positionController property Null safety
Used by subclasses to manipulate the visual value of the control.
Some controls respond to user input by updating their visual value. For example, the thumb of a switch moves from one position to another when dragged. These controls manipulate this animation controller to update their position and eventually trigger an onChanged callback when the animation reaches either 0.0 or 1.0.
AnimationController get positionController => _positionController;