js文件合并，压缩，缓存，延迟加载

做web前段也有一段时间了，对于web中js文件的加载有些体会想跟大家一起分享一下。

1.首先说说js文件的合并和压缩吧

为了便于集中式管理js的合并和压缩我们创建一个Js.ashx文件来专门处理合并压缩

代码如下：

public class Js : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
        context.Response.ContentType = "text/javascript";
        HttpRequest request = context.Request;
        HttpResponse response = context.Response;
        if (!request.QueryString.AllKeys.Contains("href"))
        {
            response.Write("No Content");
        }
        else
        {
            string href = context.Request.QueryString["href"].Trim();
            string[] files = href.Split(new string[] { ",", "，?" }, StringSplitOptions.RemoveEmptyEntries);
            foreach (string fileName in files)
            {
                string filePath = context.Server.MapPath(fileName);
                if (File.Exists(filePath))
                {
                    string content = File.ReadAllText(filePath, Encoding.UTF8);
                    content = JavaScriptCompressor.Compress(content);
                    response.Write(content);
                }
            }
        }
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }
}

返回结果如图：

但是在实际开发中很多项目为了最求js的合并和压缩，开发很不友好把js的引用放在一个地方，写了很长的一串啊，如上面js引用。
下面说说如何改善吧：

public static class Extensions
{
    const string jsFileKey = "JSFileKey";
    static string jshandlerUrl = string.Empty;
    public static string JsHandlerUrl
    {
        get
        {
            if (string.IsNullOrEmpty(jshandlerUrl))
            {
                jshandlerUrl = ConfigurationManager.AppSettings["jsHandlerUrl"] ?? string.Empty;
            }
            return jshandlerUrl;
        }
    }

    public static void AppendJsFile(this HtmlHelper htmlHelper, string jsFile, int group = 1)
    {
        NameValueCollection jsFiles = null;
        if (htmlHelper.ViewContext.HttpContext.Items.Contains(jsFileKey))
        {
            jsFiles = htmlHelper.ViewContext.HttpContext.Items[jsFileKey] as NameValueCollection;
        }
        else
        {
            jsFiles = new NameValueCollection();
            htmlHelper.ViewContext.HttpContext.Items.Add(jsFileKey, jsFiles);
        }
        if (jsFiles.AllKeys.Contains(group.ToString()))
        {
            string fileUrl = jsFiles[group.ToString()];
            if (!fileUrl.Contains(jsFile))
                jsFiles.Add(group.ToString(), jsFile);
        }
        else
        {
            jsFiles.Add(group.ToString(), jsFile);
        }

        htmlHelper.ViewContext.HttpContext.Items[jsFileKey] = jsFiles;
    }

    public static MvcHtmlString RenderJsFile(this HtmlHelper htmlHelper)
    {
        NameValueCollection jsFiles = null;
        StringBuilder content = new StringBuilder();
        if (htmlHelper.ViewContext.HttpContext.Items.Contains(jsFileKey))
        {
            jsFiles = htmlHelper.ViewContext.HttpContext.Items[jsFileKey] as NameValueCollection;
            List<string> jsKeys = jsFiles.AllKeys.OrderBy(x => x).ToList<string>();

            string jsFormat = "<script type="text/javascript" src="{0}"></script>";
            foreach (string key in jsKeys)
            {
                string jsFile = jsFiles[key];
                content.AppendFormat(jsFormat, JsHandlerUrl + jsFile);
                //htmlHelper.ViewContext.HttpContext.Response.Write(string.Format(jsFormat, JsHandlerUrl + jsFile));
            }
        }
        return new MvcHtmlString(content.ToString());
    }
}

这样在开发的时候我们书写代码就很方便了如：
[csharp]
@{Html.AppendJsFile("Scripts/jquery.lazyload.js");}
下面我们来看看js的延迟加载，为了实现js延迟加载我们需要引用相关的js，在这里我用的是lazyload.js，具体请参考http://www.2cto.com/kf/201205/130023.html
延迟加载后的效果

在文档加载前只加载了一个2.2k的lazyload.js文件，其他的js文件都在ready后加载。
下面再来看看js的缓存吧，缓存涉及到要做服务端和客户端缓存

修改后的代码：

class CacheItem
      {
          public string Content { set; get; }
          public DateTime Expires { set; get; }
      }
      public void ProcessRequest(HttpContext context)
      {
          context.Response.ContentType = "text/javascript";
          HttpRequest request = context.Request;
          HttpResponse response = context.Response;
          if (!request.QueryString.AllKeys.Contains("href"))
          {
              response.Write("No Content");
          }
          else
          {
              string href = context.Request.QueryString["href"].Trim();
              string[] files = href.Split(new string[] { ",", "，?" }, StringSplitOptions.RemoveEmptyEntries);
              CacheItem item = null;
              object obj = HttpRuntime.Cache.Get(href);//服t务?端?缓o存?
              if (null == obj)
              {
                  StringBuilder allText = new StringBuilder();
                  foreach (string fileName in files)
                  {
                      string filePath = context.Server.MapPath(fileName);
                      if (File.Exists(filePath))
                      {
                          string content = File.ReadAllText(filePath, Encoding.UTF8);
                          content = JavaScriptCompressor.Compress(content);
                          //response.Write(content);
                          allText.Append(content);
                      }
                      else
                      {
                          // response.Write("rn未找到源文件"+filePath+"rn");
                          allText.Append("rn未找到源文件" + filePath + "rn");
                      }
                  }//end foreach
                   item = new CacheItem() { Content = allText.ToString(), Expires = DateTime.Now.AddHours(1) };
                   HttpRuntime.Cache.Insert(href, item, null, item.Expires, TimeSpan.Zero);
              }
              else
              {
                  item = obj as CacheItem;
              }
              if (request.Headers["If-Modified-Since"] != null && TimeSpan.FromTicks(item.Expires.Ticks - DateTime.Parse(request.Headers["If-Modified-Since"]).Ticks).Seconds < 100)
              {
                  response.StatusCode = 304;
                 // response.Headers.Add("Content-Encoding", "gzip");
                  response.StatusDescription = "Not Modified";
              }
              else
              {
                  response.Write(item.Content);
                  SetClientCaching(response, DateTime.Now);
              }
          }//end else href
      }
      private void SetClientCaching(HttpResponse response, DateTime lastModified)
      {
          response.Cache.SetETag(lastModified.Ticks.ToString());
          response.Cache.SetLastModified(lastModified);
          //public 以指定响应能由客户端和共享（代理）缓存进行缓存。
            response.Cache.SetCacheability(HttpCacheability.Public);
          //是允许文档在被视为陈旧之前存在的最长绝对时间。
            response.Cache.SetMaxAge(new TimeSpan(7, 0, 0, 0));
          //将缓存过期从绝对时间设置为可调时间
          response.Cache.SetSlidingExpiration(true);
      }

}

运行效果如图：

代码下载地址：http://www.2cto.com/uploadfile/2012/0503/20120503021534861.zip