I'm just learning the ropes of the MVVM pattern in WPF applications, and I can imagine that this sounds like an incredibly stupid question, but here goes anyway:
I already have a model in one assembly, which is a simple class library. In a different assembly, I've created a simple view in xaml. Now the books all tell the same: link them together with a viewmodel. My question is though, where does this viewmodel belong:
Is it more or less part of the view, and should it be in that assembly?
Is the viewmodel meant to be universal, so it belongs together with the model assembly?
Does the viewmodel get its own assembly?
I know the MVVM pattern is merely a design guideline and not a strict set of rules, but I feel it's better to learn things the right way.
EDIT
Follow-up question: is a viewmodel meant to be re-usable? I can imagine a scenario where it would be usefull if you could use the same viewmodel for a WPF desktop application and a silverlight web application.
It facilitates building the view, so it belongs in the view assembly.
Think of it like this: could you take your model assembly and use it in a different style of application, e.g. a Windows service or a web application? Is there anything that is irrelevant to that style of application in that assembly? If the answers are yes and no, you've built yourself a useful re-usable component that is independent of the type of user interface.
Depending on the size of the project, I put the ViewModels either in the same assembly as the views, or in their own assembly, but never in the model assembly. The model shouldn't contain anything related to the UI.
Related
I have a WPF GUI application based on MVVM design and data binding. Now, I want to reuse the core code (i.e. the data model) in a Windows service, or a console UI app, or a WinForms app.
Is such a design reasonable? If yes, what are the pitfalls?
Or should I make a standalone data model instead, and interface WPF via wrappers?
UPDATE:
Sorry, I should have been more precise. Let me clarify: I don't doubt the very modularity thing =) My concern boils down to having my current DataModel implement INotifyPropertyChanged, use DispatcherTimers, etc. -- all that non-GUI but still WPF stuff. The model's business logic is based on it.
Is this (non-GUI WPF) design acceptable for reuse in the aforementioned cases, or should I abstract further, until no references to WPF would be required at all?
Yes, this is perfectly acceptable and most of the time it is desired!
When you build an MVVM app, it should be in at least 3 formal layers:
Presentation WPF, UI, xaml, behaviors. All that stuff. Not reusable
Application The view models and structure that supports your application rules. All that stuff. Not intended for reuse
Foundation Database access, business objects. Domain specific algorithms. Ideally this bit should be reusable anywhere
The foundation layer is the clever bit. This is where the meat in your application sandwich is. It makes perfect sense for this to be totally agnostic of UI technology. WPF, winforms, ASP. It shouldn't even need a UI.
Edit for question update:
Removing all references to WPF is hard because sometimes you need a CollectionViewSource on your view models for grouping/filtering of results. That is a WPF class.
It is very tempting to view your seperation-of-concerns as 'just dont reference wpf' and that helps but it can make life difficult. Instead, try to be disciplined with the type of behaviors you are putting in. If you find yourself writing 'clever' (domain) code on a view model, shift it to the foundation layer as a business object method or extension. Similarly, if you find yourself implementing IValueConverter often, perhaps you should make better use of view models.
One thing is for sure, your foundation layer should never, ever, ever reference WPF.
Such a design is very reasonable! You can create a portable C# library for all .NET technologies including WPF, WinRT, ASP MVC, etc which can contain your models. Obviously you'll need to wrap these portable models into a viewmodel anyway, but IPropertyChanged is implemented in all XAML flavors.
I'm very new to XAML. To utilize MVC architecture and the Command Pattern while taking advantage of XAML, I have started binding static ICommands to Buttons. I'm working on a fairly large project with over a hundred buttons. My questions are: are there different approaches for binding commands to buttons to avoid static objects. With regards to C#, WPF, and XAML, are statics commonly used? I'm sure someone has already worked on a project using MVC, Command Pattern, and XAML, what was your approach?
I should have probably edited this sooner, but while working on the project, I've realized how much I didn't know about c#, WPF, and XAML when I asked this question. Apparently, in WPF, instance properties make it convenient binding methods and data members to controls.
As far as MVC / MVVM are concerned, I guess I was hesitant to expose my model to VM before I even know what it is.
I think you mean the MVVM pattern as it applies to WPF. That is Model View ViewModel
Model = used to construct the form model for the data being manipulated
View = Presentation (usually a main form and many user control forms)
ViewModel = code container for presenter (classes that contain the code)
Generally your binding take the place of ICommand methods that implement a RelayCommand from a base. There is a lot to learn before implementing the MVVM model, I would suggest reading Josh Smith's article and downloading his example to get started on learning it:
There are special rules and principles to learn and this example will go through a lot of it.
http://msdn.microsoft.com/en-us/magazine/dd419663.aspx
I would also think it would be wise to learn how WPF performs bindings a little bit. There are many special setups you can do with bindings to help with performing operations at different events and other places. I do not even know all the bindings by heart but I know the more you learn them the more time you save in the end in your XAML coding as you can reuse, event trigger, inherit and do many things with bindings to make your applications more powerful than static creation.
http://msdn.microsoft.com/en-us/library/ms752347.aspx
This question already has answers here:
Why use MVVM? [closed]
(13 answers)
Closed 2 years ago.
