I am investigating WPF's MVVM design pattern. But am unsure where to put the Data Acess code?
In some examples I have looked at, data access is performed directly in the ViewModel. It seems odd to put something like linq to sql in the ViewModel? Other examples have a seperate project for Data Access, this seems more like it?
Is this there a general approach? I feel like I am missing something here!
Thanks
Here's how I've been organizing my MVVM w/ LINQ projects:
Model - I think of the Model as the state of the system. It provides an interface to the data, and it keeps track of system status. The Model does not know about the ViewModel or View--it just provides a public interface to its data and various events to let the consumers (usually ViewModels) know when the state has changed.
ViewModel - The ViewModel is in charge of organizing or structuring all the data needed by the View, keeping track of the status of the view (such as the currently selected row of a data grid), and responding to actions on the view (such as button pushes). It knows what the view needs, but it doesn't actually know about the view.
View - The View is the actual look and feel of the UI. It contains all the built-in and custom controls, how they arranged, and how they are styled. It knows about the ViewModel, but only for the purpose of binding to its properties.
Gateway - This is the part that directly addresses your question. The Gateway (which is basically my way of saying "DataAccessLayer") is its own separate layer. It contains all the code (including LINQ queries) to CRUD or select, insert, update, and delete data from/to your data source (database, XML file, etc.). It also provides a public interface to the Model, allowing the Model to focus on maintaining system state without having to concern itself with the details (i.e., the queries) needed to update the data source.
DataAccess Classes - In C#, these are very simple classes that model your elemental data objects. When you select something using a LINQ query, you will usually create an IEnumerable<T> or List<T> where T is one of your data objects. An example of a data object would be:
public class Person
{
public string Name { get; set; }
public int Age { get; set; }
}
The big advantage of a design like this is that it really separates your concerns. Everything has a specialized job, and it's (usually) pretty easy to know what kind of thing goes where.
The disadvantage is that it may be overkill for small projects. You end up creating a lot of infrastructure for public interfaces that basically pass a single wish through several layers. So, you might end up with a scenario like this: [user clicks Submit, ViewModel tells Model to AddNewPerson, Model tells Gateway to InsertPerson] instead of a scenario like this [user clicks Submit, ViewModel adds new record to the database directly].
Hope that helps.
I would add another layer, essentially what you want is a data factory. You want to create a set of classes that will CRUD to the database for you and return clean POCO objects to the ViewModel.
A good example to look at would the Nerd Dinner book. It covers MVC not MVVM but the patterns are very similar and the way they access data in that solution would be good starting point.
Hope this helps.
Data access should not be in the view model, as this is supposed to be a view specific (possibly simplified) representation of the domain model.
Use a mapper of some sort to map your view model (the VM in MVVM) to your model (the first M). New objects in your model can be created using the factory pattern. Once created, you can store them in a database using the repository pattern. The repositories would then represent your data access layer. In your repository you could use an O/R mapper like NHibernate or Entity Framework.
EDIT:
I see that GraemeF suggests putting the data access code in the model. This is a NOT a good approach, as this would force you to update your domain model if you were to move from e.g. SQL Server to Oracle or XML files. The domain objects should not have to worry about how they are persisted. The repository pattern isolates the domain from its persistence.
MVVM stands for Model, View, and ViewModel. The piece you are missing is the Model, which is where your data access code lives.
The ViewModel takes the Model and presents it to the View for display, so typically you would have something like this:
class PersonModel : IPerson
{
// data access stuff goes in here
public string Name { get; set; }
}
class PersonViewModel
{
IPerson _person;
public PersonViewModel(IPerson person)
{
_person = person;
}
public Name
{
get { return _person.Name; }
set { _person.Name = value; }
}
}
The PersonView would then bind to the properties of the PersonViewModel rather than directly to the model itself. In many cases you might already have a data access layer that knows nothing about MVVM (and nor should it) but you can still build ViewModels to present it to the view.
Your ViewModel should be a thin layer that just services the view. My rule of thumb: if it has to do with the presentation of the UI, then it belongs in the ViewModel, otherwise it should be in the Model.
The WPF Application Framework (WAF) contains a sample application that shows how the Model-View-ViewModel (MVVM) pattern might be used in combination with the Entity Framework.
Related
So far I have yet to see the value of having models in WPF. All my ViewModels, by convention, have an associated Model. Each of these Models is a virtual clone of the their respective ViewModel. Both the ViewModel and Model classes implement INotifyPropertyChanged and the ViewModel just delegates everything to the Model anyway.
