private void Frame_Navigating(object sender, NavigatingCancelEventArgs e)
        {
            if (Content != null && !_allowDirectNavigation)
            {
                e.Cancel = true;
                _navArgs = e;
                this.IsHitTestVisible = false;
                DoubleAnimation da = new DoubleAnimation(0.3d, new Duration(TimeSpan.FromMilliseconds(100)));
                da.Completed += FadeOutCompleted;
                this.BeginAnimation(OpacityProperty, da);

            }
            _allowDirectNavigation = false;
        }

        private void FadeOutCompleted(object sender, EventArgs e)
        {
            (sender as AnimationClock).Completed -= FadeOutCompleted;

            this.IsHitTestVisible = true;

            _allowDirectNavigation = true;
            switch (_navArgs.NavigationMode)
            {
                case NavigationMode.New:
                    if (_navArgs.Uri == null)
                    {
                        this.myFrame.NavigationService.Navigate(_navArgs.Content);
                    }
                    else
                    {
                        this.myFrame.NavigationService.Navigate(_navArgs.Uri);
                    }
                    break;
                case NavigationMode.Back:
                    this.myFrame.NavigationService.GoBack();
                    break;

                case NavigationMode.Forward:
                    this.myFrame.NavigationService.GoForward();
                    break;
                case NavigationMode.Refresh:
                    this.myFrame.NavigationService.Refresh();
                    break;
            }

            Dispatcher.BeginInvoke(DispatcherPriority.Loaded,
                (ThreadStart)delegate()
                {
                    DoubleAnimation da = new DoubleAnimation(1.0d, new Duration(TimeSpan.FromMilliseconds(200)));
                    this.BeginAnimation(OpacityProperty, da);
                });
        }


        private bool _allowDirectNavigation = false;
        private NavigatingCancelEventArgs _navArgs = null;

   private void myFrame_Navigated(object sender, NavigationEventArgs e)
        {
            Page page = this.myFrame.Content as Page;
            if (page == null)
                return;
            if (page.Visibility == Visibility.Collapsed)
            {
                this.Visibility = Visibility.Collapsed;
                this.myFrame.NavigationService.GoBack();
            }
        }