正在为windowsphone应用程序删除网页源代码

Question

我对使用C#为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，并在按下按钮1时用该页面的源代码更新textBlock1中的文本。

我一直关注的部分是如何将DownloadStringCallback2中的Result传递回LoadSiteContent函数，以便将其作为变量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

您不能随心所欲，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，并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。

我可以补充一点，如果你遇到跨线程异常，或者如果你想在执行任务时保持UI的可用性，你可以使用以下命令：

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），并在不同于UI线程的线程上执行代码，从而维护了稳定的UI。

EDIT我认为您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));
       } 
    }