路由事件问题-在子元素之前击中根UI元素

我的路由事件在子元素之前击中根UI元素。这是意料之中的吗?如何让路由事件首先击中子元素?

目的:如果文本输入到"自定义文本框"以外的任何地方，请将文本输入到"默认文本框"

结果:Window_PreviewTextInput在custom_PreviewTextInput之前被击中，即使我的光标焦点是在"自定义文本框"

我应该怎么做?

---

XAML

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" SizeToContent="WidthAndHeight"
        PreviewTextInput="Window_PreviewTextInput"
        >
    <Grid Margin="100,100,100,100">
        <StackPanel>
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="default" Width="100"/>
                <TextBox x:Name="defaultTB" Width="300" Height="50"/>
            </StackPanel>
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="custom" Width="100"/>
                <TextBox x:Name="custom" PreviewTextInput="custom_PreviewTextInput" Width="300" Height="50"/>
            </StackPanel>
        </StackPanel>
    </Grid>
</Window>

---

代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace WpfApplication2
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        //goal:  if text is typed anywhere except custom textbox, put text in default textbox
        private void Window_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            Keyboard.Focus(defaultTB); 
        }
        //goal:  if text is typed in custom TB, put text there, and end the event routing
        private void custom_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            e.Handled = true; 
        }
    }
}