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
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.
These are mine specific to MVVM
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.
"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".
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.
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.
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
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.
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...
The ability of XAML code to databind, as well as the existance of triggers will break the MVP and MVC Patterns.
Im writing a wpf project and using the MVVM paradigm, what i was wondering is if i should write my view model classes in their own project.
Advantages i can see is that your ui project would never have to know about your business logic. (not have a reference to it)
however if i want to use the ICommand interface in my view model i still need a reference to PresentationCore which may indicate that i should be in my ui project.
comments suggestions most appreciated.
As soon as you reference PresentationCore in your ViewModel (which is currently unavoidable if you want to use ICommand) you are leaking all manner of undesirable View-related features into your ViewModel. For example, MessageBox.Show, when you see this called in a unit test it drives home why it's bad.
To answer the question, yes keep your View and ViewModel in separate projects. I wondered this myself but after going down the separate projects route it took me awhile to appreciate but it has been really valuable in making me stick to a clean MVVM solution, and the lessons from this have greatly improved my overall solution architecture. It's all about reducing dependencies on unnecessary assemblies using interfaces and if necessary the Adapter pattern. One example is that my View project, being an entrypoint has a reference to Ninject, but I don't want my ViewModel to have that reference. Otherwise someone might come along and use the Ninject static Kernel directly.
Regarding ICommand, I don't know the history having only used WPF since 4, but it feels like MVVM was an afterthought. I'm not sure why else Microsoft would have put this interface in PresentationCore. I'm hoping this will be addressed in a future release with a separate assembly for the ViewModel layer.
I don't think there's an overwhelmingly compelling way to do it one way or the other. I tend to keep VMs and their views in the same assembly, but within a different folder structure. For example, I might have ViewModels/Foo/Bar/CustomerViewModel and Views/Foo/Bar/CustomerView.xaml.
I don't think there's a problem splitting out the views and view models either. Having the VM assembly reference view-related assemblies such as PresentationCore is only natural. After all, your view models are part of your view layer.
Well this question relates to MVVM pattern and i could good and fast answers on this forum so I thought to ask and clear the confusions i had about the pattern.
I am quite new to MVVM approach. I appreciate the pattern and understand the principals behind it. Maybe I have not worked that much with the pattern that’s why there are a few confusions.
If there is a scenario in which I want to load few parts of my WPF page dynamically with XAML and still want to be compliant with MVVM approach.
The confusion is:
Where the logic of loading a view dynamically with XAML reside.
Whether I should have a single ViewModel for my WPF page or each seperate part have its own viewmodel with interactions with other viewmodel classes.
What if I had to build control tree displayed on the GUI using C# code in the codebehind itself.
For the controls created using code should I do the commandbindings in the codebehind of the view itself.
Where the logic for loading goes is something not really addressed by the pattern itself. There's an interesting blog post about this by Ward Bell. There's any number of ways to skin this cat, and they're all compatible with MVVM. Not really the answer you're looking for, I know, but it's honest :). Check out Ward's blog post... you'll get a much more in depth discussion of this topic.
As for whether or not to have a single VM for the page, or one for each control, that just depends. Generally, I have one for the page. If there's some portion that reusable elsewhere, I break it out into a user control with it's own VM, which means we have a VM within a VM. I don't agree with rockeye on this one. There isn't a one-to-one relationship between V-VM-M. You're Models are designed according to business needs, with NO regard to presentation at all. You're ViewModels are designed according to your presentation needs, and may encapsulate more than one Model. In fact, it's very common for them to encapsulate many models.
Like rockeye, I don't understand your last question.
I am also quite new to mvvm, but i will try to answer :
Where the logic of loading a view dynamically with XAML reside
If you mean "how can i show the view associated with my business object?", IMHO, you don't have to care about this. Usually, your VMs have corresponding views. With dataTemplate, you use only VM in the code but Views are displayed automatically.
2 Whether i should have a single ViewModel for my WPF page or each seperate part have its own viewmodel with interactions with other viewmodel classes
It seems you have a top-bottom approach. I see the mvvm more as bottom-up : models (business objects) -> ViewModels -> Views. Every model should have its own ViewModel and view. So you can't have a whole WPF page in a viewModel unless you model represents a page.
3 What if i had to build control tree displayed on the GUI using C# code in the codebehind itself. For the controls created using code should i do the commandbindings in the codebehind of the view itself.
Don't understand. I think you may take a look at dataTemplate, it might be helpfull.