How to hide part of SliverAppBar when it is behind a widget?

Question

import 'package:flutter/material.dart';

class HomePage extends StatefulWidget {
  const HomePage({super.key});

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  var opacity = 0.0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.purple.shade100,
      body: SafeArea(
        child: Stack(
          children: [
            NotificationListener<ScrollUpdateNotification>(
              onNotification: (notification) {
                final pixels = notification.metrics.pixels / 100;

                setState(() {
                  if (pixels > 0.4) {
                    return;
                  }

                  opacity = pixels;
                });

                return true;
              },
              child: CustomScrollView(
                slivers: [
                  SliverAppBar(
                    expandedHeight: 0,
                    pinned: true,
                    backgroundColor: Colors.black.withOpacity(opacity),
                    shadowColor: Colors.transparent,
                    title: const Text(
                      'Stacked Container',
                      style: TextStyle(
                        color: Colors.white,
                        fontSize: 22.0,
                        fontWeight: FontWeight.bold,
                      ),
                    ),
                    toolbarHeight: 60,
                  ),
                  SliverAppBar(
                    clipBehavior: Clip.none,
                    floating: true,
                    toolbarHeight: 25,
                    shadowColor: Colors.transparent,
                    backgroundColor: Colors.transparent,
                    bottom: PreferredSize(
                      preferredSize: Size(double.infinity, 10),
                      child: Row(
                        mainAxisAlignment: MainAxisAlignment.center,
                        children: [
                          SizedBox(width: 15.0),
                          Opacity(
                            opacity: 1 - opacity, // Adjust the opacity based on scrolling
                            child: Text(
                              'Sliver AppBar',
                              style: TextStyle(
                                fontSize: 20.0,
                                color: Colors.green,
                              ),
                            ),
                          ),
                          SizedBox(width: 15.0),
                        ],
                      ),
                    ),
                  ),
                  SliverList(
                    delegate: SliverChildListDelegate(
                      [
                        ListView.builder(
                          itemCount: 10,
                          shrinkWrap: true,
                          physics: const NeverScrollableScrollPhysics(),
                          itemBuilder: (context, index) {
                            return Container(
                              margin:
                                  const EdgeInsets.fromLTRB(10.0, 20, 10, 10),
                              color: Colors.deepPurple,
                              height: 300,
                              width: double.infinity,
                            );
                          },
                        )
                      ],
                    ),
                  )
                ],
              ),
            ),
          ],
        ),
      ),
    );
  }
}

I'm trying to recreate Netflix appbar behavior. In this code one SliverAppBar followed by another SliverAppBar. First one is pinned second one is not. I want the part of second SliverAppBar to disappear that gets under the first SliverAppBar.

Current

Desired: The top half of text Sliver AppBar should disappear as it is under the first SliverAppBar.

Answer 1

Changes made are:

1, Listen to ScrollController.offset instead of ScrollUpdateNotification for layouts.

2, Use SliverList.builder for building sliver children of CustomScrollView. So it can be rendered lazily.

3, Keep opacity between 0 and 1. So the assertion doesn't throw.

4, Clip the content inside SliverAppBar instead of constraining the SliverAppBar. It's easier to work with RenderBox widget comparing to RenderSliver widget.

// ignore_for_file: prefer_const_constructors

import 'package:flutter/material.dart';

class HomePage extends StatefulWidget {
  const HomePage({super.key});

  @override
  State<HomePage> createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  var opacity = 0.0;
  var clipHeight = 25.0;
  late final ScrollController _scrollController;

  @override
  void initState() {
    super.initState();
    _scrollController = ScrollController();
    _scrollController.addListener(() {
      setState(() {
        var appBarOffset = _scrollController.offset;
        if (appBarOffset > 100) {
          appBarOffset = 100;
        }
        if (appBarOffset < 0) {
          appBarOffset = 0;
        }
        opacity = appBarOffset / 100; // keep it between 0,1
        clipHeight = 25 - _scrollController.offset;
        if (clipHeight > 25) {
          clipHeight = 25;
        }
        if (clipHeight < 0) {
          clipHeight = 0;
        }
      });
    });
  }

