ScrollController.dart

class ScrollControllerPlayground extends StatefulWidget {
  @override
  _ScrollControllerPlaygroundState createState() =>
      _ScrollControllerPlaygroundState();
}

class _ScrollControllerPlaygroundState
    extends State<ScrollControllerPlayground> {
  AppBar firstAppbar = AppBar(
    title: Text('FIRST APPBAR'),
  );
  AppBar secondAppbar = AppBar(
    title: Text('SECOND APPBAR'),
  );

  AppBar mAppBar;
  ScrollController _scrollController;

  @override
  void initState() {
    super.initState();
    mAppBar = firstAppbar;
    _scrollController = ScrollController()
      ..addListener(() {
        print('OFFSET: ${_scrollController.offset}');
        if (_scrollController.offset >= 105) {
          setState(() {
            mAppBar = secondAppbar;
          });
        } else {
          setState(() {
            mAppBar = firstAppbar;
          });
        }
      });
  }

  @override
  void dispose() {
    _scrollController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: mAppBar,
      body: SafeArea(
        child: ListView.builder(
          controller: _scrollController,
          itemCount: 100,
          itemBuilder: (context, index) {
            return Text('Position: $index');
          },
        ),
      ),
    );
  }
}