title: INotifyPropertyChanged does't work when field of property change internally
body: <p>I try to binding textblock usercontrol with property of my class, but it only works at initial stage, I have implement IPropertyChnaged in my class.</p>
  
  <p>In my class, _Feedbackpos (field of property) would change in background, I don't know how to solve this problem.</p>
  
  <p>my class</p>
  
  <pre><code>public class TestControl : INotifyPropertyChanged
  {
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(string propertyname)
    {
       if(PropertyChanged != null)
       {
          PropertyChanged(this, new PropertyChangedEventArgs(propertyname));
       }
    }
  
    private double _Feedbackpos;
    public double Feedbackpos
    {
       get 
       {
         return _Feedbackpos;
       }
       set
       {
         _Feedbackpos = value;
         NotifyPropertyChanged("Feedbackpos");
       }
    }
  
    //it's a callback function, it would excute when detect feedback position of controller change
    private void ReadFeedbackpos()
    {
      _Feedbackpos = Controller.Read();
    }
  
  }
  </code></pre>
  
  <p>application windows</p>
  
  <pre><code>TestControl TestDll = new TestControl();
  
  Binding BindingTxtBlk = new Binding(){Source= TestDll, Path = new Property("Feedbackpos")};
  
  FeedbackPosTxtBlk.Setbinding(Textblock.TextProperty,BindingTxtBlk);
  
  </code></pre>