ViewModelBehavior.cs

public static class ViewModelBehavior
{
    public static readonly DependencyProperty LoadUnloadProperty =
        DependencyProperty.RegisterAttached("LoadUnload", typeof(Boolean),
        typeof(ViewModelBehavior),
        new FrameworkPropertyMetadata(false,
            new PropertyChangedCallback(OnLoadUnloadChanged)));
 
    public static void SetLoadUnload(FrameworkElement element, Boolean value)
    {
        element.SetValue(LoadUnloadProperty, value);
    }
 
    public static Boolean GetLoadUnload(FrameworkElement element)
    {
        return (Boolean)element.GetValue(LoadUnloadProperty);
    }
 
    public static void OnLoadUnloadChanged(DependencyObject obj,
        DependencyPropertyChangedEventArgs args)
    {
        FrameworkElement element = obj as FrameworkElement;
 
        if (element == null)
            throw new InvalidOperationException();
 
        element.DataContextChanged += (sender, e) =>
        {
            if (!element.IsLoaded)
                return;
 
            if (e.OldValue is IViewModel)
            {
                IViewModel viewModel = ((IViewModel)e.OldValue);
                if (viewModel.Initialized)
                {
                    viewModel.Unload(element);
                    viewModel.Initialized = false;
                }
            }
 
            if (e.NewValue is IViewModel)
            {
                IViewModel viewModel = ((IViewModel)e.NewValue);
                if (!viewModel.Initialized)
                {
                    viewModel.Initialized = true;
                    viewModel.Load(element);
                }
            }
        };
 
        element.Loaded += (sender, e) =>
        {
            IViewModel viewModel =
                element.GetValue(FrameworkElement.DataContextProperty) as IViewModel;
 
            if (viewModel != null && !viewModel.Initialized)
            {
                viewModel.Initialized = true;
                viewModel.Load(element);
            }
        };
 
        element.Unloaded += (sender, e) =>
        {
            IViewModel viewModel =
                element.GetValue(FrameworkElement.DataContextProperty) as IViewModel;
 
            if (viewModel != null && viewModel.Initialized)
            {
                viewModel.Unload(element);
                viewModel.Initialized = false;
            }
        };
    }
}