apply method
override
Poses both sides at progress by writing onto the routes, whose size is
the region being routed. outgoing is null on a push, where nothing is
leaving and the covered route is its Backdrop's to pose. Runs once when
the navigation starts and once per tick after the clock moves, so
rendering always sees the pose.
Implementation
@override
void apply(progress, incoming, outgoing) {
incoming.opacity = progress;
if (crossFade) outgoing?.opacity = 1 - progress;
}