为windows phone应用程序刮去网页源代码

Question

我对用WP7开发C＃是非常新的。我正在尝试编写一个简单的应用程序，它将从textBox1中获取URL，并在button1被按下时使用该页面的源代码更新textBlock1中的Text。

我坚持的部分是如何将DownloadStringCallback2中的结果传递回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

你不能做到像你希望的，因为在WP7（Silverlight的）人的Web请求是异步的。 这意味着代码在下载网页时不会停止，并在完成相同的行和函数时继续执行，而是创建一个新线程，下载文件并调用回调函数。

你将不得不继续在回调函数（DownloadStringCallback2在你的情况下）。 在这个函数中，你必须把源代码（e.Result）放到文本框中。

我要补充到，如果你得到一个跨线程异常或者，如果你想保持UI体面可用在执行任务时，可以使用这个命令：

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

此命令修复了十字 - 线程异常（如果我没有记错的话）并在与UI线程不同的线程上执行代码，从而维护稳定的UI。

编辑我觉得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)); 
     } 
    }