Hooks.cs

namespace Nancy.Jsonp
{
    using System;
    using System.IO;
    using Nancy.Bootstrapper;

    public static class Hooks
    {
        /// <summary>
        /// Enable JSONP support in the application
        /// </summary>
        /// <param name="pipeline">Application Pipeline to Hook into</param>
        public static void Enable(IApplicationPipelines pipeline)
        {
            pipeline.AfterRequest.AddItemToEndOfPipeline(PrepareJsonp);
        }

        /// <summary>
        /// Transmogrify original response and apply JSONP Padding
        /// </summary>
        /// <param name="context">Current Nancy Context</param>
        private static void PrepareJsonp(NancyContext context)
        {   
            string callback = null;
            if (IsJsonp(context, out callback))
            {
                // grab original contents for running later
                Action<Stream> original = context.Response.Contents;

                // set content type to application/javascript so browsers can handle it by default
                // http://stackoverflow.com/questions/111302/best-content-type-to-serve-jsonp
                context.Response.ContentType = "application/javascript";
                context.Response.Contents = stream =>
                {
                    // stolen @grumpydev UnclosableStreamWrapper
                    using(Stream unclosable = new UnclosableStreamWrapper(stream))
                    {
                        using (StreamWriter writer = new StreamWriter(unclosable) { AutoFlush = true })
                        {
                            // apply padding
                            writer.Write("{0}(", callback);
                            original(stream);
                            writer.Write(");");
                        }
                    }
                };
            }
        }

        /// <summary>
        /// Determine if current request is a JSONP request.  Current rules 
        /// around this may need tweaked,
        ///   - Response is an application/json response
        ///   - Request includes a "callback" parameter
        /// </summary>
        /// <param name="context">The Nanacy context</param>
        /// <param name="callbackName">Name of the callback the JSONP response should call</param>
        /// <returns>True if the response is a JSONP response</returns>
        private static bool IsJsonp(NancyContext context, out string callbackName)
        {
            bool isJson = context.Response.ContentType == "application/json";
            bool hasCallback = context.Request.Query["callback"].HasValue;

            callbackName = hasCallback ? context.Request.Query["callback"].Value : "";

            return isJson && hasCallback;
        }
    }
}