Why MVVM and what are it's core benefits?

Question

Why we go for MVVM over MVC or MVP while dealing with WPF?

What extra benefit we get by using this?

Edit:

To be honest , today I had an interview and I have been asked this question. I answered like INotifyPropertyChanged , ICommand,IValue Convertor.. but he was not satisfied. Henceforth I have put up this question

Thanks in advance

Answer 1

I'll point you to a particularly useful video by Jason Dolinger.

Coming from a WinForms world, implementing any MVX style pattern seemed like more hassle than it was worth but after working with WPF for a couple of years now, I can honestly say that I wouldn't consider anything less. The whole paradigm is supported out-of-the-box.

First off, the key benefit is enabling true separation between the view and model. What that means in real terms is that if/when your model needs to change, it can without the view needing to and vice-versa.

Secondly, while your model may contain all the data you might need in your view, you may want to abstract that data in such a way that your model doesn't support. For example, say your model contains a date property. In the model it can exist solely as a DateTime object but your view might want to present it in a completely different way. Without the viewmodel you'd either have to duplicate the property in the model to support the view or modify the property which could seriously obfuscate the 'model'.

You can also use a viewmodel to aggregate parts of your model that exist in separate classes/libraries to facilitate a more fluent interface for the view to deal with. It's very unlikely that you'll want to work with data in your code in the same way that a user will want to or will want that data presented to them.

On top of that, you get support for automatic two-way data binding between the view and viewmodel.

There really is a whole bunch of extra stuff that I could bang on about but Jason say's it far better that I could so my advice is watch the video. After a few days of working like this, you'll wonder how you ever got by without it.

Good luck.

Answer 2

The ability of XAML code to databind, as well as the existance of triggers will break the MVP and MVC Patterns.

Answer 3

Baked in support for ICommand and INotifyPropertyChanged are the two biggest benefits. Using MVVM makes it really easy to wire up the commands and plug data into the WPF UI. Things just work.

Answer 4

WPF has better databinding than any other UI framework, which MVVM would be unruly without

MVVM provides unit testability and excellent view-agnosticism, which make it a good thing to use

Answer 5

These are mine specific to MVVM

1. Increases the "Blendability" of your views (ability to use Expression Blend to design views). This enables a separation of responsibilities on teams that are lucky enough to have a designer and a programmer... each can work independent of the other.
2. "Lookless" view logic. Views are agnostic from the code that runs behind them, enabling the same view logic to be reused across multiple views or have a view easily retooled or replaced. Seperates concerns between "behavior" and "style".
3. No duplicated code to update views. In code-behind you will see a lot of calls to "myLabel.Text = newValue" sprinkled everywhere. With MVVM you can be assured the view is updated appropriately just by setting the underlying property and all view side-effects thereof.
4. Testability. Since your logic is completely agnostic of your view (no "myLabel.Text" references), unit testing is made easy. You can test the behavior of a ViewModel without involving its view. This also enabled test-driven development of view behavior, which is almost impossible using code-behind.

The other two patterns are really sort of separate in terms of the concerns they address. You can use MVVM with MVP and MVC (most good samples out there do some form of this).

In fact, MVP (w/ a Passive View, rather than a Supervising Controller) is really just a variant of MVVM, in my opinion.

Answer 6

I personnaly see MVVM not as a benefit, but as an obligation for those who want to use WPF cool features.

WPF is very very heavily built with data binding at the core, to enable separation of UI from Model. But the way data binding is technically done in WPF is somewhat special, as it's tied to classes like:

• DependencyProperty
• INotifyPropertyChanged
• ObservableCollection

Because of this you just can't really write a model the way you want using standard .NET technology. For example, the WPF TreeView is almost impossible to use w/o using data binding and templates. You just can't populate it simply like you would from a generic model in Winforms for example. It must be bound to a hierarchical model using ObservableCollection to represent a node's children.

So let's say V represents the XAML code and it's code-behind counterpart (so it's tied to WPF as a technology), and let's say M represents your model (so it's not tied to WPF UI technology in anyway).

Well, you'll never have this working properly under WPF with only these V & M.

You must add something between the two. Something that's WPF-compatible and understands your model. Something that speaks DependencyProperty, ObservableCollection and INotifyPropertyChanged. That's what's called VM.

As a side note, an alternative to MVVM is to build a V & M (w/o VM plumbing) combination with M being WPF-compatible but still with a reasonable UI independency. Historically, ObservableCollection was in the WindowsBase.dll assembly (that was shipped with WPF), so it really looked weird to bind a generic model to something tied to a UI technology. It's been moved back to System.dll since. Even then, it's sometimes hard to keep a pure VM model w/o tweaking the M specifically for WPF...