JQuery Ajax通过Handler访问外部XML数据

　　JQuery是一款不错的Javascript脚本框架，相信园子里的很多朋友对它都不陌生，我们在开发Web应用程序时难免会使用到Javascript脚本，而使用一款不错的脚本框架将会大大节省我们的开发时间， 并可以毫不费力地实现很多非常酷的效果。JQuery的使用非常简单，我们只需要从其官方网站上下载一个脚本文件并引用到页面上即可，然后你就可以在你的脚本代码中任意使用JQuery提供的对象和功能了。

　　在JQuery中使用Ajax方法异步获取服务器资源非常简单，读者可以参考其官方网站上提供的例子http://api.jquery.com/category/ajax/。当然，作为客户端脚本，JQuery也会遇到跨域访问资源的问题，什么是跨域访问呢？简单来说就是脚本所要访问的资源属于网站外部的资源，脚本所在的位置和资源所在的位置不在同一区域。默认情况下，浏览器是不允许直接进行资源的跨域访问的，除非客户端浏览器有设置，否则访问会失败。在这种情况下，我们一般都会采用在服务器端使用handler来解决，就是说在脚本和资源之间建立一个桥梁，让脚本访问本站点内的handler，通过handler去访问外部资源。这个是非常普遍的做法，而且操作起来也非常简单，因为会经常使用到，所以在此记录一下，方便日后使用！

　　首先需要在网站中创建一个handler，在Visual Studio中新建一个Generic Handler文件，拷贝下面的代码：

<%@ WebHandler Language="C#" Class="WebApplication1.Stock" %>

namespace WebApplication1
{
    using System;
    using System.IO;
    using System.Net;
    using System.Text;
    using System.Web;
    using System.Collections.Generic;
    using System.Linq;

    /// <summary>
    /// Asynchronous HTTP handler for rendering external xml source.
    /// </summary>
    public class Stock : System.Web.IHttpAsyncHandler
    {
        private static readonly SafeList safeList = new SafeList();
        private HttpContext context;
        private WebRequest request;

        /// <summary>
        /// Gets a value indicating whether the HTTP handler is reusable.
        /// </summary>
        public bool IsReusable
        {
            get { return false; }
        }

        /// <summary>
        /// Verify that the external RSS feed is hosted by a server on the safe list
        /// before making an asynchronous HTTP request for it.
        /// </summary>
        public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
        {
            var u = context.Request.QueryString["u"];
            var uri = new Uri(u);
            if (safeList.IsSafe(uri.DnsSafeHost))
            {
                this.context = context;
                this.request = HttpWebRequest.Create(uri);
                return this.request.BeginGetResponse(cb, extraData);
            }
            else
            {
                throw new HttpException(204, "No content");
            }
        }

        /// <summary>
        /// Render the response from the asynchronous HTTP request for the RSS feed
        /// using the response's Expires and Last-Modified headers when caching.
        /// </summary>
        public void EndProcessRequest(IAsyncResult result)
        {
            string expiresHeader;
            string lastModifiedHeader;
            string rss;
            using (var response = this.request.EndGetResponse(result))
            {
                expiresHeader = response.Headers["Expires"];
                lastModifiedHeader = response.Headers["Last-Modified"];

                using (var stream = response.GetResponseStream())
                using (var reader = new StreamReader(stream, true))
                {
                    rss = reader.ReadToEnd();
                }
            }

            var output = this.context.Response;
            output.ContentEncoding = Encoding.UTF8;
            output.ContentType = "text/xml;"; // "application/rss+xml; charset=utf-8";
            output.Write(rss);

            var cache = output.Cache;
            cache.VaryByParams["u"] = true;

            DateTime expires;
            var hasExpires = DateTime.TryParse(expiresHeader, out expires);

            DateTime lastModified;
            var hasLastModified = DateTime.TryParse(lastModifiedHeader, out lastModified);

            cache.SetCacheability(HttpCacheability.Public);
            cache.SetOmitVaryStar(true);
            cache.SetSlidingExpiration(false);
            cache.SetValidUntilExpires(true);

