What is the difference between MVC and MVVM?

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asked Mar 20, 2009 by bjorn-reppen

Is there a difference between the standard "Model View Controller" pattern and Microsoft's Model/View/ViewModel pattern?

19 Answers

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answered Jan 20, 2009 by chris-ballance

For one thing, MVVM is a progression of the MVC pattern which uses XAML to handle the display. This article outlines some of the facets of the two.

The main thrust of the Model/View/ViewModel architecture seems to be that on top of the data (”the Model”), there’s another layer of non-visual components (”the ViewModel”) that map the concepts of the data more closely to the concepts of the view of the data (”the View”). It’s the ViewModel that the View binds to, not the Model directly.

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answered Jan 20, 2009 by tstamper

MVVM Model-View ViewModel is similar to MVC, Model-View Controller

The controller is replaced with a ViewModel. The ViewModel sits below the UI layer. The ViewModel exposes the data and command objects that the view needs. You could think of this as a container object that view goes to get its data and actions from. The ViewModel pulls its data from the model.

Russel East does a blog discussing more in detail Why is MVVM is different from MVC

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answered Jan 20, 2009 by eglasius

MVVM adds the view model into the mix. This is important, as it allows you to use a lot of the binding approach of WPF, without putting all that UI specific pieces in your regular model.

I may be wrong, but I am not sure MVVM really forces the controller into the mix. I find the concept to be more in line with: http://martinfowler.com/eaaDev/PresentationModel.html. I think that people choose to combine it with MVC, not that it is built in into the pattern.

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answered Jan 27, 2009 by wekempf

MVVM is a refinement (debatable) of the Presentation Model pattern. I say debatable, because the only difference is in how WPF provides the ability to do data binding and command handling.

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answered Jan 9, 2010 by der-martin

MVVMC, or perhaps MVC+, seems to be a viable approach for enterprise as well as rapid application development. While it is nice to separate the UI from business and interaction logic, the 'pure' MVVM pattern and most available examples work best on singular views.

Not sure about your designs, but most of my applications, however, contain pages and several (reusable) views and thus the ViewModels do need to interact to some degree. Using the page as controller would defeat the purpose of the MVVM altogether, so not using a "VM-C" approach for the underlying logic might result in .. well .. challenging constructs as the application matures. Even in VB-6 most of us probably stopped coding business logic into the Button event and started 'relaying' commands to a controller, right? I recently looked at many emerging framworks on that topic; my favorite clearly is the Magellan (at codeplex) approach. Happy coding!

http://en.wikipedia.org/wiki/Model_View_ViewModel#References

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answered Jan 21, 2010 by george-r

I thought one of the main differences was that in MVC, your V reads your M directly, and goes via the C to manipulate the data, whereas in MVVM, your VM acts as an M proxy, as well as providing the available functionality to you V.

If I'm not full of junk, I'm surprised no one has created a hybrid, where your VM is merely a M proxy, and C provides all functionality.

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answered Jan 22, 2010 by se-thoughts

From what I can tell, the MVVM maps to the MV of MVC - meaning that in a traditional MVC pattern the V does not communicate directly with the M. In the second version of MVC, there is a direct link between M and V. MVVM appears to take all tasks related to M and V communication, and couple it to decouple it from the C. In effect, there's still the larger scope application workflow (or implementation of the use scenarios) that are not fully accounted for in MVVM. This is the role of the controller. By removing these lower level aspects from the controllers, they are cleaner and makes it easier to modify the application's use scenario and business logic, also making controllers more reusable.

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answered Jan 23, 2010 by danice

The viewmodel is an "abstract" model for your user interface elements. It must allow you to execute the commands, and actions in your view in a non-visual way (for example to test it).

If you have worked with MVC, you probably have sometime found useful to create model objects to reflect the state of your view, for example, to show and hide some edit dialog, etc. In that case you are using a viewmodel.

The MVVM pattern is simply the generalization of that practice to all the UI elements.

And it's not a Microsoft pattern, what appends is that WPF / Silverlight data-bindings are specially well-suited to work with this pattern. But nothing stops you to use it with java server faces, for example.

