WPF Listbox selectionchanged MVVM

Question

I have a list box in my WPF application. I know how to use the selectionchanged event. However I am trying to follow the MVVM design. However I am not sure how to do this.

Answer 1

Example of binding ListBox SelectionChanged Event to command in your ViewModel

<ListBox x:Name="myListBox" ItemsSource="{Binding SomeCollection}">
   <ie:Interaction.Triggers>
      <ie:EventTrigger EventName="SelectionChanged">
        <ie:InvokeCommandAction Command="{Binding SelectedItemChangedCommand}"  CommandParameter="{Binding ElementName=myListBox, Path=SelectedItem}"/>
    </ie:EventTrigger>
  </ie:Interaction.Triggers>
</ListBox >

In your ViewModel :

public class myViewModel
{

    public myViewModel()
    {
        SelectedItemChangedCommand = new DelegateCommand<object>((selectedItem) => 
        {
             // Logic goes here
        });
    }

    public List<SomeData> SomeCollection { get; set; }

    public DelegateCommand<object> SelectedItemChangedCommand { get; set; }
}

This particular example uses Prism MVVM Framework, but you can apply the same idea with any other MVVM framework you are using as well.

Hope this helps

Answer 2

using SelectionChanged with MVVM is quite simple:

An approach could be to bind to the SelectedIndex property of the ListBox and in the property setter in the VM, act accordingly as it would be triggered whenever the property changes.

Example: Here

In this example whenever the selected index changes, the value of that item is increased by one.

the main bit is:

public int SelectedIndex {
  get {
    return _selectedIndex;
  }

  set {
    if (_selectedIndex == value) {
      return;
    }

    // At this point _selectedIndex is the old selected item's index

    _selectedIndex = value;

    // At this point _selectedIndex is the new selected item's index

    RaisePropertyChanged(() => SelectedIndex);
  }
}

xaml would just be:

<ListBox ItemsSource="{Binding Items}" SelectedIndex="{Binding SelectedIndex}" />

Items is the collection of items we bind to.