Instantiate AnimationController:

controller

AnimationController

lowerBound:

upperBound:

100

duration:

const

Duration

seconds:

),

vsync:

this

// Don't worry about vsync for now.

);

When you instantiate AnimationController,
you define an animation in terms of its starting value
(lowerBound), its ending value (upperBound),
and the amount of time it takes to change
from one to the other (duration).

The default values for lowerBound and upperBound
are 0 and 1, respectively; duration doesn’t have a default value.

Animation concepts section uses an