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
{/// <summary>/// MainWindow.xaml 的交互逻辑/// </summary>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; }}
}