tween property

Animatable<T> tween
final

Defines the value of the TweenSequence for the interval within the animation's duration indicated by weight and this item's position in the list of items.

The value of this item can be "curved" by chaining it to a CurveTween. For example to create a tween that eases from 0.0 to 10.0: 
Tween<double>(begin: 0.0, end: 10.0)
  .chain(CurveTween(curve: Curves.ease))

Implementation 

final Animatable<T> tween