【WPF】淡入淡出切换页面

<NavigationWindow x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Switch page with transition" Height="512" Icon="icon1.ico"
                  Width="958" ShowsNavigationUI="False" Source="Index.xaml" 
                  Navigating="NavigationWindow_Navigating" ResizeMode="NoResize">
</NavigationWindow>

using System;
using System.Threading;
using System.Windows;
using System.Windows.Media.Animation;
using System.Windows.Navigation;
using System.Windows.Threading;


namespace WpfApplication1
{
    /// <summary>
    /// Window1.xaml 的交互逻辑
    /// </summary>
    public partial class Window1 : NavigationWindow
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void NavigationWindow_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(300)));
                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)
                    {
                        NavigationService.Navigate(_navArgs.Content);
                    }
                    else
                    {
                        NavigationService.Navigate(_navArgs.Uri);
                    }
                    break;
                case NavigationMode.Back:
                    NavigationService.GoBack();
                    break;

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