title: How can a class notify somebody that its data have been altered?
body: <p>Lets say there is a <code>Person</code> class that holds some attributes and some other classes <code>Address</code> etc. (I am just giving an example of class{class{data}})</p>
  
  <p>An instance of this is deserialized and loaded in the ui. The used might change the data (the name, the address or something).</p>
  
  <p>If the instance is the datasource in the ui(in every control), how can I know if the data has been changed in order to notify the user to Save before exit?</p>