For each property that animates in an interval, create a
Tween. The Tween specifies the start and end
values for that property.

The Tween produces an Animation
object that is managed by the controller.

The following diagram shows the Intervals used in the
basic_staggered_animation example.
You might notice the following characteristics:

The opacity changes during the first 10% of the timeline.

A tiny gap occurs between the change in opacity,
and the change in width.

Nothing animates during the last 25% of the timeline.

Increasing the padding makes the widget appear to rise upward.

Increasing the border radius to 0.5,
transforms the square with rounded corners into a circle.

The padding and height changes occur during
the same exact interval, but they don’t have to.

To set up the animation: