Sending data from view to viewmodel with command binding

Answer 1

I usually expose a CollectionView from the view model and set the IsSynchronizedWithCurrentItem property on the ItemsControl displaying the list in the view. Then when the command is executed, I can inspect the CollectionView.CurrrentItem propety to see what is currently selected.

EDIT: This answer addresses the first question in your, um, question. Rather than your view sending the currently selected item to the ViewModel, the ViewModel keeps track of the currently selected item. So using this technique you don't need to work out how to send that information.

Something like this in your view model:

class ApplicationViewModel
{
    // Exposes a list of ShipmentViewModels.
    public CollectionView Shipments { get; private set; }

    // A DelegateCommand or similar, that when executed calls AddPallet().
    public ICommand AddPalletCommand { get; private set; }

    void AddPallet()
    {
        ShipmentViewModel shipment = (ShipmentViewModel)Shipments.CurrentItem;
        shipment.Pallets.Add(new PalletViewModel(...));
    }
}

And then this in your xaml:

<ListBox ItemsSource="{Binding Shipments}" IsSynchronizedWithCurrentItem="True"/>
<Button Command="{Binding AddPalletCommand}>Add Pallet</Button>

This way you can also track the selection of the Shipments collection from your ViewModel and update the command's CanExecute state.

Does that help any?

Answer 2

For keeping track of the currently selected item I do something similar to Groky, maybe this example make a little more sense.

In your ViewModel that contains the collection that your list is bound to (I'm using a ListBox in this example) expose a property that relates to the selected item.

// Assuming your using the MVVM template from Microsoft
public class PalletListViewModel : ViewModelBase
{
   // The collection our list is bound to
   private ObservableCollection<Pallet> _palletList;
   // The current selected item
   private Pallet _selectedPallet;
   // Our command bound to the button
   private DelegateCommand _processCommand;

   public ObservableCollection<Pallet> PalletList
   {
      get { return _palletList; }
   }

   public Pallet SelectedPallet
   {
      get { return _selectedPallet; }
      set
      {
         if(value == _selectedPallet)
            return;

         _selectedPallet = value;

         // INotifyPropertyChanged Method for updating the binding
         OnPropertyChanged("SelectedPallet");
      }
   }

   public ICommand ProcessCommand
   {
      get
      {
         if(_processCommand == null)
            _processCommand = new DelegateCommand(Process);
         return _processCommand;
      }
   }

   private void Process()
   {
      // Process the SelectedPallet
   }
}

<Window ...>
   <Grid x:Name="LayoutRoot">
      <Button Content="Process Pallet" Command="{Binding ProcessCommand}" />
      <ListBox ItemsSource="{Binding PalletList}" SelectedItem="{Binding SelectedPallet}">
         ...
      </ListBox>
   </Grid>
</Window>

Hopefully this is what your looking for.