begin and

end values. Many types have specific

ColorTween interpolates between colors and

RectTween interpolates between rects.
You can define your own interpolations by creating
your own subclass of

lerp function.

By itself, a tween just defines how to interpolate
between two values. To get a concrete value for the
current frame of an animation, you also need an
animation to determine the current state.
There are two ways to combine a tween
with an animation to get a concrete value:

You can evaluate the tween at the current
value of an animation. This approach is most useful
for widgets that are already listening to the animation and hence
rebuilding whenever the animation changes value.