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SelectionChanged event and Attached Behavior

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ComboBox

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James asked on 16 Sep 2009, 08:45 AM

I use the build 2009.2.812. I have an attached behavior to handle the RadComboBox.SelectionChanged event to implement command and command parameter binding like this:
<telerik:RadComboBox x:Name="rcbActionResponse"
ItemsSource="{Binding ActionResponseLookup}" DisplayMemberPath="StdActionDescription" SelectedValuePath="StdActionID"
Behavior:SelectionChanged.CommandParameter="{Binding SelectedItem, ElementName=rcbActionResponse}
Behavior:SelectionChanged.Command="{Binding ActionChangedCommand}"/>

in constuctor of the viewmodel:
           ActionChangedCommand = new DelegateCommand<object>(ActionChanged);

private void ActionChanged(object arg)
    {
//logic to handle SelectionChanged event, expect arg is the current selected item of the RadComboBox
}

While I noticed the ActionChanged method was always fired before the property changed callback of the attached property CommandParameter - that make the arg parameter is not the current selected item but the old selected item

Any idea of this problem?

4 Answers, 1 is accepted

answered on 23 Sep 2009, 01:46 AM

Kaloyan,
the xaml about he RadBomBox has been pasted in my first post. the code-behide is not related. the other code is in the ViewModel.
Below is the code of attached behavior which uses Prism:

using Microsoft.Practices.Composite.Presentation.Commands;
using Telerik.Windows.Controls;

public class RadComboBoxSelectionChangedBehavior : CommandBehaviorBase<RadComboBox>
{
    public RadComboBoxSelectionChangedBehavior(RadComboBox control)
      : base(control)
    {
      control.SelectionChanged += new Telerik.Windows.Controls.SelectionChangedEventHandler(control_SelectionChanged);
    }

    void control_SelectionChanged(object sender, Telerik.Windows.Controls.SelectionChangedEventArgs e)
    {

      base.ExecuteCommand();
    }

}

public static class SelectionChanged
{
    private static readonly DependencyProperty RadComboBoxSelectionChangedBehaviorProperty
      = DependencyProperty.RegisterAttached(
      "RadComboBoxSelectionChangedBehavior",
      typeof(RadComboBoxSelectionChangedBehavior),
      typeof(SelectionChanged),
      null);

    public static readonly DependencyProperty CommandProperty
       = DependencyProperty.RegisterAttached(
       "Command",
       typeof(ICommand),
       typeof(SelectionChanged),
       new PropertyMetadata(OnSetCommandCallback));

    public static ICommand GetCommand(RadComboBox control)
    {
      return control.GetValue(CommandProperty) as ICommand;
    }

    public static void SetCommand(RadComboBox control, ICommand value)
    {
      control.SetValue(CommandProperty, value);
    }

    public static readonly DependencyProperty CommandParameterProperty
      = DependencyProperty.RegisterAttached(
     "CommandParameter",
     typeof(object),
     typeof(SelectionChanged),
     new PropertyMetadata(OnSetCommandParameterCallback));

    public static void SetCommandParameter(RadComboBox control, object parameter)
    {
      control.SetValue(CommandParameterProperty, parameter);
    }

    public static object GetCommandParameter(RadComboBox control)
    {
      return control.GetValue(CommandParameterProperty);
    }

    private static void OnSetCommandCallback
      (DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
      RadComboBox control = dependencyObject as RadComboBox;
      if (control != null)
      {
        RadComboBoxSelectionChangedBehavior behavior = GetOrCreateBehavior(control);
        behavior.Command = e.NewValue as ICommand;
      }
    }

    private static void OnSetCommandParameterCallback
       (DependencyObject dependencyObject, DependencyPropertyChangedEventArgs e)
    {
      RadComboBox control = dependencyObject as RadComboBox;
      if (control != null)
      {
        RadComboBoxSelectionChangedBehavior behavior = GetOrCreateBehavior(control);
        behavior.CommandParameter = e.NewValue;
      }
    }

    private static RadComboBoxSelectionChangedBehavior GetOrCreateBehavior(RadComboBox control)
    {
      RadComboBoxSelectionChangedBehavior behavior =
          control.GetValue(RadComboBoxSelectionChangedBehaviorProperty) as RadComboBoxSelectionChangedBehavior;
      if (behavior == null)
      {
        behavior = new RadComboBoxSelectionChangedBehavior(control);
        control.SetValue(RadComboBoxSelectionChangedBehaviorProperty, behavior);
      }
      return behavior;
    }

}

Here is the property in ViewModel:
public DelegateCommand<object> ActionChangedCommand { get; private set; }

in the constructor of the ViewModel:
ActionChangedCommand = new DelegateCommand<object>(ActionChanged);

Accepted

answered on 25 Sep 2009, 10:55 AM

Hello James,

This is the expected behavior, due to the Binding system in the Silverlight CLR. To overcome this issue you can set the CommandParameter in the control_SelectionChanger eventhandler:

    
            private void control_SelectionChanged(object sender, Telerik.Windows.Controls.SelectionChangedEventArgs e) 
        
                    { 
        
                        this.CommandParameter = e.AddedItems[0]; 
        
                        base.ExecuteCommand(); 
        
                    }

Regards,
Kaloyan
the Telerik team

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