我应该将服务注入我的MVC视图吗？

Question

We're working on some kind of Cloud CMS using ASP.NET MVC technology, and have found some obstacles on the way. There is a number of parameters user could change thru the control panel that we need to end up in Views. For example, Facebook application id to initialize the Facebook JS API. Or additional text to be shown on the page. Or background picture. For now we're not using DI to transfer this parameters, instead we're adding them to the ViewModel, but this ruin the ASP.NET MVC way of working with models (eg form validation, bindings etc.)

It looks like that using DI to inject services for providing parameters, texts and pictures could make my views less dependent on controllers specific, and there is even some Microsoft technique to do it http://www.asp.net/mvc/tutorials/hands-on-labs/aspnet-mvc-4-dependency-injection#Exercise2 . However, there are a lot of answers on forums against injecting services into Views using DI.

So the question: what is a right way to inject some services into Views? Or I shouldn't do it at all and something is wrong in the application design?

UPDATE : some real code examples (now we're using Model to inject the services)

Injecting texts from database (they have to be user-editable, as it is CMS):

<div class="steps">@Html.Raw(Model.Texts["Main", "Step2"]</div>

Injecting translations from database (actually, it is localization):

<div class="gonfalon">@Model.Translations["Title_Winners"]</div>

Injecting parameters (from database, could be request-specific; for example, if the site has different domains, facebook application should be per-domain):

Facebook.Initialize(Model.Parameters["FbApplicationId"], Model.Parameters["FbApplicationSecret"]);

The problem of current approach is that this code has taken from contest mechanic. It is definitely out of contest business scope to deal with custom texts, translations or facebook application Id. Also it ruins the Model as model models not actual business domain but deals with a lot of things actually belongs to View (like translations and custom texts)

UPDATE 2 : Have modified the snippet from the answer below to be a bit more generic:

public static class WebViewPageExtensions 
{ 
    public static I ResolveService<I>(this WebViewPage page) 
    { 
         return DependencyResolver.Current.GetService<I>(); 
    } 
} 

Answer 1

No, you shouldn't inject services into Views, but...

For scenarios such as theming where you want to give the theme developer more power, just one model isn't enough. If your model contains the current post for example, how can a theme designer asks for a list of categories for the sidebar? Or for a widget?

In asp.net mvc you can use extension methods to offer that functionality.THe extension method will use the dependency resolver to get the service. This way, you can have the needed functionality in the view without actually injecting a service.

Note that calling the business layer to update the model is still a violation of Separation of Concerns. THe services made available to the view should contain only read model or general utility functionality.

An example

 public static IMyViewServices MyServices(this WebViewPage view)
     {
         return DependencyResolver.Current.GetService<IMyViewServices>();
     }

IMyViewServices lifetime configured in the DI Container should be per http (scope) request

Answer 2

No, end of story. Why? Here is why:

Your view only needs to know what the view model it's going to be working with to present that model. There are couple of reasons for this but the biggest one is the separation of concerns. Keep your view as stupid as possible. You will see that this seperation will give you a clean application structure throughout the way.

There is a number of parameters user could change through the control panel that we need to end up in Views.

I'm not sure what you exactly mean here but this is why there are view models. Your business layer will shape models up, your controller will simply map them to your view models and pass them into the view (presentation layer).

Answer 3

This really depends on how much or little you want your controllers to do and to what degree of separation you want to achieve.

In my world, the "controller" in an MVC app does as little as possible because I have a service layer handling all of the business logic and a data layer above that handling all of the database interaction.

On a GET, the controller will simply call a service method that will build the view model and hands it back to the controller and the controller passes it on to the view. On POST, the view posts data to the controller which sends it off to the service layer for validation, saving to DB, etc. The service is injected into the controller's constructor.

I'd be more than happy to post code examples if you'd like to see them.