flutter AnimatedPositioned

Positioned in the animated version.

Only a child to work when the Stack.

If the child's size will change during the animation, it is a good choice AnimatedPositioned

    double indicatorWidth = (width * 2 / 3).ceilToDouble();
    double indicatorHeight = 2;
    double indicatorTopMargin = height - indicatorHeight*2;
    double left = (currentIndex * width + indicatorWidth / 4).ceilToDouble();
  Widget _indicator() {
      return AnimatedPositioned(
        left: left,
        child: Container(
          height: indicatorHeight,
          width: indicatorWidth,
          margin: EdgeInsets.only(top: indicatorTopMargin),
          color: kGridTextColor,
        ),
        duration: Duration(milliseconds: 200),
      );