WPF 静态资源的方式绑定DataContext

<Window x:Class="MyWPFSimple5.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:MyWPFSimple5"
        mc:Ignorable="d" DataContext="{Binding Source={StaticResource vmLocator}, Path=HelloObj}"
        Title="MainWindow" Height="250" Width="400">
    <Grid>
        <StackPanel>
            <TextBox Text="{Binding Say}" BorderBrush="Black" Margin="10"/>
            <Button Content="改变内容" Click="Button_Click" Margin="10"/>
        </StackPanel>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.ComponentModel;
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 MyWPFSimple5
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Hello hello = DataContext as Hello;
            hello.Say += "+1";
        }
    }

    public class Locator
    {
        public Locator()
        {
            //HelloObj = new Hello();
        }

        private Hello m_HelloObj;
        public Hello HelloObj
        {
            get
            {
                if (m_HelloObj == null)
                {
                    m_HelloObj = new Hello();
                }
                return m_HelloObj;
            }
            set
            {
                m_HelloObj = value;
            }
        }
    }

    public class Hello : INotifyPropertyChanged
    {
        public Hello()
        {
            Say = "Hello World!";
        }
        private string m_Say;
        public string Say { get { return m_Say; } set { m_Say = value; RaisedPropertyChanged(nameof(Say)); } }

        public event PropertyChangedEventHandler PropertyChanged;
        protected virtual void RaisedPropertyChanged(string name)
        {
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(name));
        }
    }
}

<Application x:Class="MyWPFSimple5.App"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:local="clr-namespace:MyWPFSimple5"
             StartupUri="MainWindow.xaml">
    <Application.Resources>
        <ResourceDictionary>
            <local:Locator x:Key="vmLocator"/>
        </ResourceDictionary>
    </Application.Resources>
</Application>