taskCompletionSource.SetResult(e.Url + ": " + webBrowser1.Document.Title);
        //    });
        //    webBrowser1.DocumentCompleted += handler;
        //    taskCompletionSource.Task.ContinueWith(s => { webBrowser1.DocumentCompleted -= handler; });

        //    webBrowser1.Navigate(url);
        //    return taskCompletionSource.Task;
        //}

下一个示例包含 Eric Lippert 正在谈论的 awaiter struct 的示例实现 here .

public partial class Form1 : Form
    {
        public struct WebBrowserAwaiter
        {
            private readonly WebBrowser _webBrowser;
            private readonly string _url;

            private readonly TaskAwaiter<string> _innerAwaiter;

            public bool IsCompleted
            {
                get
                {
                    return _innerAwaiter.IsCompleted;
                }
            }

            public WebBrowserAwaiter(WebBrowser webBrowser, string url)
            {
                _url = url;
                _webBrowser = webBrowser;
                _innerAwaiter = ProcessUrlAwaitable(_webBrowser, url);
            }

            public string GetResult()
            {
                return _innerAwaiter.GetResult();

            }

            public void OnCompleted(Action continuation)
            {
                _innerAwaiter.OnCompleted(continuation);
            }

            private TaskAwaiter<string> ProcessUrlAwaitable(WebBrowser webBrowser, string url)
            {
                TaskCompletionSource<string> taskCompletionSource = new TaskCompletionSource<string>();
                var handler = new WebBrowserDocumentCompletedEventHandler((s, e) =>
                {
                    // TODO: put custom processing of document here
                    taskCompletionSource.SetResult(e.Url + ": " + webBrowser.Document.Title);
                });
                webBrowser.DocumentCompleted += handler;
                taskCompletionSource.Task.ContinueWith(s => { webBrowser.DocumentCompleted -= handler; });

                webBrowser.Navigate(url);
                return taskCompletionSource.Task.GetAwaiter();
            }
        }

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            ProcessUrlsAsync(new[] { "http://google.com", "http://microsoft.com", "http://yahoo.com" })
                .Start();
        }

        private Task ProcessUrlsAsync(string[] urls)
        {
            return new Task(() =>
            {
                foreach (string url in urls)
                {
                    var awaiter = new WebBrowserAwaiter(webBrowser1, url);
                    string result = awaiter.GetResult();

                    MessageBox.Show(result);
                }
            });
        }
    }   
        }

希望这对您有所帮助。