createAnimation method
Called to create the animation that exposes the current progress of the transition controlled by the animation controller created by createAnimationController().
Implementation
Animation<Alignment> createAnimation() {
assert(!_transitionCompleter!.isCompleted,
'Cannot reuse a $runtimeType after disposing it.');
assert(_controller != null);
return AlignmentTween(begin: _initialAlignment, end: _endAlignment).animate(
CurvedAnimation(
parent: _controller!,
curve: snackbar!.forwardAnimationCurve!,
reverseCurve: snackbar!.reverseAnimationCurve,
),
);
}