Sliver Magic - by Simon Lightfoot - #HumpdayQandA - 19th July 2023 - #Flutter #Dart - https://www.youtube.com/live/zNOZD0JSFQY?feature=share&t=7315

sliver_magic.dart

// MIT License
//
// Copyright (c) 2023 Simon Lightfoot
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in all
// copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
// SOFTWARE.
//
import 'dart:math' as math;

import 'package:flutter/gestures.dart';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

void main() {
  runApp(const RevealApp());
  // runApp(const HideLucasApp());
}

@immutable
class RevealApp extends StatelessWidget {
  const RevealApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData.dark(useMaterial3: true),
      home: Material(
        child: ScrollConfiguration(
          behavior: const ScrollBehavior().copyWith(
            dragDevices: {
              PointerDeviceKind.mouse,
              PointerDeviceKind.touch,
            },
            overscroll: false,
          ),
          child: const CustomScrollView(
            slivers: [
              SliverItem(color: Colors.green),
              SliverItem(color: Colors.green),
              SliverScrollReveal(
                height: 400.0,
                child: BoxItem(color: Colors.red),
              ),
              SliverItem(color: Colors.blue),
              SliverItem(color: Colors.blue),
            ],
          ),
        ),
      ),
    );
  }
}

class SliverScrollReveal extends StatelessWidget {
  const SliverScrollReveal({
    super.key,
    required this.height,
    required this.child,
  });

  final double height;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return SliverLayoutBuilder(
      builder: (BuildContext context, SliverConstraints constraints) {
        final width = constraints.crossAxisExtent;
        final actualHeight = math.min(
          height,
          math.max(constraints.remainingPaintExtent, 0.0),
        );
        return SliverToBoxAdapter(
          child: ClipRect(
            child: Transform.translate(
              offset: Offset(0.0, constraints.scrollOffset),
              child: SizedBox(
                height: actualHeight,
                child: OverflowBox(
                  alignment: constraints.scrollOffset > 0
                      ? Alignment.topLeft
                      : Alignment.bottomLeft,
                  minWidth: width,
                  maxWidth: width,
                  minHeight: height,
                  maxHeight: height,
                  child: Stack(
                    fit: StackFit.expand,
                    children: [
                      child,
                      Positioned.fill(
                        child: Align(
                          alignment: constraints.scrollOffset > 0
                              ? Alignment.topLeft
                              : Alignment.bottomLeft,
                          child: Padding(
                            padding:
                                const EdgeInsets.symmetric(horizontal: 12.0),
                            child: Text(
                              'axisDirection: ${constraints.axisDirection}\n'
                              'growthDirection: ${constraints.growthDirection}\n'
                              'userScrollDirection: ${constraints.userScrollDirection}\n'
                              'scrollOffset: ${constraints.scrollOffset.toStringAsFixed(3)}\n'
                              'precedingScrollExtent: ${constraints.precedingScrollExtent.toStringAsFixed(3)}\n'
                              'overlap: ${constraints.overlap.toStringAsFixed(3)}\n'
                              'remainingPaintExtent: ${constraints.remainingPaintExtent.toStringAsFixed(3)}\n'
                              'crossAxisExtent: ${constraints.crossAxisExtent.toStringAsFixed(3)}\n'
                              'crossAxisDirection: ${constraints.crossAxisDirection}\n'
                              'viewportMainAxisExtent: ${constraints.viewportMainAxisExtent.toStringAsFixed(3)}\n',
                              style: const TextStyle(
                                color: Colors.black,
                                fontWeight: FontWeight.w500,
                                fontSize: 16.0,
                              ),
                            ),
                          ),
                        ),
                      ),
                    ],
                  ),
                ),
              ),
            ),
          ),
        );
      },
    );
  }
}

@immutable
class BoxItem extends StatelessWidget {
  const BoxItem({
    super.key,
    required this.color,
  });

  final Color color;

  @override
  Widget build(BuildContext context) {
    return ColoredBox(
      color: color,
      child: const AspectRatio(
        aspectRatio: 1.3,
        child: Placeholder(
          color: Colors.white,
        ),
      ),
    );
  }
}

@immutable
class SliverItem extends StatelessWidget {
  const SliverItem({
    super.key,
    required this.color,
  });

  final Color color;

  @override
  Widget build(BuildContext context) {
    return SliverPadding(
      padding: const EdgeInsets.symmetric(horizontal: 12.0),
      sliver: SliverToBoxAdapter(
        child: BoxItem(color: color),
      ),
    );
  }
}

@immutable
class HideLucasApp extends StatefulWidget {
  const HideLucasApp({super.key});

  @override
  State<HideLucasApp> createState() => _HideLucasAppState();
}

class _HideLucasAppState extends State<HideLucasApp> {
  final _controller = ScrollController();
  final _items = [...Colors.primaries, ...Colors.accents];
  late final _keys = List.generate(
    _items.length,
    (index) => GlobalKey(),
  );
  bool _hidden = false;

  @override
  void initState() {
    super.initState();
    _controller.addListener(_onScrollChanged);
  }

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

  void _onScrollChanged() {
    final offset = _controller.hasClients && _controller.position.hasPixels
        ? _controller.position.pixels
        : 0.0;
    //print(offset);
    if (_hidden == false && offset > 1400.0) {
      double height = 0;
      for (int i = 0; i < 3; i++) {
        final render =
            _keys[i].currentContext!.findRenderObject() as RenderSliver;
        final childHeight = render.geometry?.scrollExtent ?? 0.0;
        height += childHeight;
      }
      setState(() {
        _hidden = true;
        _controller.position.correctBy(-height);
        print('trigger $height');
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      theme: ThemeData.dark(useMaterial3: true),
      home: Material(
        child: CustomScrollView(
          controller: _controller,
          slivers: [
            for (final (index, color)
                in _items.skip(_hidden ? 3 : 0).indexed) //
              if (color == Colors.blue)
                SliverToBoxAdapter(
                  child: Builder(
                    builder: (BuildContext context) {
                      final scrollable = Scrollable.of(context);
                      return AnimatedBuilder(
                        animation: scrollable.position,
                        builder: (BuildContext context, Widget? child) {
                          // print('scrollable: ${scrollable.position.pixels}');
                          return child!;
                        },
                        child: BoxItem(color: color),
                      );
                    },
                  ),
                )
              else
                SliverItem(
                  key: _keys[index],
                  color: color,
                ),
          ],
        ),
      ),
    );
  }
}