MVC 3.0 Editing a variable length list and using the PRG pattern

Question

I created a view with a variable length list as described here: http://blog.stevensanderson.com/2010/01/28/editing-a-variable-length-list-aspnet-mvc-2-style/.

I am trying to use the PRG pattern with action filters as described at point 13 here: http://weblogs.asp.net/rashid/archive/2009/04/01/asp-net-mvc-best-practices-part-1.aspx.

I have an Edit action:

    [HttpGet, ImportModelStateFromTempData]
    public ActionResult Edit(int id)
    {
    }

And the post action:

    [HttpPost, ExportModelStateToTempData]
    public ActionResult Edit(int id, FormCollection formCollection)
    {
        if (!TryUpdateModel<CategoryEntity>(category, formCollection))
        {
            return RedirectToAction("Edit", new { id = id });
        }

        // succes, no problem processing this...
        return RedirectToAction("Edit", new { id = id });
    }

All works fine including validation and error messages.

The only problem I have is that newly added items and deleted items (client side deleted/added) are not preserved after the redirect. I am trying to find a way to update my model after the redirect with the new items. I changed the ImportModelStateFromTempData attribute to use the OnActionExecuting override instead of the OnActionExecuted override to have the ModelState available in the action but I don't see a clean way to update my model from the passed in ModelState.

Changed ImportModelStateFromTempData:

public class ImportModelStateFromTempData : ModelStateTempDataTransfer
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        ModelStateDictionary modelState = filterContext.Controller.TempData[Key] as ModelStateDictionary;

        if (modelState != null)
        {
            filterContext.Controller.ViewData.ModelState.Merge(modelState);
        }
        base.OnActionExecuting(filterContext);
    }

    public override void OnActionExecuted(ActionExecutedContext filterContext)
    {
        //ModelStateDictionary modelState = filterContext.Controller.TempData[Key] as ModelStateDictionary;

        //if (modelState != null)
        //{
        //    //Only Import if we are viewing
        //    if (filterContext.Result is ViewResult)
        //    {
        //        filterContext.Controller.ViewData.ModelState.Merge(modelState);
        //    }
        //    else
        //    {
        //        //Otherwise remove it.
        //        filterContext.Controller.TempData.Remove(Key);
        //    }
        //}
        base.OnActionExecuted(filterContext);
    }
}

Any input on this is much appreciated, thanks.

Harmen

UPDATE: Thought I might add some more of my (pseudo) code to make it more clear:

public class CategoryEntity
{
    public int Id;
    public string Name;
    public IEnumerable<CategoryLocEntity> Localized;
}

public class CategoryLocEntity
{
    public int CategoryId;
    public int LanguageId;
    public string LanguageName;
    public string Name;
}

My Edit view:

@model CategoryEntity

@{
    ViewBag.Title = Views.Category.Edit;
}

<h2>@Views.Category.Edit</h2>

<script src="@Url.Content("~/Scripts/jquery.validate.min.js")" type="text/javascript"></script>
<script src="@Url.Content("~/Scripts/jquery.validate.unobtrusive.min.js")" type="text/javascript"></script>
<script type="text/javascript"><!--

    $(document).ready(function () {
        $('#addItem').click(function () {
            var languageId = $('#languageId').val();
            var index = $('#editor-rows').children().size() - 1;
            $.ajax({
                url: this.href + '?languageId=' + languageId + '&index=' + index,
                cache: false,
                error: function (xhr, status, error) {
                    alert(error);
                },
                success: function (html) {
                    $('#editor-rows').append(html);
                }
            });
            return false;
        });

        $("a.removeItem").live("click", function () {
            $(this).parents("div.editor-row:first").remove();
            return false;
        });
    });

