
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 on the form, which can make for a pretty frustrating