How can I realize SelectionChanged in MVVM ListBox Silverlight

Question

The ListBox control does not implement a Command property. I have to attach some functionality to the SelectionChanged event. Somebody knows how can I do it? Please help me

Answer 1

I prefer using a binding to the SelectedItem and implementing any functionality in the setting of the binding property.

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

...

public class ViewModel
{
    public IEnumerable<Item> Items { get; set; } 

    private Item selectedItem;
    public Item SelectedItem
    {
        get { return selectedItem; }
        set
        {
            if (selectedItem == value)
                return;
            selectedItem = value;
            // Do logic on selection change.
        }
    }
}

Answer 2

This is the way where You can Reach the Selection changed events in Your MVVM Application First Of all i tell you that Command Property only work in Button now we have to Explicitly binding that property in our Selection Changed event like List box or combo box in Your XMAL file

<ListBox Name="MyListBox" ItemsSource="{Binding ListItems}" Height="150" Width="150" Margin="281,32,-31,118">

        <Local:Interaction.Triggers>
            <Local:EventTrigger EventName="SelectionChanged">
                <Local:InvokeCommandAction Command="{Binding MyCommand}" CommandParameter="{Binding ElementName=MyListBox,Path=SelectedItem}"/>
            </Local:EventTrigger>
        </Local:Interaction.Triggers>
    </ListBox>

for this you have to add dll Syatem.Windows.Interactivity now u have to add references in your xaml file namespace like

 xmlns:Local="clr-namespace:System.Windows.Interactivityassembly=System.Windows.Interactivity"

in your ViewModel Class you have to define your Command in Con structure

 public ViewModel123()
    {
         MyCommand = new RelayCommand<string>(TestMethod);

    }

now create the TestMethod method which can handle the selection changed event

 private void TestMethod(string parameter)
    {
        MessageBox.Show(parameter);
    }

i hope this may help u.

Answer 3

Basically you have a few options:

1. Use the property SelectedItem of ListBox to bind to a property in the backend (ie in view model) and perform logic in the setter as described by Cameron MacFarland.
2. Use a third party library that has a generic event to command behavior like in the link posted by Pedro Lamas.
3. If you don't want to use third party libraries or writing logic inside property setter is somehow unacceptable you can create your own behavior for ListBox control. It would subscribe to control's SelectionChanged event and execute a command (the command could be a dependency property exposed by the behavior).

Answer 4

Think this post from Laurent Bugnion will help you solve the problem:

http://geekswithblogs.net/lbugnion/archive/2010/05/19/handling-datagrid.selecteditems-in-an-mvvm-friendly-manner.aspx

The post above mentions the DataGrid but I do think it will work with the ListBox too!

Best regards and Happy New Year!! :)

Answer 5

I would suggest using RelayCommand. Either use the MVVM Light Toolkit or just use the RelayCommand and CommandManager classes from Josh Smith's implementations. I personally use just the two classes, so I don't need the entire toolkit.

While this will definitely work, there might be an easier way depending on what you are doing. It might just be easier to bind an object to the SelectedValue of the ListBox and listen for that value to change.