Question

My current problem is that I have a partial view that I want to determine what model is being used by it.

I have had to deal with a few strange scenarios for my project so I will try to outline it here, maybe someone can offer a better way to do this.

I am designing something like the Google iGoogle page. A main page with multiple widgets that are able to move around or be configured as needed. The current system loads the actual widget’s data asynchronously view a POST to a controller within my application. That controller will either render a partial view to HTML that can be returned (and then loaded into the page view JQUERY) or just straight HTML/JavaScript that is stored in a database.

This was working fine for me, I had a model for the widgets that holds a dictionary of options that are described via the database, and then used by the partial view. The problem came when I wanted to pass data to a partial view. The best solution I could come up with was having the controller determine which model the partial view in question uses, have some function that will fill the model, and then pass it, along with the partial view, to the function that will render it to HTML within the controller.

I realize this is an odd scenario for MVC (the layers are blending…) and any advice on fundamental design, or implementation of this would be greatly appreciated.

I am currently using MVC3/Razor. Feel free to ask any other questions.

Answer 1

I prototyped a possible solution to this, because it seemed like a fun problem. I hope it’s useful to you.

Models

First, the models. I decided to create two ‘widgets’, one for news, and one for a clock.

public class NewsModel
{
    public string[] Headlines { get; set; }

    public NewsModel(params string[] headlines)
    {
        Headlines = headlines;
    }
}

public class ClockModel
{
    public DateTime Now { get; set; }

    public ClockModel(DateTime now)
    {
        Now = now;
    }
}

Controller

My controller doesn’t know anything about the views. What it does is returns a single model, but that model has the ability to dynamically fetch the right model as required by the view.

public ActionResult Show(string widgetName)
{
    var selector = new ModelSelector();
    selector.WhenRendering<ClockModel>(() => new ClockModel(DateTime.Now));
    selector.WhenRendering<NewsModel>(() => new NewsModel("Headline 1", "Headline 2", "Headline 3"));
    return PartialView(widgetName, selector);
}

Delegates are used so that the correct model is only created/fetched if it is actually used.

ModelSelector

The ModelSelector that the controller uses is pretty simple – it just keeps a bag of delegates to create each model type:

public class ModelSelector
{
    private readonly Dictionary<Type, Func<object>> modelLookup = new Dictionary<Type, Func<object>>();

    public void WhenRendering<T>(Func<object> getter)
    {
        modelLookup.Add(typeof(T), getter);
    }

    public object GetModel(Type modelType)
    {
        if (!modelLookup.ContainsKey(modelType))
        {
            throw new KeyNotFoundException(string.Format("A provider for the model type '{0}' was not provided", modelType.FullName));
        }

        return modelLookup[modelType]();
    }
}

The Views – Simple solution

Now, the easiest way to implement a view would be:

@model MvcApplication2.ModelSelector
@using MvcApplication2.Models
@{
    var clock = (ClockModel) Model.GetModel(typeof (ClockModel));
}

<h2>The time is: @clock.Now</h2>

You could end here and use this approach.

The Views – Better solution

That’s pretty ugly. I wanted my views to look like this:

@model MvcApplication2.Models.ClockModel
<h2>Clock</h2>
@Model.Now

And

@model MvcApplication2.Models.NewsModel
<h2>News Widget</h2>
@foreach (var headline in Model.Headlines)
{
    <h3>@headline</h3>
}

To make this work, I had to create a custom view engine.

Custom view engine

When a Razor view is compiled, it inherits a ViewPage<T>, where T is the @model. So we can use reflection to figure out what type the view wanted, and select it.

public class ModelSelectorEnabledRazorViewEngine : RazorViewEngine
{
    protected override IView CreateView(ControllerContext controllerContext, string viewPath, string masterPath)
    {
        var result = base.CreateView(controllerContext, viewPath, masterPath);

        if (result == null)
            return null;

        return new CustomRazorView((RazorView) result);
    }

    protected override IView CreatePartialView(ControllerContext controllerContext, string partialPath)
    {
        var result = base.CreatePartialView(controllerContext, partialPath);

        if (result == null)
            return null;

        return new CustomRazorView((RazorView)result);
    }

    public class CustomRazorView : IView
    {
        private readonly RazorView view;

        public CustomRazorView(RazorView view)
        {
            this.view = view;
        }

        public void Render(ViewContext viewContext, TextWriter writer)
        {
            var modelSelector = viewContext.ViewData.Model as ModelSelector;
            if (modelSelector == null)
            {
                // This is not a widget, so fall back to stock-standard MVC/Razor rendering
                view.Render(viewContext, writer);
                return;
            }

            // We need to work out what @model is on the view, so that we can pass the correct model to it. 
            // We can do this by using reflection over the compiled views, since Razor views implement a 
            // ViewPage<T>, where T is the @model value. 
            var compiledViewType = BuildManager.GetCompiledType(view.ViewPath);
            var baseType = compiledViewType.BaseType;
            if (baseType == null || !baseType.IsGenericType)
            {
                throw new Exception(string.Format("When the view '{0}' was compiled, the resulting type was '{1}', with base type '{2}'. I expected a base type with a single generic argument; I don't know how to handle this type.", view.ViewPath, compiledViewType, baseType));
            }

            // This will be the value of @model
            var modelType = baseType.GetGenericArguments()[0];
            if (modelType == typeof(object))
            {
                // When no @model is set, the result is a ViewPage<object>
                throw new Exception(string.Format("The view '{0}' needs to include the @model directive to specify the model type. Did you forget to include an @model line?", view.ViewPath));                    
            }

            var model = modelSelector.GetModel(modelType);

            // Switch the current model from the ModelSelector to the value of @model
            viewContext.ViewData.Model = model;

            view.Render(viewContext, writer);
        }
    }
}

The view engine is registered by putting this in Global.asax.cs:

ViewEngines.Engines.Clear();
ViewEngines.Engines.Add(new ModelSelectorEnabledRazorViewEngine());

Rendering

My home view includes the following lines to test it all out:

@Html.Action("Show", "Widget", new { widgetName = "Clock" })
@Html.Action("Show", "Widget", new { widgetName = "News" })