Question

I am trying to get my head around MVVM. For a simple list->data view it’s no problem. But I am struggling to understand how multiple layers work. I sort of have something working but it’s very hit and miss as to which bits work and which bits don’t. For example, some data updates, some doesn’t. Anything in a deeper level which should affect a list at an upper level sometimes updates the list, sometimes doesn’t. There must be a pattern but I have yet to spot it. Does anybody know of any tutorials with more than just a list->data type of view?

Example:

List of widgets
+- Widget name
+- Widget description
+- List of Widget parts
   +- Part ID
   +- Colour

In that example I could have a three-column approach – list of widgets in the left, widget information in the middle including the parts list, and then the part detail on the right.

Answer 1

You should have multiple ViewModels, one for each level.
Then you can provide events to let the upper levels update on change.

For example you can have a

public class WidgetListViewModel 
{
    public ObservableCollection<WidgetViewModel> Widgets {get; set; } 
}

public class WidgetViewModel
{
    public string WidgetName { get; set; }
    public string WidgetDescription { get; set; }
    public ObservableCollection<WidgetPartViewModel> Parts { get; set; }
}

public class WidgetPartViewModel
{
    public int PartId { get; set; }
    public System.Windows.Media.Color Color { get; set; }
}

Having events (including a simple pattern) is described here Events in .Net

Furthermore, I recommend watching this excellent video tutorial on MVVM:
Jason Dollinger on MVVM

The video covers some issues of Unity also! (which could be very valuable for you)

The sourcecode he developes is also available:
Lab49 Sourcecode by Jason Dollinger