In my opinion, if you are using WPF or Silverlight you should be using the MVVM design pattern. It is perfectly suited to the technology and allows you to keep your code clean and easy to maintain.
The problem is, there are a lot of online resources for MVVM, each with their own way of implementing the design pattern and it can be overwhelming. I would like to present MVVM in the simplest way possible using just the basics.
So lets start at the beginning.
MVVM
MVVM is short for Model-View-ViewModel.
Models are simple class objects that hold data. They should only contain properties and property validation. They are not responsible for getting data, saving data, click events, complex calculations, business rules, or any of that stuff.
Views are the UI used to display data. In most cases, they can be DataTemplates which is simply a template that tells the application how to display a class. It is OK to put code behind your view IF that code is related to the View only, such as setting focus or running animations.
ViewModels are where the magic happens. This is where the majority of your code-behind goes: data access, click events, complex calculations, business rules validation, etc. They are typically built to reflect a View. For example, if a View contains a ListBox of objects, a Selected object, and a Save button, the ViewModel will have an ObservableCollection ObectList, Model SelectedObject, and ICommand SaveCommand.
MVVM Example
I’ve put together a small sample showing these 3 layers and how they relate to each other. You’ll notice that other than property/method names, none of the objects need to know anything about the others. Once the interfaces have been designed, each layer can be built completely independent of the others.
Sample Model
For this example I’ve used a Product Model. You’ll notice that the only thing this class contains is properties and change notification code.
Usually I would also implement IDataErrorInfo here for property validation, however I have left this out for now.
public class ProductModel : ObservableObject
{
#region Fields
private int _productId;
private string _productName;
private decimal _unitPrice;
#endregion // Fields
#region Properties
public int ProductId
{
get { return _productId; }
set
{
if (value != _productId)
{
_productId = value;
OnPropertyChanged("ProductId");
}
}
}
public string ProductName
{
get { return _productName; }
set
{
if (value != _productName)
{
_productName = value;
OnPropertyChanged("ProductName");
}
}
}
public decimal UnitPrice
{
get { return _unitPrice; }
set
{
if (value != _unitPrice)
{
_unitPrice = value;
OnPropertyChanged("UnitPrice");
}
}
}
#endregion // Properties
}
The class inherits from ObservableObject, which is a custom class I use to avoid having to rewrite the property change notification code repeatedly. I would actually recommend looking into Microsoft PRISM’s NotificationObject or MVVM Light’s ViewModelBase which does the same thing once you are comfortable with MVVM, but for now I wanted to keep 3rd party libraries out of this and to show the code.
public abstract class ObservableObject : INotifyPropertyChanged
{
#region INotifyPropertyChanged Members
/// <summary>
/// Raised when a property on this object has a new value.
/// </summary>
public event PropertyChangedEventHandler PropertyChanged;
/// <summary>
/// Raises this object‘s PropertyChanged event.
/// </summary>
/// <param name="propertyName">The property that has a new value.</param>
protected virtual void OnPropertyChanged(string propertyName)
{
this.VerifyPropertyName(propertyName);
if (this.PropertyChanged != null)
{
var e = new PropertyChangedEventArgs(propertyName);
this.PropertyChanged(this, e);
}
}
#endregion // INotifyPropertyChanged Members
#region Debugging Aides
/// <summary>
/// Warns the developer if this object does not have
/// a public property with the specified name. This
/// method does not exist in a Release build.
/// </summary>
[Conditional("DEBUG")]
[DebuggerStepThrough]
public virtual void VerifyPropertyName(string propertyName)
{
// Verify that the property name matches a real,
// public, instance property on this object.
if (TypeDescriptor.GetProperties(this)[propertyName] == null)
{
string msg = "Invalid property name: " + propertyName;
if (this.ThrowOnInvalidPropertyName)
throw new Exception(msg);
else
Debug.Fail(msg);
}
}
/// <summary>
/// Returns whether an exception is thrown, or if a Debug.Fail() is used
/// when an invalid property name is passed to the VerifyPropertyName method.
