How do you handle a ComboBox SelectionChanged in MVVM?

Question

For those doing pure MVVM, how do you handle a ComboBox SelectionChanged event without reverting to code behind?

I tried eg AttachedBehaviors but Event="SelectedChanged" is not supported:

<ComboBox>
    <ComboBoxItem Content="Test1">
        <c:CommandBehaviorCollection.Behaviors>
            <c:BehaviorBinding Event="SelectionChanged" 
                               Command="{Binding SelectedChanged}"
                               CommandParameter="MainBorder123"/>
        </c:CommandBehaviorCollection.Behaviors>
    </ComboBoxItem>
    <ComboBoxItem Content="Test2"/>
    <ComboBoxItem Content="Test3"/>
</ComboBox>

Answer 1

This post is quite old, but since I got the same issue. Here is how I solved it (using framework 4.0) : the idea is to use System.Windows.Interactivity.

In the XAML :

<ComboBox ItemsSource="{Binding Items}">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="SelectionChanged">
            <i:InvokeCommandAction Command="{Binding SelectionChangedCommand}"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</ComboBox>

Then you just need to implement the SelectionChangedCommand in your viewmodel.

Answer 2

I'm not sure if what you're after is possible, but the way I do it is to simply bind the SelectedItem to a property on view model. Then within the property setter, I call any custom code that I want to happen ie setting other properties based on rule. If I need the selected item to be bound to an object aswell (for other bound controls to update) I set this in the setter too and send out a notification.

Answer 3

You would use a data trigger to trigger an event on a different UI element such as "enable / disable, or visible /invisible"

If you want the selected element to show the object data in other UI elements then you would use data binding and set the datacontext of the UI data display elements to be bound to the currently selected item in the combo box.