Is it OK to use my Domain Model as Model in the MVVM pattern, in the context of a WPF application? Or is the Model (in MVVM) supposed to be some POCO? I personally don't see any reason against using the Domain Model as Model of the MVVM WPF pattern.
But then the XAML elements will bind to what? Properties in the VM, which underneath are mapped to the Domain Model, right?
To answer the question posed in your title: neither.
In MVVM, the "model" is generally everything the other side of the viewmodel (i.e. between the viewmodel and the repository). The model can (and usually is) composed of several different additional architectures and patterns, it is normal to have services, micro services, DALs and DILs, domain models, POCOs/data entities etc all within the "model".
The domain objects and POCOs you speak of are not the model, they are part of the model. The perspective of the model that you talk of has been brought over from other patterns like MVC, where a data entity or domain object is indeed the model. The model is a little more complex than that in MVVM.
You can bind directly to a POCO in MVVM, but it's not usually a good idea unless it is a very simple one that is not subject to change (like a lookup list that is sourced from a repository). And it is usually quite a bad idea to bind to domain objects because these will contain extra functionality or information that is related to storage and retrieval but is overkill for anything shown in the UI. The other problem with domain objects is they usually don't serialize nicely (if at all - often domain objects are generated), so unless you have a very simple model it is best to transform the domain objects to POCOs as soon as practical.
So as mentioned in the answer from Owen, you should wrap your POCOs with viewmodel classes before they are made available to the UI. The only time you shouldn't is when you don't need any property change notifications and you also don't want to leverage any further functionality on the objects (for example you won't need a context menu on the displayed item).
But then the XAML elements will bind to what? Properties in the VM, which underneath are mapped to the Domain Model, right?
Yes, you could reference your POCO model entity classes in your WPF application and wrap them in view model classes that implement the INotifyPropertyChanged interface to provide support for dynamic change notifications. This is perfectly fine.
Please refer to my answer to the below question for more information about this.
Correct way to update property in ViewModel from Model
You'll likely want to wrap your domain model objects with view models. The purpose of your view model is to expose data from your model in a way that is useful to the view. For very simple applications it might seem cumbersome to add that extra layer just for change notification, but as your application becomes more complex, having a view model layer to bind to can help keep your model clean. If you are doing something quick and dirty, there is nothing stopping you from binding directly to your domain model objects.
Yes as long as your domain model implements inotifypropertychanged you can use and bind xaml with VM properties
Related
I'm currently facing one of the most discussed problems in MVVM: I have a complex Model in my WPF application and I'm not sure how I should display its data to the View.
According to many answers here on StackOverflow and also to this article there are two ways:
to wrap the Model inside the ViewModel by adding a property in the ViewModel for each property in the Model
to expose the Model directly to the view without replicating the properties.
What I understood so far is that the first approach is better from a theoretical point of view, while the second one is a quick shortcut that should be avoided.
In the same article I previously linked, the author writes the following:
In reviewing the sample application from the Caliburn framework, they implement the VM using option 2.
I took a look at the Caliburn.Micro documentation and unfortunately it just uses a simple ViewModel without a real Model, so I don't know how to verify this statement.
Is the author right? Since I'm using Caliburn.Micro should I use the second approach instead of the first one in order to be more "compliant" with the framework implementation?
Since I'm using Caliburn.Micro should I use the second approach instead of the first one in order to be more "compliant" with the framework implementation?
No. Caliburn.Micro is just an MVVM library. How you implement the actual MVVM pattern is entirely up to you.
I agree with #Marek Dzikiewicz that you should wrap the model in a view model class that may implement the INotifyPropertyChanged interface and provide any other UI specific functionality. This code doesn't belong to a business object. You could refer to my answer here for more information:
Reuse the same models in ASP.NET MVC and WPF MVVM
Obviously if the model class is indeed a UI specific class that is not used in any other application and doesn't contain any business logic that is used on the server side, you could modify this class and bind to it directly. But then it is kind of a (sub) view model after all.
Usually it is better to expose view models because that allows you to add additional properties to control the way the data is displayed (for example formatting or concatenating data). However, if you don't need that, there is nothing wrong in exposing the model classes directly.
I'm working on a Windows Phone 7 app with a designer. I've done C# development with XNA so I know C# but was not familiar with the Model/View/ViewModel architecture.
Our first crack at it had multiple ViewModels active for any given view. Each ViewModel was an in-between layer for each object in our model.
For example: We had a "Friends" page that has a ListBox that displays the list of Friends. So we made a FriendsListViewModel that would handle getting an ObservableCollection<Friend> from the Model that the XAML would bind to. There were other functions available in the page (navigating to other pages, activating semi-related features, etc.) so that was contained in the FriendsPageViewModel.
