gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using System.Text;
using System.Web.WebPages;

namespace StackOverflow.Helpers
{
    public static class DeferJS 
    {
        const string builderKey = "SCRIPT_BUILDER";
        const string builderEnabledKey = "SCRIPT_BUILDER_ENABLED";

        class ScriptBuilder
        {
            List<string> LayoutScripts = new List<string>();
            List<string> Scripts = new List<string>();

            bool layoutStarted = false;

            public void Add(string script)
            {
                if (layoutStarted)
                {
                    LayoutScripts.Add(script);
                }
                else
                {
                    Scripts.Add(script);
                }
            }

            public void MarkLayoutStart()
            { 
                // once the template starts we need to add scripts to a second list, templates are rendered after internal views
                layoutStarted = true;
            }
            public void MarkLayoutBodyRendered()
            {
                layoutStarted = false;  
            }

            public string ToString()
            {
                return string.Join("", LayoutScripts) + string.Join("", Scripts);
            }
        }

        public static bool Enabled 
        {
            get 
            {
                return (bool?)HttpContext.Current.Items[builderEnabledKey] == true;
            }
            set 
            {
                HttpContext.Current.Items[builderEnabledKey] = value;
            }
        }

        private static ScriptBuilder GetScriptBuilder()
        {
            var builder = HttpContext.Current.Items[builderKey] as ScriptBuilder;
            if (builder == null)
            {
                HttpContext.Current.Items[builderKey] = builder = new ScriptBuilder();
            }
            return builder;
        }

        public static HelperResult AppendJs(this WebPageBase page, Func<object, HelperResult> stuff)
        {
            if (Enabled)
            {
                var builder = GetScriptBuilder();
                builder.Add(stuff(null).ToHtmlString());
                return new HelperResult(_ => {});
            }
            else
            {
                return stuff(null);
            }
        }

        public static IHtmlString AppendJs(this WebPageBase page, IHtmlString stuff)
        {
            if (Enabled)
            {
                var builder = GetScriptBuilder();
                builder.Add(stuff.ToString());
                return "".AsHtml();
            }
            else
            {
                return stuff;
            }
        }

        public static void MarkLayoutStart()
        {
            if (Enabled)
            {
                GetScriptBuilder().MarkLayoutStart();
            }
        }

        // called when the body is done rendering
        public static void MarkLayoutBodyRendered()
        {
            if (Enabled)
            {
                GetScriptBuilder().MarkLayoutBodyRendered();
            }
        }

        public static IHtmlString RenderJs(this WebPageBase page)
        {
            if (Enabled)
            {
                return new MvcHtmlString(GetScriptBuilder().ToString());
            }
            else 
            {
                return "".AsHtml();
            }
        }

    }
}