WPF，这段代码啥意思？
<UserControl x:Class="WPFRoutedEvent.UserControl1"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
             xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
             mc:Ignorable="d" 
             d:DesignHeight="300" d:DesignWidth="300">
    <Grid>
        <StackPanel>
            <Button x:Name="testButton" Content="testButton" Click="TestButton_Click" />
        </StackPanel>
    </Grid>
</UserControl>
    public partial class UserControl1 : UserControl
    {
        public UserControl1()
        {
            InitializeComponent();
        }
        private void TestButton_Click(object sender, RoutedEventArgs e)
        {
            MessageBox.Show(string.Format("sender 类型:{0} \n\r source 类型:{1} \n\r originalSource 类型: {2}",
                sender.GetType().Name, e.Source.GetType().Name, e.OriginalSource.GetType().Name), "子窗体消息");
        }
    }


<Window x:Class="WPFRoutedEvent.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525"
        xmlns:local="clr-namespace:WPFRoutedEvent">
    <Grid>
        <local:UserControl1 x:Name="test1"/>