This was starting to look crazy to me, so I made the relationship between View and ViewModel 1:1.
With all that described, I've got a question with two components:
With MVVM, what is the common relationship between Views and ViewModels? (focusing on Windows Phone 7 development here in case it's any different from ASP.NET, WPF, or Silverlight)
And as a possible addon to that question: Say the 1:1 relationship is the generally correct or accepted one: If you're dealing with something like a Pivot control or Panorama control, would you typically give each PivotItem or PanoramaItem its own ViewModel?
In MVVM, you typically have one ViewModel for each View (exceptions exist). The View typically 'binds' to the ViewModel which is the glue between the view and the data model. Your view can contain multiple controls and each control will bind to a particular property (i.e. data source) on your ViewModel. The ViewModel will then notify the View once one of these properties is updated (via the INotifyPropertyChanged interface in C#).
When thinking about ViewModels, don't think of it as a single ViewModel per control. Think of the control binding to a single property of the shared ViewModel.
The ViewModel provides data from the model to the View. The View should only be used to display the data it gets from the ViewModel. Keep code in the View to a mimimum and only pertaining to rendering control elements. The ViewModel is responsible for querying data from whatever the data source may be and then providing public properties the View can hook into.
This MSDN link has a pretty detailed article on it, but you can get a good synopsis on wikipedia.
The question is stated in the subject: are the advantages of Model-View-ViewModel (MVVM) pattern worth the overhead?
In many cases, implementing the view model involves quite significant overhead of duplicating the Model properties and sometimes synchronization between Model and ViewModel data members. For example, currently in Silverlight 4 & WCF RIA, View Models are not generated (if the developer follows the MVVM pattern, it is up to him to create the view models, often duplicating the corresponding Model's properties at ViewModel, that do nothing significant but refer to Model as the storage).
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
Effectively that is what the PresentationModel is for. Which MVVM is strongly based on. The difference is that the ViewModel is the model for the view and not the model for the data. So you are concerned around more how the view behaves with the data.
If you have a simple UI that all it does is present the model then I would suggest expose the Model on a property of the ViewModel and bind to that. Make sure though the model does implement INotifyPropertyChanged etc.
The power of the ViewModel is when you have things to do in response to a user action. The ViewModel can then support Commands, calling out to services and validation and thus leaving the Model as a data container
Why not extending the Model class, providing additional properties to make it easy to be consumed by the View instead?
In the simple cases, this is all the ViewModel is doing - wrapping up the Model so that its extended in a way that's consumable by the View. If your Model can be bound directly, you're welcome to do so.
That being said, there is more to the ViewModel layer than just wrapping the model - this is also where the application specific logic - ie: the application's plumbing, will occur. Something has to make the requests from the Model classes correctly and compose together the logic.
If you are concerned about extra work, you can always create a ViewModelBase (INotifyPropertyChanged , Errors/Validation, generic stuff) to be inherited by your ViewModel, it will minimize things that you think may cost you time to replicate. And also, Silverlight/Wpf provides us with binding which greatly reduces our coding, besides the fact that XAML also does that by providing functionalities through markup. Besides that you can further the design by using screens, controllers, etc.
For me, I do not see any "overhead" with regards to using MVVM; if there were, it'd be worth it. It properly deals with the Separation of Concerns. It provides a good platform for development especially in teams where people may take care of different aspects of the application without affecting other team members codes (especially between developers and designers).
Hope this helps
Benefits of MVVM
Reduced complexity.
Isolation of Designing and Development.
Dependency injection.
Major advantage is when you have a Well MVVM structured Windows Phone application and want to develop same for Windows Metro Desktop, Only thing u want to concentarte on design as the same view model can be used as it is.
Hope it helps.
If I am generating POCO objects from EntityFramework, and using these to go to/from the WCF server, is there any reason to create client-side Models for the Views & ViewModels to use instead of just using the POCOs directly?
Almost all the MVVM examples I have looked at bind straight to the object returned from the WCF service. Is this good practice? Are there arguments that can be made for actually mapping the POCO to a Model and having the Views/ViewModels working with the Model object instead of the POCO?
The main reason I could think of is validation, however since the EF POCOs are partial classes, they can be expanded on to include validation.
EDIT
Most answers so far have brought up INotifyPropertyChanged as the main reason to build a separate Model. Does your answer change if you are using Self-Tracking entities instead of POCOs which already includes INotifyPropertyChanged? STEs are also partial classes which can be expanded upon to include validation.
Validation is the main reason not to bind directly to a POCO. In addition, if the POCO doesn't already implement INotifyPropertyChanged and other required interfaces, the experience working with the object on the WPF side may be less desirable, and implementing a ViewModel to wrap this makes sense.
