title: OnSizeAllocated, OnMeasure, InvalidateMeasure in Xamarin Froms
body: <p>I can't really find any documentation with examples how to correctly override and use OnSizeAllocated, OnMeasure, InvalidateMeasure methods. Can someone give any examples?</p>
  
  <p>As a background I'd like to implement Editor that has a height of 200 when it's empty, but when it's full of text it will grow.</p>