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.purple.shade100,
      body: SafeArea(
        child: CustomScrollView(
          controller: _scrollController,
          slivers: [
            SliverAppBar(
              expandedHeight: 0,
              pinned: true,
              backgroundColor: Colors.black.withOpacity(
                opacity,
              ),
              shadowColor: Colors.transparent,
              title: Text(
                'Stacked Container',
                style: const TextStyle(
                  color: Colors.white,
                  fontSize: 22.0,
                  fontWeight: FontWeight.bold,
                ),
              ),
              toolbarHeight: 60,
            ),
            SliverAppBar(
              clipBehavior: Clip.none,
              floating: true,
              toolbarHeight: 25,
              shadowColor: Colors.transparent,
              backgroundColor: Colors.transparent,
              bottom: PreferredSize(
                preferredSize: Size(double.infinity, 10),
                child: SizedBox(
                  height: 25,
                  child: Column(
                    children: [
                      SizedBox(height: 25 - clipHeight),
                      Container(
                        height: clipHeight,
                        color: Colors.amber,
                        child: SingleChildScrollView(
                          reverse: true,
                          child: Row(
                            mainAxisAlignment: MainAxisAlignment.center,
                            children: [
                              SizedBox(width: 15.0),
                              Opacity(
                                opacity: 1 - opacity,
                                // Adjust the opacity based on scrolling
                                child: Text(
                                  'Sliver AppBar',
                                  style: TextStyle(
                                    fontSize: 20.0,
                                    color: Colors.green,
                                  ),
                                ),
                              ),
                              SizedBox(width: 15.0),
                            ],
                          ),
                        ),
                      ),
                    ],
                  ),
                ),
              ),
            ),
            SliverList.builder(
              itemCount: 10,
              itemBuilder: (context, index) {
                return Container(
                  margin: const EdgeInsets.fromLTRB(10.0, 20, 10, 10),
                  color: Colors.deepPurple,
                  height: 300,
                  width: double.infinity,
                );
              },
            )
          ],
        ),
      ),
    );
  }
}

Answer 2

import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  late ScrollController _scrollController;
  bool isScrolledUp = false;
  double appBarHeight = 50.0;
  double _previousScrollPosition = 0.0;

  void _scrollListener() {
    final currentScrollPosition = _scrollController.position.pixels;
    final currentScrollVelocity = _scrollController.position.activity?.velocity;

    debugPrint(currentScrollVelocity.toString());

    if (currentScrollPosition < _previousScrollPosition) {
      debugPrint('Scrolling Up');
      setState(() {
        if (currentScrollPosition == 0) {
          appBarHeight = 50.0;
        }
        if (appBarHeight + 1 > 50) {
          appBarHeight = 50;
        } else {
          if (currentScrollVelocity != 0) {
            appBarHeight += 10;
          } else {
            appBarHeight += 1;
          }
        }
      });
    } else {
      debugPrint('Scrolling Down');
      setState(() {
        if (appBarHeight - 1 < 0) {
          appBarHeight = 0;
        } else {
          if (currentScrollVelocity != 0) {
            appBarHeight -= 10;
          } else {
            appBarHeight -= 0.6;
          }
        }
      });
    }
    _previousScrollPosition = currentScrollPosition;
  }

  @override
  void initState() {
    super.initState();

    _scrollController = ScrollController();
    _scrollController.addListener(_scrollListener);
  }

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: Stack(
            alignment: Alignment.topCenter,
            children: [
              ListView.builder(
                itemCount: 30,
                shrinkWrap: true,
                addRepaintBoundaries: true,
                controller: _scrollController,
                padding: const EdgeInsets.only(top: 100.0),
                itemBuilder: (context, index) {
                  return Container(
                    margin: const EdgeInsets.only(bottom: 20.0),
                    color: Colors.deepPurple,
                    height: 200.0,
                  );
                },
              ),
              Column(
                crossAxisAlignment: CrossAxisAlignment.stretch,
                children: [
                  MyAppBar(
                    title: 'STATIC APPBAR',
                    color: Colors.black.withOpacity(0.5),
                    height: 50.0,
                  ),
                  MyAppBar(
                    title: 'DYNAMIC APPBAR 1',
                    color: Colors.red.withOpacity(0.5),
                    height: appBarHeight,
                  ),
                ],
              )
            ],
          ),
        ),
      ),
    );
  }
}

class MyAppBar extends StatelessWidget {
  const MyAppBar({
    super.key,
    required this.title,
    required this.color,
    required this.height,
  });
  final String title;
  final Color color;
  final double height;

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      child: AnimatedContainer(
        duration: Duration(milliseconds: 1),
        constraints: const BoxConstraints(maxHeight: 50.0, minHeight: 0.0),
        color: color,
        height: height,
        child: Text(
          title,
          style: const TextStyle(
            color: Colors.white,
            fontSize: 20.0,
            fontWeight: FontWeight.bold,
          ),
          textAlign: TextAlign.center,
        ),
      ),
    );
  }
}

After trying different ways to implement this is the closest that I could get to right now. Feel free to share if If it can be done more efficiently. With or without SliverAppBar!