How do multiple layers work in the MVVM pattern?

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.

Not enough to go by, also this place is **not** for book/tutorial/etc suggestions or link requests. — H.B., Jul 16 '12 at 17:03
Are you using any frameworks such as Prism, Caliburn Micro, etc? — myermian, Jul 16 '12 at 17:03
This is not a direct answer to your question, but more of a side note, you might want to look at "Event Aggregation" patterns. This is a good way for disconnected parts of an MVVM application to communicate with each other. — CodingGorilla, Jul 16 '12 at 17:06
m-y - I'm not but maybe that's what I need to be looking at. CG - I'll have a look. Thanks, you two - it's given me pointers of what I should be looking at. — GeoffM, Jul 16 '12 at 17:17
@GeoffM: Look at this StackOverflow question (http://stackoverflow.com/questions/1409553/what-framework-for-mvvm-should-i-use) to decide which framework to use... They're all good overall, some offer a more narrow scope of features, while others are more broad. Personally I chose to learn Prism because it is, in my opinion, ideal for modularized/component designs. I do warn you though, Prism has a **very** steep learning curve. — myermian, Jul 16 '12 at 17:27

score 3 · Accepted Answer · edited Jul 16 '12 at 18:09

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

The multiple view models question was nagging in my brain. Time for more reading and watching! Thanks. — GeoffM, Jul 16 '12 at 17:43

How do multiple layers work in the MVVM pattern?

1 Answers1