【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;
}
}