MVC: pass model / model data to a view from a controller?

Question

If a view needs to acces data from a model, do you think the controller should:

a) pass the model to the view
b) pass the data of the model to the view
c) neither; it shouldn't be the controllers concern. Let the view access the model directly to retrieve the data. Only let the controller give some parameters the view needs to filter the data from the model.
d) it depends on the situation.
e) none of the above, but [...]

Thanks

After some debate in the comments to an answer that was deleted by the user, maybe this needs clarification. My view of the MVC architecture is biased towards that of the Zend Framework (php) in which an action in a controller by default has a default view assigned to it. So it's not so much the model that dictates which view is approporiate, but rather the controller. Do you feel the model should dictate what view is appropriate? The only way I see fit to let the view be build based on a model, is by letting the controller pass the model to the view. Are there other techniques to let a view access a model without the controller being involved? Or is it perfectly fine to let a controller pass the model to a view, so that the view can be build based on the models attributes?

Answer 1

e) None of the above; pass in a view-optimised "ViewModel".

Example in ASP.NET MVC:-

public ActionResult Details(int id)
{
  Product p = ProductService.GetProductById(id);

  if(p == null) { return RedirectToAction("Index"); }

  ProductViewModel model = new ProductViewModel(p);
  return View(model);
}

both a) and b) "will do" subject to d). Never ever c).

Typically, the ViewModel just encapsulates the Model (if nothing complicated is going on, your view could access the model directly via ProductViewModel.Product). If the view needs to do anything complicated with the Model however, it's the ViewModel's responsibility to do that, rather than the responsibility of the Controller, or the View.

This keeps your concerns nice and segregated. Your Controller doesn't care exactly what specific data your View needs (beyond the fact that it's rendering some Details of a Product), or especially what format your View needs that data in. Your View doesn't depend on the implementation details of your Model. Your Model doesn't have to concern itself with how it's being Viewed. If you have two Views rendering Products (e.g. Create, Edit, Summary, MoreDetails etc), those Views can have different ViewModels to expose only the data that each specific View needs, so your Views aren't depending on eachother. Lovely :)

Some further reading from various viewpoints:-

http://www.thoughtclusters.com/2007/12/datamodel-and-viewmodel/

http://stephenwalther.com/blog/archive/2009/04/13/asp.net-mvc-tip-50-ndash-create-view-models.aspx

http://www.nikhilk.net/Silverlight-ViewModel-MVC.aspx

I think ViewModels are a particularly .NET thing, but I see no reason at all why the pattern can't be used in PHP.

Hope this helps.

Answer 2

Ideally, it should "pass the data of the model to the view" so the view doesn't need to know any explicit structure of the model and thus be more reusable and designer-friendly.

But practically, "pass the model to the view" works as just fine. Most of the time you will need a new view anyway because clients never share favorite colors (if you know what I mean :-) so views re-usability doesn't justify having a lot of tedious code required to copy data from the model to the view.

What you should concern more about is the modularity of the controller itself, since many websites do share common functionalities (controllers) such as web forums or a news listing but not looks (views)