Why bother having Models then? Why can't I just move my Model logic up into the ViewModel and call it a day?
It seems rather redundant (that is, not DRY) to have MVVM, and just use VVM by default unless some special edge case demands a Model.
If using explicit model classes better supports unit testing, for example, or some other best practice, I can see the value.
The model is just the low level application data.
The view model is more specific.
It's like a window tapping into the data tailored for the view.
It also augments the model with details needed for the view. A good example is pagination.
A model can have more than one view model. These view models would offer different aspects of the data.
You can create a mashup of different data sources. A view model cleanly façades the models involved.
That means there is no 1:1 relationship between models and view models.
So, if you just display the low level data without a lot of additional logic, features, summaries, aggregations, filters, etc. you can do away with view models and just use the model directly. This seems to be the case with your project.
The model can be auto-generated (Entity Framework), or it might not be a specific class at all (think of a DataTable).
I assume that if you say "I don't use a model", you do in fact use a model, you just don't call it that way.
First of all, even in MVVM you can expose your Model directly in the VM and bind through it to the Model. {Binding MyModel.MyModelsProperty} where DataContext = ViewModel that way you don't have to necessarily wrap everything unless that is just your style to do so.
The ViewModel and Model have different responsibilities. For example, consider designing a file explorer with a tree view of folders. Each node in the tree view is a Directory/Folder. The directories are the models and they have properties related to the file system. The ViewModel may wrap or expose these properties for the TreeView nodes to display (For example the Name of the directory), but it also adds additional information such as "IsEditing" and "IsExpanded" to determine the state that the node is in.
You're just talking about one pattern in MVVM, actually there are more.
In Stateful viewmodel you don't actually delegate things to Models, Viewmodel itself maintains its state, so as you said in this pattern you can almost ignore the model as you can have state in VM itself.
To create isolation between business logic and presentation, data
should be removed from the view. The stateful view model pattern moves
data into the view model using XAML data binding. This allows the view
model to be tested without constructing a view, and it allows the view
to change with minimal impact on the business logic.
In Stateless viewmodel you delegate the calls to Model, which is what you're probably referring to.
Note that you don't necessarily implement INotifyPropertyChanged in Model. Just implementing it in VM is fine as long as you don't change the properties directly in Model.
So why do we need VM and Model? VM is for supporting view like providing Commands, etc and Model is just to hold the piece of data abstract the same.
If you think of it as an abstraction, let's say you need to build a screen to display a list of Employees and make a Selection, Search or Filter. Then we break it up in components. You will require:
An Employee class (Model)
An EmployeeManagementViewModel to prepare and present your list of Employees and manage state changes for your View (e.g. can contain a SelectedEmployee, Filter Search text, etc) to be used by your EmployeeManagementView
A list of Employees (Which will live in your EmployeeManagementViewModel)
Most likely you will already have an Employee class. If that's the case then you just need to expose that model in your EmployeeManagementViewModel as an ObservableCollection of Employees.
In case you don't already have an Employee class you may decide to create an EmployeeViewModel and add your Employee properties there like FirstName, LastName, etc.
Technically this will work but conceptually it bothers me because an EmployeeViewModel is not an Employee (it contains an employee). If you're abstracting reality then, the blueprint of an Employee should not include properties or methods to be used by a View. To me Employee should be a POCO which could implement INotifyPropertyChanged and nothing more than that. You're separating View state from the Model itself. Having an Employee POCO makes it easier to UnitTest, create mock employees, map it to a database table through an ORM, etc.
As the name implies the ViewModel is the model for your View and the Model is the model for your business domain
Anyway that's how I see it. When I started doing MVVM work I had that same question but over the years seems like it makes sense.
In general, my Models end up being a Data Access Layer, be it through Entity Framework, a WCF proxy, or some other class.
Depending on concurrency issues, the class could be static or instanced. If you can separate the behavior enough, you could "split" the DAL classes into a separate model for each view model, but duplicate code could become a problem.
Wikipedia: MVVM
Model: as in the classic MVC pattern, the model refers to either (a) a domain model which represents the real state content (an object-oriented approach), or (b) the data access layer that represents that content (a data-centric approach).
Your ViewModel isn't a substitute for your domain Model. It acts as an intermidiate between your view and the model, including commands, binding collections to your view, validation, etc.
Your domain model should also be reusable across ViewModels. If you implement it all in the VM you will end up abusing the DRY principle by sharing specific VM logic across multiple VMs.
