sliding_cards.dart

class _SlidingCardsViewState extends State<SlidingCardsView> {
  PageController pageController;
  double pageOffset = 0;

  @override
  void initState() {
    super.initState();
    pageController = PageController(viewportFraction: 0.8);
    pageController.addListener(() {
      setState(() => pageOffset = pageController.page); //<-- add listener and set state
    });
  }
  
  @override
  Widget build(BuildContext context) {
    return ...
        [
          SlidingCard(
            name: 'Shenzhen GLOBAL DESIGN AWARD 2018',
            date: '4.20-30',
            assetName: 'steve-johnson.jpeg',
            offset: pageOffset, //<-- pass offset
          ),
          SlidingCard(
            name: 'Dawan District, Guangdong Hong Kong and Macao',
            date: '4.28-31',
            assetName: 'rodion-kutsaev.jpeg',
            offset: pageOffset - 1, //<-- pass offset - indexOfCard
          ),
        ]
      ...;
  }
}