Inheritance: System.Windows.Controls.ContentControl
        /// <summary>
        /// IsTransitioningProperty property changed handler.
        /// </summary>
        /// <param name="d">TransitioningContentControl that changed its IsTransitioning.</param>
        /// <param name="e">Event arguments.</param>
        private static void OnIsTransitioningPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            TransitioningContentControl source = (TransitioningContentControl)d;

            if (!source._allowIsTransitioningWrite)
            {
                source.IsTransitioning = (bool)e.OldValue;
                throw new InvalidOperationException("IsTransitioning property is read-only.");
            }
        }
        /// <summary>
        /// TransitionProperty property changed handler.
        /// </summary>
        /// <param name="d">TransitioningContentControl that changed its Transition.</param>
        /// <param name="e">Event arguments.</param>
        private static void OnTransitionPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            TransitioningContentControl source = (TransitioningContentControl)d;
            string oldTransition = e.NewValue as string;
            string newTransition = e.NewValue as string;

            if (source.IsTransitioning)
            {
                source.AbortTransition();
            }

            // find new transition
            Storyboard newStoryboard = source.GetStoryboard(newTransition);

            // unable to find the transition.
            if (newStoryboard == null)
            {
                // could be during initialization of xaml that presentationgroups was not yet defined
                if (VisualTreeHelperEx.TryGetVisualStateGroup(source, PresentationGroup) == null)
                {
                    // will delay check
                    source.CurrentTransition = null;
                }
                else
                {
                    // revert to old value
                    source.SetValue(TransitionProperty, oldTransition);

                    throw new ArgumentException(
                              string.Format(CultureInfo.CurrentCulture, "Transition '{0}' was not defined.", newTransition));
                }
            }
            else
            {
                source.CurrentTransition = newStoryboard;
            }
        }