In my experience when you just use Domain models as the ViewModel (as a property of the VM) you end up with a lot of Keys that require you to either go get the text value and store somewhere else or you end up adding the property to the VM anyway. Your view typically has more info than just one single domain model (e.g. related objects, display values, text status values etc..), something that the Domain model has no need for and would ultimately weigh it down. The view model is dedicated to the needs of the view, it keeps coding the View simple and non-complex.
I'm creating an application that enables a user to insert, update and delete data that has been entered and then shown in a data-grid (CRUD operations).
In my View Model, it contains properties which are bound to the xaml (Firstname for example). It also contains a navigation property as well as validation attributes.
[Required(ErrorMessage = "First Name is a required field")]
[RegularExpression(#"^[a-zA-Z''-'\s]{1,20}$", ErrorMessage = "First Name must contain no more then 20 characters and contain no digits.")]
public string FirstName
{
get { return _FirstName; }
set
{
if (_FirstName == value)
return;
_FirstName = value;
OnPropertyChanged("FirstName");
}
}
Furthermore, it contains commands for the xaml to execute, which creates an instance of the CRUD operation;
private void UpdateFormExecute()
{
var org = new OrganisationTypeDetail();
UpdateOrganisationTypeDetail(org);
}
And lastly, it contains the CRUD operations as well. Such as the Insert, Update and Delete.
Which leads me to my question. if I want to implement the correct MVVM way, is all this code too much for the view model to contain?
Should I use the model and create a collection within my View-model and bound that to my xaml? Would this be the correct way of doing it?
Should I use a Repository system for the CRUD operations? If so, how would I pass the data from the text fields through to the model to get updated?
Im new to WPF, MVVM and finding it hard to adapt without proper guidance.
I would say that this is a correct way to implement MVVM, but not the correct way to implement MVVM.
What I mean by this is that there is no one correct way to implement this pattern. if you have created a ViewModel that can be bound to your View, without having any extra logic within your View (i.e. code-behind) then you have captured the essence of MVVM.
Whether or not you add more patterns and structure to your code is entirely up to you. If this is a simple application, I would keep the patterns light. Go ahead and have your ViewModel talk directly with a repository. You current code looks just fine to me in that respect.
If this is a large application, you might want to add further layers, like a service layer, data access layer. You might want to think about dependency injection.
But don't just adopt a pattern, or add an extra layer just because you think you should. Dependency Injection sounds cool, but in many cases it is more hassle than it is worth!
For me it’s not the correct way, I think defining the properties like FirstName in view model is not good idea. view should contain model only and your view model should be wrap the model which should be bounded to XAML(if required).
Also model object creation should be completely independent of view model. View model should know only about unit operations on models and validations should be inside the model e.g in your case FirstName validations are in ViewModel means you are only limiting GUI to validate the FirstName property, but what if someone set it from other place.
Whats the difference between Model and ViewModel? I should use both of them or I better skip one of them? who grabs the data from the database?
I wonder whats the best/right way to take my data from the database.
One option is to have a DAL (Data Access Layer) and instantiate it in every controller,
fill with it the viewmodels like:
var viewmodel = Dal.GetArticles();
Another option is to let the model itself grab the information from the Database:
var model = new ArticlesModel();
model.GetArticles();
public void GetArticles()
{
var context = new DbContext();
_articles = context.Articles
}
Another similar option is to have a static DAL so you can access it inside every model,
so each model will have a method to grab the data using the static DAL class (Which contain a DbContext class inside to access the Database)
public void GetArticles()
{
_articles = DAL.GetArticles();
}
So the general question is if the model itself needs to grab the data from the database or the controller itself can have access to the DAL.
While someone is writing a more useful answer, I will quickly address your points.
Model is the data you want to display.
More often than not, you will want to use object relational mapping so most of your business object classes correspond to database tables and you don't have to manually construct queries.
There are plenty of ORM solutions available, including Entity Framework, NHibernate and (now dying) LINQ to SQL.
There is also an awesome micro-ORM called Dapper which you may like if bigger frameworks feel unneccessarily bloated for your solution.
Make sure you learn about the differences between them.
DAL is more idiomatic in .NET than classes that “know” how to load themselves.
(Although in practice your solution will very likely be a mixture of both approaches—the key is, as usual, to keep the balance.)
My advice is to try keeping your models plain old CLR objects as long as your ORM allows it and as long as this doesn't add extra level of complexity to the calling code.
These objects, whenever possible (and sensible—there are exceptions for any rule!), should not be tied to a particular database or ORM implementation.
