In WPF, how to add an EventHandler for a FrameworkElement designed in Template?

Question

I have defined the following DataTemplate for ListBox items in an external resource dictionary:

Answer 1

Using the OnApplyTemplate approach will work if you if you're working with the ControlTemplate for a Control. For example, if you've subclassed TextBox you could do this like

public class MyTextBox : TextBox
{
    public override void OnApplyTemplate()
    {
        MySlider MySlider = GetTemplateChild("MySlider") as MySlider;
        if (MySlider != null)
        {
            MySlider.ValueChanged += new RoutedPropertyChangedEventHandler(MySlider_ValueChanged);
        }
        base.OnApplyTemplate();
    }
    void MySlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs e)
    {
        //...
    }
}

I don't think this approach will work in your situation however. You could use the Loaded event for ListBoxItem and find the Slider in the visual tree in the event handler

Code behind

private void ListBoxItem_Loaded(object sender, RoutedEventArgs e)
{
    ListBoxItem listBoxItem = sender as ListBoxItem;
    Slider MySlider = GetVisualChild(listBoxItem);
    MySlider.ValueChanged += new RoutedPropertyChangedEventHandler(MySlider_ValueChanged);
}
void MySlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs e)
{

}

GetVisualChild

private static T GetVisualChild(DependencyObject parent) where T : Visual
{
    T child = default(T);

    int numVisuals = VisualTreeHelper.GetChildrenCount(parent);
    for (int i = 0; i < numVisuals; i++)
    {
        Visual v = (Visual)VisualTreeHelper.GetChild(parent, i);
        child = v as T;
        if (child == null)
        {
            child = GetVisualChild(v);
        }
        if (child != null)
        {
            break;
        }
    }
    return child;
}