title: Managing events in flutter's TextFormField
body: <p>In Flutter project, I need to listen to the input text in TextFormField and do certain actions, especially when user put some character (eg. space) in this filed or when he request focus. When this kind of event happen, I need to modify filed's value.
  I know there is a property <code>called controller</code> but I don't know how to use it in this case.</p>
  
  <p>Thank you in advance.</p>