title: who subscribes to PropertyChanged event
body: <p>usually when a class exposes an event :</p>
  
  <pre><code>SomeClass.OnSomeEvent += Handler ;
  </code></pre>
  
  <p>what i'm trying to understand : (correct me if i'm wrong )</p>
  
  <p>when implementing INotifyPropertyChanged the implementing class exposes PropertyChanged event</p>
  
  <p>(1)who registers to that event ? </p>
  
  <p>(2) is there a way to observe the underlying code of the wpf framework ? </p>