This widget uses an AnimatedBuilder to animate the
progress indicator to the latest value.

Run the app.
Type anything into the three fields to verify that
the animation works, and that clicking the
Sign up button brings up the Welcome screen.

Complete sample

Observations

You can use an AnimationController to run any animation.

AnimatedBuilder rebuilds the widget tree when the value
of an Animation changes.

Using a Tween, you can interpolate between almost any value,
in this case, Color.

What next?

Congratulations!
You have created your first web app using Flutter!

If you’d like to continue playing with this example,
perhaps you could add form validation.
For advice on how to do this,
see the Building a form with validation
recipe in the Flutter cookbook.