WPF中ElementName与RelativeSource绑定的局限性以及对策

完全来源于十月的寒流，感谢大佬讲解

<Window x:Class="Test_01.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Test_01"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <StackPanel>
        <TextBlock Text="{Binding ElementName=txt, Path=Text, StringFormat='Input: {0}'}" FontSize="30"></TextBlock>
        <TextBlock Text="{Binding RelativeSource={RelativeSource AncestorType=StackPanel}, Path=Children[2].Text, StringFormat='Input: {0}'}" FontSize="30"></TextBlock>
        <TextBox x:Name="txt" FontSize="30"></TextBox>
    </StackPanel>
</Window>

RelativeSource解析不到

<Window x:Class="Test_01.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:Test_01"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800" Loaded="Window_Loaded">
    <StackPanel x:Name="Spanel">
        <TextBlock Text="{Binding ElementName=txt, Path=Text, StringFormat='Input: {0}'}" FontSize="30"></TextBlock>
        <TextBlock Text="{Binding RelativeSource={RelativeSource AncestorType=StackPanel}, Path=Children[2].Text, StringFormat='Input: {0}'}" FontSize="30"></TextBlock>
        <!--<TextBox x:Name="txt" FontSize="30"></TextBox>-->
    </StackPanel>
</Window>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
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 Test_01
{
    /// 

    /// MainWindow.xaml 的交互逻辑
    /// 
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            var box = new TextBox
            {
                Name = "txt",
            };
            this.RegisterName(box.Name, box);
            Spanel.Children.Add(box);
        }
    }

    public class MainWindowViewModel
    {
        public string Message { get; set; }
    }
}