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answered Mar 22, 2010 by gone-coding

MVC/MVVM is not an either/or choice.

The two patterns crop up, in different ways, in both ASP.Net and Silverlight/WPF development.

For ASP.Net, MVVM is used to two-way bind data within views. This is usually a client-side implementation (e.g. using Knockout.js). MVC on the other hand is a way of separating concerns on the server-side.

For Silverlight and WPF, the MVVM pattern is more encompassing and can appear to act as a replacement for MVC (or other patterns of organising software into separate responsibilities). One assumption, that frequently came out of this pattern, was that the ViewModel simply replaced the controller in MVC (as if you could just substitute VM for C in the acronym and all would be forgiven)...

The ViewModel does not necessarily replace the need for separate Controllers.

The problem is: that to be independently testable*, and especially reusable when needed, a view-model has no idea what view is displaying it, but more importantly no idea where its data is coming from.

*Note: in practice Controllers remove most of the logic, from the ViewModel, that requires unit testing. The VM then becomes a dumb container that requires little, if any, testing. This is a good thing as the VM is just a bridge, between the designer and the coder, so should be kept simple.

Even in MVVM, controllers will typically contain all processing logic and decide what data to display in which views using which view models.

From what we have seen so far the main benefit of the ViewModel pattern to remove code from XAML code-behind to make XAML editing a more independent task. We still create controllers, as and when needed, to control (no pun intended) the overall logic of our applications.

The basic MVCVM guidelines we follow are:

  • Views display a certain shape of data. They have no idea where the data comes from.
  • ViewModels hold a certain shape of data and commands, they do not know where the data, or code, comes from or how it is displayed.
  • Models hold the actual data (various context, store or other methods)
  • Controllers listen for, and publish, events. Controllers provide the logic that controls what data is seen and where. Controllers provide the command code to the ViewModel so that the ViewModel is actually reusable.

We also noted that the Sculpture code-gen framework implements MVVM and a pattern similar to Prism AND it also makes extensive use of controllers to separate all use-case logic.

Don't assume controllers are made obsolete by View-models.

I have started a blog on this topic which I will add to as and when I can. There are issues with combining MVCVM with the common navigation systems, as most navigation systems just use Views and VMs, but I will go into that in later articles.

An additional benefit of using an MVCVM model is that only the controller objects need to exist in memory for the life of the application and the controllers contain mainly code and little state data (i.e. tiny memory overhead). This makes for much less memory-intensive apps than solutions where view-models have to be retained and it is ideal for certain types of mobile development (e.g. Windows Mobile using Silverlight/Prism/MEF). This does of course depend on the type of application as you may still need to retain the occasional cached VMs for responsiveness.

Note: This post has been edited numerous times, and did not specifically target the narrow question asked, so I have updated the first part to now cover that too. Much of the discussion, in comments below, relates only to ASP.Net and not the broader picture. This post was intended to cover the broader use of MVVM in Silverlight, WPF and ASP.Net and try avoid people replacing controllers with ViewModels.

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answered Jan 12, 2013 by mat

You can see an explanation of the MVVM Pattern in the Windows environment:

In the Model-View-ViewModel design pattern, an app is composed of three general components. enter image description here

  • Model: This represents the data model that your app consumes. For example, in a picture sharing app, this layer might represent the set of pictures available on a device and the API used to read and write to the picture library.

  • View: An app typically is composed of multiple pages of UI. Each page shown to the user is a view in MVVM terminology. The view is the XAML code used to define and style what the user sees. The data from the model is displayed to the user, and it’s the job of the ViewModel to feed the UI this data based on the current state of the app. For example, in a picture sharing app, the views would be the UI that show the user the list of albums on the device, the pictures in an album, and perhaps another that shows the user a particular picture.

  • ViewModel: The ViewModel ties the data model, or simply the model, to the UI, or views, of the app. It contains the logic with which to manage the data from the model and exposes the data as a set of properties to which the XAML UI, or views, can bind. For example, in a picture sharing app, the ViewModel would expose a list of albums, and for each album expose a list of pictures. The UI is agnostic of where the pictures come from and how they are retrieved. It simply knows of a set of pictures as exposed by the ViewModel and shows them to the user.

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