cdn-with-fallback.cs

using System;
using System.Web.Mvc;
using System.Web.Mvc.Html;
using System.Web.Routing;

public static class HtmlHelperExtensions
{
    /// <summary>
    /// render script tag to include javascript file from cdn, but fallback to local if cannot be loaded
    /// </summary>
    /// <param name="helper"></param>
    /// <param name="cdnPath">start with // to handle either http or https</param>
    /// <param name="localPath">start with ~/</param>
    /// <param name="javascriptCondition">javascript condition, if true then your browser will load the local file</param>
    /// <returns>
    ///    <script src="//ajax.googleapis.com/ajax/libs/jquery/1.4.4/jquery.min.js"></script>
    ///    <script>!window.jQuery && document.write(unescape('%3Cscript src="/Scripts/jquery-1.4.4.min.js"%3E%3C/script%3E'))</script>
    /// </returns>
    /// <seealso cref="http://paulirish.com/2010/the-protocol-relative-url/"/>
    public static MvcHtmlString JavascriptIncludeFromCdnWithFallback(this HtmlHelper helper, string cdnPath, string localPath, string javascriptCondition)
    {
        var urlHelper = new UrlHelper(helper.ViewContext.RequestContext);
        var cdnBuilder = new TagBuilder("script");
        var localBuilder = new TagBuilder("script");

        // src for cdn resource
        cdnBuilder.Attributes.Add("src", cdnPath);
        // js to build a local script tag if needed
        localBuilder.InnerHtml = String.Format
        (
            "{0} && document.write(unescape('%3Cscript src=\"{1}\"%3E%3C/script%3E'))",
            javascriptCondition,
            urlHelper.Content(localPath)
        );
        // return markup
        var built = String.Format("{0}\n\t{1}", cdnBuilder.ToString(TagRenderMode.Normal), localBuilder.ToString(TagRenderMode.Normal));
        return MvcHtmlString.Create(built);
    }
}