WPF依赖项属性回调

Question

我有一个具有依赖项属性的用户控件，并且正在wpf窗口中使用此控件。当我从窗口视图模型更改为绑定到该依赖项属性的属性的值时，我希望该依赖项属性的回调方法将被调用，但不会发生任何事情。

MainWindow.xaml

 <test:TestView Grid.Column="0" TestString="{Binding TestString, Mode=TwoWay}"></test:TestView>

MainWindow.xaml.cs

public MainWindow()
    {
        InitializeComponent();

        this.DataContext = new MainWindowViewModel();
    }

MainWindowViewModel.cs

 private string _testString;

    public string TestString
    {
        get { return _testString; }
        set
        {
            if (_testString!= value)
            {
                _testString= value;
                OnPropertyChanged();
            }
        }
    }

TestView.xaml.cs

public TestView()
    {
        InitializeComponent();
        this.DataContext = new TestViewModel();
    }

    public static readonly DependencyProperty TestStringProperty =
        DependencyProperty.Register("TestString", typeof(string), typeof(TestView), new PropertyMetadata(null, OnTestStringPropertyChanged));
    public string TestString
    {
        get { return (string)GetValue(TestStringProperty); }
        set
        {
            SetValue(TestStringProperty, value);
        }
    }

    private static void OnTestStringPropertyChanged(DependencyObject source,
    DependencyPropertyChangedEventArgs e)
    {
        TestView control = source as TestView;
        string time = (string)e.NewValue;
        // Put some update logic here...
        ((TestViewModel) control.DataContext).Merge();
    }

BaseViewModel.cs（我在每个ViewModel中都进行了扩展）

public class BaseViewModel : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;

    [NotifyPropertyChangedInvocator]
    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }
}

我在这里做错了什么，或者为什么从未调用过我的回调？