            DateTime expireBy = DateTime.Now.AddHours(1);
            if (hasExpires && expires.CompareTo(expireBy) <= 0)
            {
                cache.SetExpires(expires);
            }
            else
            {
                cache.SetExpires(expireBy);
            }

            if (hasLastModified)
            {
                cache.SetLastModified(lastModified);
            }
        }

        /// <summary>
        /// Do not process requests synchronously.
        /// </summary>
        public void ProcessRequest(HttpContext context)
        {
            throw new InvalidOperationException();
        }
    }

    /// <summary>
    /// Methods for matching hostnames to a list of safe hosts.
    /// </summary>
    public class SafeList
    {
        /// <summary>
        /// Hard-coded list of safe hosts.
        /// </summary>
        private static readonly IEnumerable<string> hostnames = new string[]
        {
            "cnblogs.com",
            "msn.com",
            "163.com",
            "csdn.com"
        };

        /// <summary>
        /// Prefix each safe hostname with a period.
        /// </summary>
        private static readonly IEnumerable<string> dottedHostnames =
            from hostname in hostnames
            select string.Concat(".", hostname);

        /// <summary>
        /// Tests if the <paramref name="hostname" /> matches exactly or ends with a
        /// hostname from the safe host list.
        /// </summary>
        /// <param name="hostname">Hostname to test</param>
        /// <returns>True if the hostname matches</returns>
        public bool IsSafe(string hostname)
        {
            return MatchesHostname(hostname) || MatchesDottedHostname(hostname);
        }

        /// <summary>
        /// Tests if the <paramref name="hostname" /> ends with a hostname from the
        /// safe host list.
        /// </summary>
        /// <param name="hostname">Hostname to test</param>
        /// <returns>True if the hostname matches</returns>
        private static bool MatchesDottedHostname(string hostname)
        {
            return dottedHostnames.Any(host => hostname.EndsWith(host, StringComparison.InvariantCultureIgnoreCase));
        }

        /// <summary>
        /// Tests if the <paramref name="hostname" /> matches exactly with a hostname
        /// from the safe host list.
        /// </summary>
        /// <param name="hostname">Hostname to test</param>
        /// <returns>True if the hostname matches</returns>
        private static bool MatchesHostname(string hostname)
        {
            return hostnames.Contains(hostname, StringComparer.InvariantCultureIgnoreCase);
        }
    }
}

 　　我给出的例子中是想通过Ajax异步取得msn站点上微软的股票信息，其外部资源地址为http://money.service.msn.com/StockQuotes.aspx?symbols=msft，我们在页面上这样使用JQuery api通过Handler来访问数据：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title></title>

    <script type="text/javascript" src="jquery-1.3.2.min.js"></script>

</head>
<body>
    <div id="con">
        <span id="loader">loading...</span>
    </div>

    <script type="text/javascript">
        function getData() {
            $("#loader").ajaxStart(function() {
                $(this).show();
            });

            $("#loader").ajaxComplete(function() {
                $(this).hide();
            });

            $.ajax({
                type: "GET",
                url: "Stock.ashx?u=http://money.service.msn.com/StockQuotes.aspx?symbols=msft",
                dataType: "xml",
                success: function(data) {
                    var last = "";
                    var change = "";
                    var percentchange = "";
                    var volume = "";
                    var cap = "";
                    var yearhigh = "";
                    var yearlow = "";
                    $(data).find('ticker').each(function() {
                        last = $(this).attr('last');
                        change = $(this).attr('change');
                        percentchange = $(this).attr('percentchange');
                        volume = $(this).attr('volume');
                        cap = $(this).attr('marketcap');
                        yearhigh = $(this).attr('yearhigh');
                        yearlow = $(this).attr('yearlow');
                        document.getElementById('con').innerHTML = '<span>name:' + last + '&nbsp;&nbsp;&nbsp;&nbsp;high:' + volume + '&nbsp;&nbsp;&nbsp;&nbsp;low:' + cap + '</span>';
                    })
                }
            });
        }

        $(window).load(getData);

    </script>

</body>
</html>

 　　下面是实现的结果：

name:25.8    high:67,502,221    low:$226,107,039,514

　　Handler的写法基本都大同小异，因此可以写成一个通用的例子，以后如遇到在脚本中需要跨域访问资源时便可以直接使用！代码记录于此，方便查阅。