e>&lt;LinkTo /&gt;</code>, <code>transitionTo</code>,
or a URL change, a <code>willTransition</code> action is fired on the currently
active routes. This gives each active route, starting with the leaf-most
route, the opportunity to decide whether or not the transition should occur.</p>
<p>Imagine your app is in a route that's displaying a complex form for the user
to fill out and the user accidentally navigates backwards. Unless the
transition is prevented, the user might lose all of the progress they
made