/// The default value is false, but subclasses used by unit tests might
/// override this property‘s getter to return true.
/// </summary>
protected virtual bool ThrowOnInvalidPropertyName { get; private set; }
#endregion // Debugging Aides
}
In addition to the INotifyPropertyChanged methods, there is also a debug method to validate the PropertyName. This is because the PropertyChange notification gets passed in as a String, and I have caught myself forgetting to change this string when I change the name of a Property.
Note: The PropertyChanged notification exists to alert the View that a value has changed so it knows to update. I have seen suggestions to drop it from the Model and to expose the Model’s properties to the View from the ViewModel instead of the Model, however I find in most cases this complicates things and requires extra coding. Exposing the Model to the View via the ViewModel is much simpler, although either method is valid.
Sample ViewModel
I am doing the ViewModel next because I need it before I can create the View. This should contain everything the User would need to interact with the page. Right now it contains 4 properties: a ProductModel, a GetProduct command, a SaveProduct command, an a ProductId used for looking up a product.
public class ProductViewModel : ObservableObject
{
#region Fields
private int _productId;
private ProductModel _currentProduct;
private ICommand _getProductCommand;
private ICommand _saveProductCommand;
#endregion
#region Public Properties/Commands
public ProductModel CurrentProduct
{
get { return _currentProduct; }
set
{
if (value != _currentProduct)
{
_currentProduct = value;
OnPropertyChanged("CurrentProduct");
}
}
}
public ICommand SaveProductCommand
{
get
{
if (_saveProductCommand == null)
{
_saveProductCommand = new RelayCommand(
param => SaveProduct(),
param => (CurrentProduct != null)
);
}
return _saveProductCommand;
}
}
public ICommand GetProductCommand
{
get
{
if (_getProductCommand == null)
{
_getProductCommand = new RelayCommand(
param => GetProduct(),
param => ProductId > 0
);
}
return _getProductCommand;
}
}
public int ProductId
{
get { return _productId; }
set
{
if (value != _productId)
{
_productId = value;
OnPropertyChanged("ProductId");
}
}
}
#endregion
#region Private Helpers
private void GetProduct()
{
// You should get the product from the database
// but for now we‘ll just return a new object
ProductModel p = new ProductModel();
p.ProductId = ProductId;
p.ProductName = "Test Product";
p.UnitPrice = 10.00;
CurrentProduct = p;
}
private void SaveProduct()
{
// You would implement your Product save here
}
#endregion
}
There is another new class here: the RelayCommand. This is essential for MVVM to work. It is a command that is meant to be executed by other classes to run code in this class by invoking delegates. Once again, I’d recommend checking out the MVVM Light Toolkit’s version of this command when you are more comfortable with MVVM, but I wanted to keep this simple so have included this code here.
/// <summary>
/// A command whose sole purpose is to relay its functionality to other
/// objects by invoking delegates. The default return value for the
/// CanExecute method is ‘true‘.
/// </summary>
public class RelayCommand : ICommand
{
#region Fields
readonly Action<object> _execute;
readonly Predicate<object> _canExecute;
#endregion // Fields
#region Constructors
/// <summary>
/// Creates a new command that can always execute.
/// </summary>
/// <param name="execute">The execution logic.</param>
public RelayCommand(Action<object> execute)
: this(execute, null)
{
}
/// <summary>
/// Creates a new command.
/// </summary>
/// <param name="execute">The execution logic.</param>
/// <param name="canExecute">The execution status logic.</param>
public RelayCommand(Action<object> execute, Predicate<object> canExecute)
{
if (execute == null)
throw new ArgumentNullException("execute");
_execute = execute;
_canExecute = canExecute;
}
#endregion // Constructors
#region ICommand Members
[DebuggerStepThrough]
public bool CanExecute(object parameters)
{
return _canExecute == null ? true : _canExecute(parameters);
}
public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}
public void Execute(object parameters)
{
_execute(parameters);
}
#endregion // ICommand Members
}
Sample View
And now the Views. These are DataTemplates which define how a class should be displayed to the User. There are many ways to add these templates to your application, but the simplest way is to just add them to the startup window’s Resources.
