title: call converter in XAML if property of a object in the observable collection changes
body: <p>I have a converter that accepts an <code>ObservableCollection</code> as a parameter, and I'd like to re-evaluate it whenever a specific property on any item in the collection changes</p>
  
  <p>For example: lets say I have bound a label to a collection of <code>Person</code> objects with a converter. The job of the converter is to count the number of Persons in the list that are female, and return "valid" for 1 female or "accepted" for 2. I'd like the converter to get called again anytime the <code>Gender</code> property on any <code>Person</code> object gets changed.</p>
  
  <p>How can I accomplish this?</p>