Migrating your code to another ORM, if needed, will be just a matter of rewriting data access layer.
You should understand, however, that this is not the main reason to separate DAL.
It is highly unlikely you'll want to change an ORM in the middle of the project, unless your initial choice was really unfit for the purpose or you suddenly gained a traction of 100,000 of users and your ORM can't handle it. Optimizing for this in the beginning is downright stupid because it distracts you from creating a great product capable of attracting even a fraction of hits you're optimizing for. (Disclaimer: I've walked this path before.)
Rather, the benefit of DAL is that you database access becomes always explicit and constrained to certain places where you want it to happen. For example, a view that received a model object to display will not be tempted to load something from the database, because in fact it is the job of controller to do so.
It's also generally good to separate things like business logic, presentation logic and database logic. Too often it results in better, less bug-ridden code. Also: you are likely to find it difficult to unit-test any code that relies on objects being loaded from the database. On the other hand, creating a “fake” in-memory data access layer is trivial with LINQ.
Please keep in mind that again, there are exceptions to this rule, like lazy properties generated by many ORMs that will load the associated objects on the go—even if called within a view. So what matters is you should make an informed decision when to allow data access and why. Syntaxic sugar might be useful but if your team has no idea about performance implications of loading 20,000 objects from ORM, it will become a problem.
Before using any ORM, learn how it works under the hood.
Choosing between Active Record-style objects and a DAL is mostly a matter of taste, common idioms in .NET, team habits and the possibility that DAL might eventually have to get replaced.
Finally, ViewModels are a different kind of beast.
Try to think of them like this:
You shouldn't have any logic in views that is more sophisticated than an if-then-else.
However, there often is some sophisticated logic in showing things.
Think pagination, sorting, combining different models in one view, understanding UI state.
These are the kinds of thing a view model could handle.
In simple cases, it just combines several different models into one “view-model”:
class AddOrderViewModel {
// So the user knows what is already ordered
public IEnumerable<Order> PreviousOrders { get; set; }
// Object being added (keeping a reference in case of validation errors)
public Order CurrentOrder { get; set; }
}
Models are just data, controllers combine the data and introduce some logic to describe data to be shown in view models, and views just render view models.
View model also serves as a kind of documentation. They answer two questions:
What data can I use in a view?
What data should I prepare in controller?
Instead of passing objects into ViewData and remembering their names and types, use generic views and put stuff in ViewModel's properties, which are statically typed and available with IntelliSense.
Also, you'll likely find it useful to create ViewModel hierarchies (but don't take it to extremes!). For example, if your site-wide navigation changes from breadcrumbs to something else, it's cool to just replace a property on base view model, a partial view to display it and the logic to construct it in the base controller. Keep it sensible.
A model represents the structure you like your data in and is not concerned about the view which may consume it. A model's intend is purely that of representing the structure.
A model may contain properties irrelevant to the view consuming it.
A view-model is designed with the view in mind. A view-model is intended for a 1-to-1 relationship to a view. A view-model only contains the basic fields and properties the view it is intended for requires.
In general you would have your controller contact a repository (In your example your DAL) obtaining the data and then populating either a model or view-model with the results, sending it down to the view.
Model (Domain Model): is the heart of the application, representing the biggest and most important business asset because it captures all the complex business entities, their relationships and their functionality.
ViewModel: Sitting atop the Model is the ViewModel:The two primary goals of the ViewModel are
1. to make the Model easily consumable by the View and
2. to separate and encapsulate the Model from the View.
Eg.
Model:
public class Product
{
...............
}
public class Category
{
...........
}
ViewModel:
public class ProductViewModel
{
public ProductViewModel(List<Product> products, List<Category> categories)
{
this.Products = products;
this.Categories = categories;
}
public List<Product> Products { get; set; }
public List<Category> Categories { get; set; }
}
I am Learning ASP.NET MVC and downloaded a couple of sample apps. MusicStore etc...
I am coming from a wpf background where we had the MVVM Pattern.
I have noticed that they used the concept of model and ViewModel.
In MVVM is pretty clear that you bind the view to the ViewModel injecting the model into the viewModel.
In MVC you have a controller but I am not sure and confused how the all ties together,as I cannot see the model injected into the ViewModel
I have the following structure
MyCompany.Entities.dll (All the models go here) EG Product
MyCompany.Dal.dll (All the repositories go here)
MyCompany.Services.dll (called by MyCompany.WebUI.Controller calls MyCompany.Dal)
MyCompany.WebUI.MyApp
MyCompany.Tests
From some of the examples I have seen your Model acts as a ViewModel.Am I correct?
