ViewModel Discovery

ViewModelDiscovery-MEF project contains a sample to decouple discovery of ViewModel using MEF and MVVMLight.

I'm using it for another project and I wanted to share it (no fireworks here, just a really simple sample).

App.xaml.cs

In this file, you'll find two things:

• A static property to return the ViewModelLocator instance

public static ViewModelLocator Locator
{
  get
	{
		return (ViewModelLocator)Current.Resources["Locator"];
	}
}

• A static variable that will contain an AggregateCatalog

public static readonly AggregateCatalog Catalog = new AggregateCatalog(new AssemblyCatalog(typeof(App).Assembly));

ExportViewModelAttribute.cs

This file contains:

• An ExportAttribute that receives a ViewModelBase type in the constructor (The ViewModelType property will be used within the indexer of the ViewModelContainer).

[MetadataAttribute]
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false)]
public sealed class ExportViewModelAttribute : ExportAttribute
{
	public Type ViewModelType { get; private set; }
	public ExportViewModelAttribute(Type viewModelType)
		: base(typeof(ViewModelBase))
	{
		ViewModelType = viewModelType;
	}
}

• An interface (IViewModelMetadata) used as metadata in the ViewModels Lazy Collection property (property defined in the ViewModelContainer)

public interface IViewModelMetadata
{
	Type ViewModelType { get; }
}

ViewModelLocator.cs

The important thing in this file is the indexer, used to provide a syntax for XAML:

//e.g.: DataContext="{Binding Source={StaticResource Locator}, Path=[MainViewModel]}"
public ViewModelBase this[string viewModel]
{
	get
	{
		var vm = _vmContainer[viewModel];

		if (!_viewModels.Contains(vm))
			_viewModels.Add(vm);

		return vm;
	}
}

The Locate method availabe to get an instance of a specific ViewModel type in our code (in a method, in code-behind,...)

public T Locate<T>() where T : ViewModelBase
{
	var vm = _vmContainer.Locate<T>();

	if (!_viewModels.Contains(vm))
		_viewModels.Add(vm);

	return vm;
}
}

And in the ViewModelContainer, there is a Collection that will contain all exported ViewModel:

[ImportMany(typeof(ViewModelBase), AllowRecomposition = true)]
public IEnumerable<Lazy<ViewModelBase, IViewModelMetadata>> ViewModels { get; set; }

CustomerViewModel.cs

In the CustomerViewModel class, I defined a Name property to display a string in my XAML page.

The Export Attribute used by MEF and the ExportViewModel Attribute used by MEF that contains the ViewModelType property.

The sample

For the sample, you'll find two way to use the ViewModelLocator:

• In the XAML (to bind the DataContext):

<Window x:Class="ViewModelDiscovery_MEF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" DataContext="{Binding Source={StaticResource Locator}, Path=[CustomerViewModel]}">
    <Grid>
        <TextBlock Text="{Binding Name}"></TextBlock>
    </Grid>
</Window>

• And in code (to get an instance of our ViewModel):

public partial class MainWindow : Window
{
	public MainWindow()
	{
		InitializeComponent();

		var vm = App.Locator.Locate<CustomerViewModel>();

		vm.Name = "Hello";
	}
}