windows phone 应用抓取网页源代码

Question

我对使用 C# for WP7 进行开发非常陌生。我正在尝试编写一个简单的应用程序，它将从 textBox1 获取 url，并在按下 button1 时使用该页面的源代码更新 textBlock1 中的文本。

我坚持的部分是如何将 DownloadStringCallback2 中的 Result 传递回 LoadSiteContent 函数，以便它可以作为变量 sourceCode 返回。

代码如下：

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Net;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;

namespace TestApp1
{
    public partial class MainPage : PhoneApplicationPage
    {
        // Constructor
        public MainPage()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            string url = textBox1.Text;
            string sourceCode = LoadSiteContent(url);
            textBlock1.Text = sourceCode;
        }

        /// <summary>
        /// method for retrieving information from a specified URL
        /// </summary>
        /// <param name="url">url to retrieve data from</param>
        /// <returns>source code of URL</returns>
        public string LoadSiteContent(string url)
        {
            //create a new WebClient object
            WebClient client = new WebClient();

            //create a byte array for holding the returned data
            string sourceCode = "Fail";
            client.DownloadStringCompleted += new    DownloadStringCompletedEventHandler(DownloadStringCallback2);
           client.DownloadStringAsync(new Uri(url));

           //use the UTF8Encoding object to convert the byte
           //array into a string
           //UTF8Encoding utf = new UTF8Encoding();

           //return the converted string
           //return utf.GetString(html, 0, html.Length);
           return sourceCode;
        }

        private static void DownloadStringCallback2(Object sender, DownloadStringCompletedEventArgs e)
        {
           // If the request was not canceled and did not throw
           // an exception, display the resource.
           if (!e.Cancelled && e.Error == null)
           {
               string textString = (string)e.Result;
           } 
        }
    }
}

Answer 1

您无法按照自己的意愿进行操作，因为 WP7 (silverlight) 中的所有 Web 请求都是异步的。 这意味着代码在下载网页时不会停止，在同一行和函数中完成后继续，而是创建一个新线程，下载文件并调用回调函数。

您必须继续使用回调函数（在您的情况下为 DownloadStringCallback2）。 在该函数中，您必须将源代码（e.Result）放入文本框中。

如果您遇到跨线程异常，或者您希望在执行任务时保持 UI 正常可用，我可以补充一下，您可以使用以下命令：

Dispatcher.BeginInvoke(new Action (() => LoadContent("http://www.google.com")));

这个命令修复了跨线程异常（如果我没记错的话）并在与 UI 线程不同的线程上执行代码，从而保持稳定的 UI。

编辑我认为你的代码应该是这样的：

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        string url = textBox1.Text;
        LoadSiteContent(url);
    }

    public string LoadSiteContent(string url)
    {
        //create a new WebClient object
        WebClient client = new WebClient();

        client.DownloadStringCompleted += new    DownloadStringCompletedEventHandler(DownloadStringCallback2);
       client.DownloadStringAsync(new Uri(url));
    }

    private static void DownloadStringCallback2(Object sender, DownloadStringCompletedEventArgs e)
    {
       // If the request was not canceled and did not throw
       // an exception, display the resource.
       if (!e.Cancelled && e.Error == null)
       {
           textBlock1.Text = (string)e.Result;
           //If you get the cross-thread exception then use the following line instead of the above
           //Dispatcher.BeginInvoke(new Action (() => textBlock1.Text = (string)e.Result));
       } 
    }