Binding to properties of ObservableCollection-items - wpf

Question

interface ICar
{
UserControl SmallView{ get; }
UserControl CompleteView{ get; }
}
class ViewModel
{
ObservableCollection<ICar> Cars{ get; set;}
ObservableCollection<UserControl> SmallViews{ get; }
ObservableCollection<UserControl> CompleteViews{ get; }
}
XAML
<ItemControl ItemsSource="{Binding SmallViews}"/>
<ItemControl ItemsSource="{Binding CompleteView}"/>
I am adding ICars instances to ViewModel.Cars collection. When that happens I want the two UserControls (small and Complete) to be added in the View (XAML).
-I can get it to work as I want, by setting the ItemsSources in CodeBehind when Cars.CollectionChanged is Raised. But I fear all the collection is redrawn for all items in ItemsSource.. I only want the changes to be added, and I would like en elegant solution without a lot of CodeBehind.
This Codebehind makes it work as intended - but I would like something cleaner somthing with real Binding.
CompleteControls and SmallControls are the names for the ItemControls above, which in this solution has no binding markup :-( .
public CarsView(ViewModel viewModel)
{
InitializeComponent();
DataContext = viewModel;
UpdateViews(viewModel.Cars);
viewModel.Cars.CollectionChanged += (caller, args) =>
UpdateViews(args.NewItems.Cast<ICar>());
}
private void UpdateViews(IEnumerable<ICar> newCars)
{
foreach (var car in newCars)
{
CompleteControls.Items.Add(car.CompleteView);
SmallControls.Items.Add(car.SmallView);
}
}

Answer 1

I see a couple of flaws in your concepts.
class ViewModel
{
ObservableCollection<ICar> Cars{ get; set;}
// This is wrong for MVVM. You don't need this.
//
// ObservableCollection<UserControl> SmallViews{ get; }
// ObservableCollection<UserControl> CompleteViews{ get; }
}
It will also get rid of this.
UpdateViews(viewModel.Cars);
viewModel.Cars.CollectionChanged += (caller, args) =>
UpdateViews(args.NewItems.Cast<ICar>());
About the ObvservableCollection you need to know 2 things.
If you create it directly from another collection it will not trigger the event.
Your args.NewItems.Cast() will always produce one item only in a collection since the ObservableCollection does not have a AddRange() method.
I see that you came from Winforms. Start by inspecting more about MVVM. it will pay off very fast. You need to remember that if you are doing anything in the code behind with controls, stop doing it, cause your doing it wrong.
You will instantiate the UserControls in XAML.
<Listbox ItemsSource={Binding Cars}>
<Listbox.ItemsTemplate>
<DataTemplate>
<StackPanel>
<my:SmallViews />
<my:CompleteViews />
</StackPanel>
</DataTemplate>
</Listbox.ItemsTemplate>
</Listbox>
You should really avoid ever dealing with any UI elements in code behind