How can I Import my ViewModels without breaking MVVM?

Question

[Export]
public sealed class MainViewModel : NotificationObject
{
    [Import]
    public ISomeService MyService { get; private set; }

    ...
}

In order to INJECT this class as the DataContext to my View, I have to mark it as Export so MEF creates an instance of it in the Catalog. The problem is that the main window needs to create other windows and pass in orders, I'm not sure how to go about that without breaking the MVVM approach.

I figure that an ICommand will trigger something on my MainViewModel to generate a new ViewModel, but then after that happens I can't really force a new Window (view) to open up from the ViewModel. Plus, I can't even really create a new ViewModel from my MainViewModel because then MEF won't really work, right?

[Export]
public sealed class MainViewModel : NotificationObject
{
    [Import]
    public ISomeService MyService { get; private set; }

    private ObservableCollection<IOrderViewModel> Orders { get; set; }

    public void OpenOrder(int id)
    {
        //Pseudo-code to ensure that duplicate orders are not opened)

        //Else create/open the new order
        var order = new OrderViewModel(id);

        OpenOrders.Add(order);
    }
}

2 problems here:

Since I "newed" the OrderViewModel services are not autoloaded via MEF.
How does this code on my ViewModel layer (appropriate layer) create the necessary view as a NEW WINDOW (child of the main window), and then link this new OrderViewModel as the DataContext?

Steve Greatrex Steve Greatrex · Accepted Answer · 2011-10-10T13:48:54

The way to avoid 'new-ing' the OrderViewModel is to use a factory:

[Export]
public class OrderViewModelFactory
{
    [Import]
    public ISomeDependency ImportedDependency { get; set; }

    public OrderViewModel Create(int id)
    {
        return new OrderViewModel(id, this.ImportedDependency);
    }
}

Then import the factory into your MainViewModel as a dependency and MEF will take care of filling everything in as required.

To get around the problem of instantiating windows, we have created a DialogService that does something like:

[Export]
public class DialogService
{
    public bool? ShowDialog(string regionName, object data = null)
    {
        var window = new Window();
        var presenter = new ContentControl();
        presenter.SetProperty(RegionManager.RegionName, regionName);
        window.Content = presenter;
        if (data != null) window.DataContext = data;
        return window.ShowDialog();
    }
}

How can I Import my ViewModels without breaking MVVM?

2 Answers