Let's take a controller i have something like
public class ProductController
{
public ProductController(IProductRepository productRepository)
{
//omitted as not relevant
}
}
public class ProductVM
{
public ProductVM()
{
// Shouldn't we inject the model here RG Product
}
}
Is there some N-tier examples out there I can refer to?
Is the concept of ViewModel a valid one in MVC?
What is the standard?
Thanks for any suggestions.
Use ViewModels to simplify the View.
For instance, you might have a deep object graph with Products, Order, Customers, etc - and some information from each of these objects are required on a particular View.
A ViewModel provides a way to aggregate the information required for a View into a single object.
ViewModels also allow for things like data annotations and validation - which does not belong on your model, as your model should stay "domain-specific".
But in reality, ViewModels are nothing more than a simple wrapper for your domain objects.
Use a tool like AutoMapper to map back and forth between your ViewModels and domain models with ease.
Personally I always bind to ViewModels in my Views, never to the domain models, even if it's a single object. Why? Well I like to decorate my ViewModels with UIHints, validation, data annotations. Just the same way your domain models are enriched with domain-specific rules and business logic, so should your ViewModels be enriched with UI-specific logic.
If you simply have a object with a 1-1 representation of your domain model, you are missing the point of ViewModels.
Add to the ViewModels only, and nothing more, what is required for a particular View.
Example controller action
public ActionResult CustomerInfo(int customerId)
{
// Fetch the customer from the Repository.
var customer = _repository.FindById(customerId);
// Map domain to ViewModel.
var model = Mapper.Map<Customer,CustomerViewModel>(customer);
// Return strongly-typed view.
return View(model);
}
The difference between MVC and MVVM is that MVC has one set of classes for the data entities. In MVVM you have 2 - one set for binding to your views, and one set for managing the data persistence (which could be in a separate WCF service for example).
The benefits of MVVM are that the model bound to the views is relevant to the UI and completely independant from the persistence Model.
Which to use? Well it depends on how closely the data structure required by your Views maps to the structure of the database. When it is similar - it is possible to bind the DataEntities from your DAL directly to your view - this is the classic MVC pattern. However, you gain much with a separate ViewModel as you can extend these classes with View specific behaviour (e.g. Validation) that your DAL should not be concerned with.
For all but the most simple applications, I would recommend use of a separate ViewModel.
Extremely simple,
ViewModel can be the combination of multiple Model classes, to display or transforming the data to and from within the Network, may be via API calls.
However, Model class is nothing but just be specific to any particular entity, Object or strict to a particular Table.
Ex- For "EmployeeViewModel" can be the combinations of Multiple Model Classes i.e. EmpBasicDetails + EmpAddressDetails +EmpExperienceSummary.
Whereas, "EmpBasicDetails" - is specific Model class which contains the Employee Details specifically.
~Cheers,
I've recently started develping an application using WPF and I'cant really wrap my mind around following thing:
I have a Domain Model of my application which is simple POCO objects serialized to/from harddisk. Then I have the WPF application and I'd like to bind it to various parts of the model. I need to be able to notify UI of changes to underlying model (eg. implement INotifyPropertyChanged) BUT I want to do that without interfeering with my model (read without modifying current implementation of the model). How can I implement changes notification other than modifying the model?
Reason why I want to do that is that I share the model among multiple projects, only one being WPF, and I don't want to add unncecessary code to the model.
One thing that came to my mind was to create "copy" of the model (with INotifyPropertyChanges and BindingLists etc.) but that seems to be hard to maintainn...
Thanks in advance.
Ondrej
Check this out MVVM
Download its source code see the hierarchy.
Basically you still keep simple POCO objects as your models. Then you create a ViewModel around the model like this:
public class CustomerViewModel : INotifyPropertyChanged
{
readonly Customer _customer;
public CustomerViewModel(Customer customer)
{
_customer = customer;
}
public string FirstName
{
get { return _customer.FirstName; }
set
{
if (value == _customer.FirstName)
return;
_customer.FirstName = value;
OnPropertyChanged("FirstName");
}
}
...
}
I see two possible solutions here:
Use a separate model for WPF screens only (MVVM pattern). This will require maintaining two different models, also be prepare for lots of mapping code.
Use PostSharp to "enhance" your model with all necessary boilerplate code. Here you can find example of automatic INotifyPropertyChanged implementation. Remember that introducing PostSharp to the project is an important decision, so I suggest getting familiar with it first.