爲windows phone應用程序颳去網頁源代碼

Question

我對用WP7開發C＃是非常新的。我正在嘗試編寫一個簡單的應用程序，它將從textBox1中獲取URL，並在button1被按下時使用該頁面的源代碼更新textBlock1中的Text。

我堅持的部分是如何將DownloadStringCallback2中的結果傳遞迴LoadSiteContent函數，以便它可以作爲變量sourceCode返回。

代碼如下：

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，因爲在WP7（Silverlight的）人的Web請求是異步的。 這意味着代碼在下載網頁時不會停止，並在完成相同的行和函數時繼續執行，而是創建一個新線程，下載文件並調用回調函數。

你將不得不繼續在回調函數（DownloadStringCallback2在你的情況下）。 在這個函數中，你必須把源代碼（e.Result）放到文本框中。

我要補充到，如果你得到一個跨線程異常或者，如果你想保持UI體面可用在執行任務時，可以使用這個命令：

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

此命令修復了十字 - 線程異常（如果我沒有記錯的話）並在與UI線程不同的線程上執行代碼，從而維護穩定的UI。

編輯我覺得你的代碼應該是這樣的：

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)); 
     } 
    }