title: Propagate state between different pages
body: <p>I've three different <code>MaterialPage</code>. So I use <code>Navigator.push</code> to go from 1 -&gt; 2 -&gt; 3.
  Now, page 1 make an HTTP request to <code>GET</code> a model that is then used in page 1, 2 and 3.
  At the end of the process an action on page 3 triggers some modification of the model on the server. How can I update the tree pages on the stack to reflect these changes? In order to do that I need to make a <code>GET</code> request again.
  Should I pass a callback from page 1 to 3 and then go back all the way up to page 1 (and then down again...), or duplicate the HTTP request in page 3? Or something else?
  I've attached a schema, in order to clarify (I hope) my explanation. Thanks in advance.</p>
  <p><a href="https://i.stack.imgur.com/C2FeL.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/C2FeL.jpg" alt="Flutter schema" /></a></p>