Flutter persistent draggable bottom widget

flutter_persistent_draggable_bottom_widget.dart

import 'package:flutter/material.dart';

class PersistentDraggableBottomWidget extends StatefulWidget {
  PersistentDraggableBottomWidget() : super();

  @override
  _PersistentDraggableBottomWidgetState createState() => _PersistentDraggableBottomWidgetState();
}

class _PersistentDraggableBottomWidgetState extends State<PersistentDraggableBottomWidget> {
  double _parentHeight = 0;
  double _childHeight = 0;
  double _ratio = 0.5;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Persistent Draggable Bottom Widget'),
      ),
      body: Container(
        color: Colors.green,
        child: Builder(
          builder: (context) {
            WidgetsBinding.instance?.addPostFrameCallback((_) {
              _parentHeight = context.size?.height ?? 0;
              _calculate();
              print('Parent height: ' + (context.size?.height.toString() ?? 'null'));
            });

            return Stack(
              children: [
                Container(
                  color: Colors.yellow,
                ),
                DraggableScrollableSheet(
                  expand: true,
                  initialChildSize: _ratio,
                  minChildSize: 0.1,
                  maxChildSize: _ratio,
                  builder: (context, scrollController) => SingleChildScrollView(
                    physics: ClampingScrollPhysics(),
                    controller: scrollController,
                    child: Builder(
                      builder: (context) {
                        WidgetsBinding.instance?.addPostFrameCallback((_) {
                          _childHeight = context.size?.height ?? 0;
                          _calculate();
                          print('Child height: ' + (context.size?.height.toString() ?? 'null'));
                        });

                        return Column(
                          children: [
                            Container(
                              height: 200,
                              color: Colors.red,
                            ),
                            Container(
                              height: 200,
                              color: Colors.cyan,
                            ),
                          ],
                        );
                      },
                    ),
                  ),
                ),
              ],
            );
          },
        ),
      ),
    );
  }

  void _calculate() {
    if (_parentHeight == 0 || _childHeight == 0) return;

    final ratio = _childHeight / _parentHeight;

    setState(() {
      _ratio = ratio;
    });
  }
}