--></script>

@using (Html.BeginForm()) 
{
    @Html.ValidationSummary(false)
    <fieldset>
        <legend>@Views.Shared.Category</legend>
        @Html.HiddenFor(model => model.Id)
        <div id="editor-rows">
            <div class="editor-row">
                <div class="editor-label">
                    @Html.LabelFor(model => model.Name, Views.Shared.NameEnglish)
                </div>
                <div class="editor-field">
                    @Html.EditorFor(model => model.Name)
                    @Html.ValidationMessageFor(model => model.Name)
                </div>
            </div>

            @for (int i = 0; i < Model.Localized.Count; i++)
            {
                @Html.EditorFor(m => m.Localized[i], "_CategoryLoc", null, null)
            }
        </div>

        <div class="editor-label"></div>
        <div class="editor-field">
            @Html.DropDownList("languageId", (IEnumerable<SelectListItem>)ViewBag.LanguageSelectList)
            @Html.ActionLink(Views.Category.AddNewLanguage, "AddNewLanguage", null, new { id = "addItem" })
        </div>

        <p class="clear">
            <input type="submit" value="@Views.Shared.Save" />
        </p>
    </fieldset>
}

<div>
    @Html.ActionLink(Views.Shared.BackToList, "Index")
</div>

Editor template for the CategoryLocEntity:

@model CategoryLocEntity

<div class="editor-row">
    @Html.HiddenFor(model => model.Id)
    @Html.HiddenFor(model => model.LanguageId)
    <div class="editor-label">
        @Html.LabelFor(model => model.LanguageName, Model.LanguageName)
    </div>
    <div class="editor-field">
        @Html.EditorFor(model => model.Name)
        <a href="#" class="removeItem">@Views.Shared.Remove</a>
        @Html.ValidationMessageFor(model => model.Name)
    </div>
</div>

Answer 1

I found a solution (probably not the most elegant one but it works for me). I created my own ModelStateValueProvider to be used with UpdateModel. The code is based on the DictionaryValueProvider. See http://www.java2s.com/Open-Source/CSharp/2.6.4-mono-.net-core/System.Web/System/Web/Mvc/DictionaryValueProvider%601.cs.htm.

public class ModelStateValueProvider : IValueProvider
{
    HashSet<string> prefixes = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
    ModelStateDictionary modelStateDictionary;

    public ModelStateValueProvider(ModelStateDictionary modelStateDictionary)
    {
        if (modelStateDictionary == null)
            throw new ArgumentNullException("modelStateDictionary");

        this.modelStateDictionary = modelStateDictionary;

        FindPrefixes();
    }

    private void FindPrefixes()
    {
        if (modelStateDictionary.Count > 0)
            prefixes.Add(string.Empty);

        foreach (var modelState in modelStateDictionary)
            prefixes.UnionWith(GetPrefixes(modelState.Key));
    }

    public bool ContainsPrefix(string prefix)
    {
        if (prefix == null)
        {
            throw new ArgumentNullException("prefix");
        }

        return prefixes.Contains(prefix);
    }

    public ValueProviderResult GetValue(string key)
    {
        if (key == null)
            throw new ArgumentNullException("key");

        return modelStateDictionary.ContainsKey(key) ? modelStateDictionary[key].Value : null;
    }

    static IEnumerable<string> GetPrefixes(string key)
    {
        yield return key;
        for (int i = key.Length - 1; i >= 0; i--)
        {
            switch (key[i])
            {
                case '.':
                case '[':
                    yield return key.Substring(0, i);
                    break;
            }
        }
    }
}

public class ModelStateValueProviderFactory : ValueProviderFactory
{
    public override IValueProvider GetValueProvider(ControllerContext controllerContext)
    {
        return new ModelStateValueProvider(controllerContext.Controller.ViewData.ModelState);
    }
}

I use it in de Edit (Get) action like:

[HttpGet, ImportModelStateFromTempData]
public ActionResult Edit(int id)
{
  var category = new CategoryEntity(id);
  if (!ModelState.IsValid)
  {    
     TryUpdateModel<CategoryEntity>(category, 
         new ModelStateValueProviderFactory().GetValueProvider(ControllerContext));
  }
  return View(category);
}

Looking forward to your comments...