Providing a ViewModel to wrap your POCO allows you to encapsulate the logic into ICommand implementations as well as implement required interfaces cleanly.
I disagree only slightly with Reed (an unusual circumstance to be sure). I would NOT implement a ViewModel to wrap the POCO. I would implement a Model class to wrap the POCO and expose the Models to the ViewModel via a Service layer.
The ViewModel's primary job is to appropriately present Model data to the View and react to its requests. The architecture I'm working on for this looks like so:
1 ViewModel for each View
The ViewModel calls a Data Service layer object to retrieve Model instances (not to be confused with a WCF service)
The Data Service layer issues the appropriate CRUD requests to the backend (this uses WCF, RIA, or RESTful Services for Silverlight but could be ADO.NET or EF directly for WPF).
The Data Service uses the returned POCOs to create Model objects.
Model objects wrap the POCO object and implement INotifyPropertyChanged. Model objects enforce business rules.
I'm still working through the details but I will be publishing something more concrete in the near future.
My Models accept a WCF object which exposes those properties which I wish to use in my ViewModel. I can then also extend the object as needed. My properties point to the WCF object's property and when I have to send the object back to the WCF service, I don't have to do any more work. The models inherit INotifyPropertyChanged and INotifyDataErrorInfo which the DTOs (mentioned here as POCOs) will not have. Your business logic / validaton exists in your Silverlight application and not in your WCF Service.
The View binds to the ViewModel which has a Model (or an observable collection of Models). The Models have a WFCObject which is a DTO (mentioned here as POCO). I use my ViewModel to communicate with the service, MVVM Light has the models communicate with the service / provider - which I don't like.
Bind to EF POCOs if you want to do simple CRUD or you want to make something fast.
Otherwise, your server-side models will tend to be very closely related to the database, which changes very slowly, as compared to user interface. For less trivial UI, you'll find yourself putting more and more kludges just to fit your database model into UI (or otherwise, which is even worse).
Also, there are performance issues (e.g. would you like to transmit whole entity when for UI you need only couple of properties?), and maintenance issues (e.g. if you would like to validate premium customer's order quite differently from ordinary one).
See also http://ayende.com/Blog/archive/2010/08/06/data-access-is-contextual-a-generic-approach-will-fail.aspx
Rachel's POCO's are just dumb objects generated by EF and used for transport (DTO). Therefore, they shouldn't have other things cluttering up their domain. This is a very nice way of designing your code because it decouples any client-side requirements from those on the server-side. That's why MVVM exists - to extend the MVC model incorporating those concerns.
There is no reason you can't bind to them in your views as long as you are not modifying them directly. You can add functionality to them by adding a partial class but I wouldn't even do that. In that case you should follow the MVVM design tenants and separate those into model objects that serve your needs in the client. This will be quite automated once you hook up INotifyPropertyChanged events to notify your views.
I'm studying WCF and WPF. I've learned a lot by doing sample projects, but I'm having trouble putting everything together. It would help if I could paraphrase my understanding of proper WCF/WPF structure and invite others to confirm or correct my ideas. Here is a very broad description of how I expect my next project to work:
My persistent data will be stored in a SQL Server database. I will create a WCF Service Library which serves as an interface to the database, solving security issues and recasting the relational data into an object-oriented entity model. My application will read data through the WCF service into a memory structure which might be customized somewhat for the needs of my application, but will basically consist of one ObservableCollection for each of the entities in my data model. Because the data will be stored in ObservableCollections, I will be able to use event procedures to respond to data changes that trigger business processes. Simple user interface elements will bind directly to the collections. More sophisticated user interface elements, like a TreeView, will require another layer, called a PresentationModel or ViewModel. In the case of a TreeView, the TreeView will bind directly to the PresentationModel, and the PresentationModel will bind directly to the collections.
Have I described everything correctly?
-TC
There is nothing technically wrong in what you wrote.
Things that feel off:
... solving security issues...
Scares me because it implies, to me at least, that you will have no security issues. I would have phrased it as
provideds a centralised system for authentication and authorisation to the data from all interfaces
I would definately make use of the MVVM pattern, allow a ViewModel to expose your collections and properties that your UI binds too, you seem to have a grasp of that pattern from what you have described.
Is WCF really required for your data layer? Have you looked into Entity Framework at all?
Simple user interface elements will bind directly to the collections.
I'd advise slightly against the above. A decent model to follow is the MVVM (Model-View-ViewModel) pattern. It sounds like you've read a bit about this considering your ListViews are going to be contained in a ViewModel. I would also have your raw data models exposed to a ViewModel, and have your View bind to that. So, for your raw data models, use them like you intend to do with ListViews.
Other than that, sounds like you're spot on.