createAnimate method
void
createAnimate()
Implementation
void createAnimate() {
// 内容动画
top = Tween<double>(begin: _boxHeight, end: 0)
.animate(CurvedAnimation(parent: _controller, curve: Curves.decelerate))
..addStatusListener(animateEnd);
// 遮罩层透明动画
opacity = Tween<double>(begin: 0.0, end: 1.0).animate(
CurvedAnimation(parent: _controller, curve: Curves.decelerate));
}