Interesting Dilemna

0.Interesting Dilemna with Progressive Enhancement.txt

Say you are in a world where you want to use progressive enhancement (sometimes for accessibility) 
 with ASP.NET MVC 3+.

Progressive Enhancement means the site WORKS WITHOUT JAVASCRIPT FOLKS.

Now say you have something like comments that will be on multiple pages. 

You want the site to be maintainable so you want to use a partial with 
 form submission. 

This means that the partial will submit to a different controller than 
 the page you are currently on. 

When Javascript is turned on, you use progressive enhancement
 to ajax the form submission (jquery post) and the only part 
 of the page that reloads is the form area. You also use 
 client side validation.

When Javascript is turned off, you are submitting to a different
 url. But when there are validation errors you need to present 
 the same page you were on. But how do you do that?

This outlines a possible solution.

You still need a way to tell the server that Javascript is on 
 and we've implemented this, but we are not happy with it, so 
 I left that part out of this example. Please see 
 http://stackoverflow.com/a/5188833 for what we based our solution
 on.

We have an explicit opt in to share the ModelState, but every action 
 implicitly will opt in to merge it automatically. And if Javascript
 is enabled, we don't even use this code.

1.ModelStateFilters.cs

// This is based loosely on http://stackoverflow.com/a/12024227 
 public class SetTempModelStateAttribute : ActionFilterAttribute
{
    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        base.OnActionExecuted(filterContext);

        filterContext.Controller.TempData[ApplicationParameters.Web.ModelState] = filterContext.Controller.ViewData.ModelState;
    }
}

public class MergeModelStateAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        base.OnActionExecuting(filterContext);
        if (!filterContext.Controller.TempData.ContainsKey(ApplicationParameters.Web.ModelState)) return;

        var tempModelState = filterContext.Controller.TempData[ApplicationParameters.Web.ModelState] as ModelStateDictionary;
        if (tempModelState != null)
        {
            filterContext.Controller.ViewData.ModelState.Merge(tempModelState);
        }
    }
}

2.OptInForHoldingModelState.cs

[HttpPost]
[SetTempModelState]
public ActionResult AddComment(CommentModel model)
{
    // One might pull the url the request is from (we will need to deal with this eventually) or pass it as a parameter. 
    // What we've done with it as part of the model is a short circuit that we are not happy with using.
    
    if(!ModelState.IsValid)
    {
        return ReturnPartialOrRedirect("_CommentAdd", model, model.Url);
    }

    //actions for saving...

    return ReturnPartialOrRedirect("_Success", new SuccessModel{Message="Some message"}, model.Url);
}

3.GlobalFilter.cs

//no explicit opt in necessary

 public static void RegisterGlobalFilters(GlobalFilterCollection filters)
{
    filters.Add(new MergeModelStateAttribute());
}

4.ReturnPartialOrRedirect.cs

//this goes in a global controller that inherits controller that all controllers for your site inherit from
 
public ActionResult ReturnPartialOrRedirect<T>(string viewName, T model, string redirectUrl)
{
    if (ApplicationParameters.IsJavaScriptEnabled)
    {
        return PartialView(viewName, model);
    }
 
    return Redirect(redirectUrl);
}

Instead of checking whether the user can use javascript (or has it enabled), Could you not simply check whether the call was an Ajax call?

Request.IsAjaxRequest(); will be true if the request was made using Ajax:
http://stackoverflow.com/questions/4523827/asp-net-mvc-what-does-isajaxrequest-actually-mean

The chances that you are using a JS library that does not set the required header is low.