<Window.Resources>
<DataTemplate DataType="{x:Type local:ProductModel}">
<Border BorderBrush="Black" BorderThickness="1" Padding="20">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<TextBlock Grid.Column="0" Grid.Row="0" Text="ID" VerticalAlignment="Center" />
<TextBox Grid.Row="0" Grid.Column="1" Text="{Binding ProductId}" />
<TextBlock Grid.Column="0" Grid.Row="1" Text="Name" VerticalAlignment="Center" />
<TextBox Grid.Row="1" Grid.Column="1" Text="{Binding ProductName}" />
<TextBlock Grid.Column="0" Grid.Row="2" Text="Unit Price" VerticalAlignment="Center" />
<TextBox Grid.Row="2" Grid.Column="1" Text="{Binding UnitPrice}" />
</Grid>
</Border>
</DataTemplate>
<DataTemplate DataType="{x:Type local:ProductViewModel}">
<DockPanel Margin="20">
<DockPanel DockPanel.Dock="Top">
<TextBlock Margin="10,2" DockPanel.Dock="Left" Text="Enter Product Id" VerticalAlignment="Center" />
<TextBox Margin="10,2" Width="50" VerticalAlignment="Center" Text="{Binding Path=ProductId, UpdateSourceTrigger=PropertyChanged}" />
<Button Content="Save Product" DockPanel.Dock="Right" Margin="10,2" VerticalAlignment="Center"
Command="{Binding Path=SaveProductCommand}" Width="100" />
<Button Content="Get Product" DockPanel.Dock="Right" Margin="10,2" VerticalAlignment="Center"
Command="{Binding Path=GetProductCommand}" IsDefault="True" Width="100" />
</DockPanel>
<ContentControl Margin="20,10" Content="{Binding Path=CurrentProduct}" />
</DockPanel>
</DataTemplate>
</Window.Resources>
The View defines two DataTemplates: one for the ProductModel, and one for the ProductViewModel. You’ll need to add a namespace reference to the Window definition pointing to your Views/ViewModels so you can define the DataTypes. Each DataTemplate only binds to properties belonging to the class it is made for.
In the ViewModel template, there is a ContentControl that is bound to ProductViewModel.CurrentProduct. When this control tries to display the CurrentProduct, it will use the ProductModel DataTemplate.
Starting the Sample
And finally, to start the application add the following on startup:
MainWindow app = new MainWindow(); ProductViewModel viewModel = new ProductViewModel(); app.DataContext = viewModel; app.Show();
This is found in the code behind the startup file – usually App.xaml.cs.
This creates your Window (the one with the DataTemplates defined in Window.Resources), creates a ViewModel, and it sets the Window’s DataContext to the ViewModel.
And there you have it. A basic look at MVVM.
UPDATE
Sample code can be found here.
Notes
There are many other ways to do the things shown here, but I wanted to give you a good starting point before you start diving into the confusing world of MVVM.
The important thing to remember about using MVVM is your Forms, Pages, Buttons, TextBoxes, etc (the Views) are NOT your application. Your ViewModels are. The Views are merely a user-friendly way to interact with your ViewModels.
So if you want to change pages, you should not be changing pages in the View, but instead you should be setting something like the AppViewModel.CurrentPage = YourPageViewModel. If you want to run a Save method, you don’t put that behind a button’s Click event, but rather bind the Button.Command to a ViewModel’s ICommand property.
I started with Josh Smith’s article on MVVM, which was a good read but for a beginner like me, some of these concepts flew right over my head.
I’ve never done a blog or tutorial before, but I noticed there is a lot of confusion about what MVVM is and how to use it. Since I struggled through the maze of material online to figure out what MVVM is and how its used, I thought I’d try and write a simpler explanation. I hope this clarifies things a bit and doesn’t make it worse :)
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