Why is the SelectTemplate Method run 2 times in debug mode?

debuging this class the SelectTemplate Method is run 2 times, but why?

The first time the item is always null.

public class PersonDataTemplateSelector : DataTemplateSelector
{
    public override DataTemplate SelectTemplate(object item,DependencyObject container)
    {
        if (item is Person)
        {
            Person person = item as Person;

            Window window = Application.Current.MainWindow;

            if (System.ComponentModel.DesignerProperties.GetIsInDesignMode( window))
                return null;

            开发者_开发百科if (person.Gender == "male")               
                return window.FindResource("boysViewTemplate") as DataTemplate;
            else                
                return window.FindResource("girlsViewTemplate") as DataTemplate;

        }
        return null;
    }
}

You can set a break point and check the stack trace to verify but I believe it's called once with a null input when the visual tree is set up and the second time is when the bindings are actually populated.

If your selector were to provide a look for "Empty" or "Loading", the first call is what gives your selector the opportunity to provide that template while the elements are loading.