Layout child sliver only when expanded

[Amir-P]

Fixes #2.

[Zekfad]

Hi, when I was fixing extent size, I've tried to fix this as well, but faces issues with layout.
It seems that your fix is also causes this issue, here's reproducible:

Collapse the list and then press regenerate.
This would cause

_AssertionError ('package:flutter/src/rendering/object.dart': Failed assertion: line 3848 pos 12: '!_needsLayout': Updated layout information required for RenderSliverList#1f474 relayoutBoundary=up17 NEEDS-LAYOUT NEEDS-PAINT to calculate semantics.)

// because
// ignore_for_file: unreachable_from_main


import 'dart:math';

import 'package:flutter/material.dart';
import 'package:sliver_expandable/sliver_expandable.dart';

void main() {
  runApp(const MainApp());
}

class MainApp extends StatefulWidget {

  const MainApp({super.key});

  @override
  State<MainApp> createState() => _MainAppState();
}

class _MainAppState extends State<MainApp> {
  final rnd = Random(1337);
  bool expanded = true;
  List<String> objects = [];

  @override
  Widget build(BuildContext context) =>
    MaterialApp(
      home: Scaffold(
        body: Center(
          child: CustomScrollView(
            shrinkWrap: true,
            slivers: [
              SliverToBoxAdapter(
                child: ElevatedButton(
                  child: const Text('Regenerate'),
                  onPressed: () => setState(() {
                    objects = List.generate(50 + rnd.nextInt(50), (i) => 'String $i - ${rnd.nextDouble()}');
                  }),
                ),
              ),
              AnimatedSliverExpandable(
                expanded: expanded,
                headerBuilder: (context, animation) => ListTile(
                  onTap: () => setState(() => expanded = !expanded),
                  title: const Text('Example'),
                  trailing: AnimatedBuilder(
                    animation: animation,
                    builder: (context, child) => Transform.rotate(
                      angle: (animation.value - 0.5) * pi,
                      child: child,
                    ),
                    child: const Icon(Icons.chevron_left),
                  ),
                ),
                sliver: SliverList.builder(
                  itemCount: objects.length,
                  itemBuilder: (context, i) => ListTile(
                    title: Text(objects[i]),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
}