How to handle attached properties events?

Question

I created an expander style that contains a checkbox in its header. The checkbox state is bound to an attached property:

<Style TargetType="{x:Type Expander}" x:Key="MyCheckboxExpander">
    <Setter Property="Template">
         <Setter.Value>
              <ControlTemplate TargetType="{x:Type Expander}">
               (...)
                   <CheckBox x:Name="ExpanderHeaderChk" VerticalAlignment="Center" Margin="4,0,0,2" 
                                          IsChecked="{Binding RelativeSource={RelativeSource TemplatedParent}, Path=(my:AP.IsChecked)}" />
                (...)

I my view, inside the expander I have a stackpanel with a ComboBox.

Whenever the user checks the expander's checkbox, I wan't that the combobox gets the first item selected, on the oher hand whenever the user unchecks it, I wan't that the selecteditem of the combobox be null.

How can I accomplish this? I'm following the MVVM pattern, but since this is more a matter of the view, I'm open to code-behind suggestions.

Answer 1

I followed the suggestion provided by @Baboon and I created a custom control with a routed event named CheckedChanged, this way I can access it through the view's xaml and code-behind:

 [TemplatePart(Name = "PART_Expander", Type = typeof(Expander))]
[TemplatePart(Name = "PART_CheckBox", Type = typeof(CheckBox))]
public class MyCustomExpander : Expander
{
    static MyCustomExpander()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(MyCustomExpander), new FrameworkPropertyMetadata(typeof(MyCustomExpander)));
    }

    public bool IsChecked
    {
        get { return (bool)GetValue(IsCheckedProperty); }
        set { SetValue(IsCheckedProperty, value); }
    }
    public static readonly DependencyProperty IsCheckedProperty =
        DependencyProperty.Register("IsChecked", typeof(bool), typeof(MyCustomExpander),
                                     new UIPropertyMetadata(false));

    #region Events

    private CheckBox chkExpander = new CheckBox();
    public CheckBox ChkExpander { get { return chkExpander; } private set { chkExpander = value; } }

    public static readonly RoutedEvent CheckedChangedEvent = EventManager.RegisterRoutedEvent("ExtraButtonClick", 
                                                                                               RoutingStrategy.Bubble, 
                                                                                               typeof(RoutedEventHandler),
                                                                                               typeof(MyCustomExpander));

    public event RoutedEventHandler CheckedChanged
    {
        add { AddHandler(CheckedChangedEvent, value); }
        remove { RemoveHandler(CheckedChangedEvent, value); }
    }

    void OnCheckedChanged(object sender, RoutedEventArgs e)
    {
        RaiseEvent(new RoutedEventArgs(CheckedChangedEvent, this));
    } 

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        CheckBox chk = base.GetTemplateChild("PART_CheckBox") as CheckBox;
        if (chk != null)
        {
            chk.Checked += new RoutedEventHandler(OnCheckedChanged);
            chk.Unchecked += new RoutedEventHandler(OnCheckedChanged);
        }
    }

    #endregion

}

I want to thank to @Baboon and @Vlad for their help.