SetBinding没有为PropertyChanged事件注册

在我正在开发的一个应用程序中，我以编程方式创建了几个具有不同数据源的frameworkelement。不幸的是，数据绑定失败了。

我设法把这个问题提炼成下面的程序:

using System.Collections.Generic;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;
using Windows.UI.Xaml.Data;
namespace TestBinding
{
    public class BindingSource : INotifyPropertyChanged
    {
        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void OnPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(propertyName));
        }
        protected bool SetField<T>(ref T field, T value, [CallerMemberName] string propertyName = null)
        {
            if (EqualityComparer<T>.Default.Equals(field, value)) return false;
            field = value;
            OnPropertyChanged(propertyName);
            return true;
        }

        private string _text = "Initial Text";
        public string Text
        {
            get { return _text; }
            set { SetField(ref _text, value); }
        }
    }
    public sealed partial class MainPage : Page
    {
        BindingSource bs = new BindingSource();
        public MainPage()
        {
            this.InitializeComponent();
            TextBlock tb = new TextBlock();
            tb.HorizontalAlignment = HorizontalAlignment.Center;
            tb.VerticalAlignment = VerticalAlignment.Center;
            tb.FontSize = 36;
            Binding bind = new Binding() { Source = bs.Text, Mode=BindingMode.OneWay, UpdateSourceTrigger=UpdateSourceTrigger.PropertyChanged };
            tb.SetBinding(TextBlock.TextProperty, bind);
            Patergrid.Children.Add(tb);
        }
        private void ClickText1(object sender, RoutedEventArgs e)
        {
            bs.Text = "First text button clicked!";
        }
        private void ClickText2(object sender, RoutedEventArgs e)
        {
            bs.Text = "Second text button stroked!";
        }
    }
}

这里是xaml:

<Page
    x:Class="TestBinding.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:TestBinding"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">
    <Grid x:Name="Patergrid" Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*"/>
            <ColumnDefinition Width="*"/>
        </Grid.ColumnDefinitions>
        <StackPanel Grid.Column="1" HorizontalAlignment="Center" VerticalAlignment="Center">
            <Button Content="Text 1" Margin="25" HorizontalAlignment="Center" Click="ClickText1" />
            <Button Content="Text 2" Margin="25" HorizontalAlignment="Center" Click="ClickText2" />
        </StackPanel>
    </Grid>
</Page>

当属性被更改时，通过逐步执行代码，我看到PropertyChanged事件没有订阅者。

为什么SetBinding不注册PropertyChanged事件?

Binding bind = new Binding()
{
    Source = bs,
    Path = new PropertyPath("Text"),
    Mode = BindingMode.TwoWay,
    UpdateSourceTrigger = UpdateSourceTrigger.PropertyChanged
};