Question

This is based on the example from the book:
Pro WPF and Silverlight MVVM by Gary McLean Hall

where the author only insists on how to create the Model for a DB structure (and how to implement the DAL for it).

I am looking for the correct way to create the ViewModel(s).

Here is the database model and the MVVM Model– I suspect it is not quite complete, but the Product is missing the ProductCode:

My Views will be: Pages displaying / editing views for Products, Customers and Orders
I am familiar with the Models / ViewModels implementing / using the INotifyPropertyChange and ObservableCollection, no need to insist on that.

My questions:

1. How to create the ViewModels in such a way that they would all share the same model
2. How do I manage the ViewModels? Do I have one Main ViewModel which aggregates all the specific ones? This relates to ViewModel state saving and restoring.

I am particularly interested in how to deal with this: the Model for Order has a List of Products. I will also have to maintain a list of Products for my ProductsViewModel which supports the displaying / editing Views for the Products. How can all be synchronized? Should the OrderModel only have a List of ProductCodes instead? What are the implications in that case?

In general, the thing I am after here is: how to create and manage the ViewModels for Models which implement DB tables with many to many relationships (like Product-Orders). Do we use only the foreign keys as part of the Model objects or do we use a reference to a whole other Model object represented by that foreign key?

Answer 1

To me it sounds like you are thinking about it the wrong way round. When you ask “How to create and manage the ViewModels for the Models which implement DB tables with many to many relationships” it sounds like you are thinking about ViewModels in terms of Models. Which isn’t right. A ViewModel is a model of a View – not the Model. You shouldn’t be thinking about creating ViewModels for your models, you should be thinking about creating VewModels of your Views.

In fact the model doesn’t even come into it until the end. Start with your UI – your View. You then create a logical representation of that View in code so that you can bind to it, your ViewModel. Then finally you implement your ViewModel by accessing your Model to do what ever needs to be done. When you come to design the next View, even though it might contain some of the same data as the first, you would still create a new model for it – another ViewModel. Now the new ViewModel could include some of the same properties at the first ViewModel, which is fine. Remember is is a model of the View not the Model.

Consider an email app, with 2 views, summary and detail. Because there are 2 different Views you have 2 different ViewModels even though the are both pulling data from the same underlying model.

Model                SummaryViewModel       DetailsViewModel
-----                ---------------        ----------------
RecipientAddress                            RecipientAddress
SenderAddress        SenderAddress          SenderAddress
Subject              Subject                Subject
Content                                     Content

Now the Summary View is only a summary and doesn’t display the RecipientAddress or the Content so consequently those properties don’t exist on the SummaryViewModel. The Details View displays more information, so has more properties. This same idea answers your question about foreign keys. Foreign keys are never visible on your View therefore have no business being members of your ViewModel. Your ViewModel cares only about what is required for the View.

In answer to your question “how do you manage the view models”: you don’t. You don’t have to. ViewModel instances usually (not not always) have a one to one relationship with the Views, therefore you don’t need to manage them. They only live as long as the view lives. You just create a new instance of the ViewModel when the View is loaded (typically in OnNavigatedTo event handler, see below) and store it in the View’s DataContext. If two instances of the View are loaded, then there are two ViewModels. When the View is GC’ed, the ViewModel is too (if it isn’t you have a memory leak).

Finally, as for how you should synchronize changes, that can be tricky in a desktop Silverlight app where many views might display concurrently. Fortunately on the Windows Phone we usually only have one view open at a time (although not always). So we can simply tell our ViewModel to refresh each time the page is navigated to:

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
    base.OnNavigatedTo(e);

    if (DataContext == null)
         DataContent = new MyViewModel();  //Create new instance of the ViewModel
    else
         (MyViewModel)DataContext.Refresh();  //Refresh the existing ViewModel
}

This works well in most cases for more advanced scenarios you could look at the messaging infrasructure provided by a toolkit like Prism or MvvmLight.

Hope this has helped.