Question

I have encountered a problem while using the MVVM model in WPF. Here are my objects:

Views

• MainWindow(View)
  (DataContext for this view binds to MainWindowViewModel)

• ItemViewWindow
  (DataContext for this binds an ItemListViewModel)

View-Models

• MainWindowViewModel
  (contains:
  ObservableCollection Items)

• ItemListViewModel
  (contains:
  ObservableCollection Items,
  ItemViewModel SelectedItem,
  Other properties/commands)

ItemViewWindow is accessed by a menu button on the MainWindow. I want to be able to pass just the collection from the MainWindow to the ItemView (note: I can’t set it as the DataContext because it needs an ItemListViewModel). Then any changes made there should be reflected back in the MainWindow.

I know one way is to have the ItemListViewModel as a property of MainWindowViewModel instead of the Collection, then I can just pass that as the DataContext, but it seems wrong to have the extra functionality of the ItemListViewModel in the MainWindowViewModel. Is there a more elegant way of doing this without using the UI code-behind?

Answer 1

Why not either:

a: Make ItemListViewModel expose a property on which you assign the collection e.g.

class ItemListViewModel 
{
    public SomeCollectionType TheCollection { get; set; } // most likely you have INotifyPropertyChanged etc so probably more to it than get/set
}

Then just set it when you instantiate the ItemVM in the MainWindowViewModel

public void OpenItemList() 
{
    var itemList = new ItemListViewModel();
    itemList.TheCollection = CollectionFromThisVM;
    // etc
}

Or…

b: Use something like the EventAggregator pattern to publish a message containing the collection for the ItemListViewModel to subscribe to

The first method seems more straightforward