Bottom Pinned Buttons with Dynamic Scroll Physics

main.dart

import 'package:flutter/material.dart';

void main() => runApp(MaterialApp(
        home: Scaffold(
      appBar: AppBar(title: Text('Bottom Pinned Button')),
      body: MyWidget(),
    )));

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  int childCount = 1;
  ScrollController _controller;
  ScrollPhysics physics = ClampingScrollPhysics();

  _addRows() {
    setState(() => childCount++);
    _checkPhysics();
  }

  _removeRows() {
    if (childCount > 0) {
      setState(() => childCount--);
    }
    _checkPhysics();
  }

  _checkPhysics() {
    if (_controller.position.maxScrollExtent > 0) {
      setState(() => physics = BouncingScrollPhysics());
    } else {
      setState(() => physics = ClampingScrollPhysics());
    }
  }

  @override
  void initState() {
    _controller = ScrollController();
    _controller.addListener(_checkPhysics);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return CustomScrollView(
      controller: _controller,
      physics: physics,
      slivers: <Widget>[
        SliverFixedExtentList(
          itemExtent: 150.0,
          delegate: SliverChildBuilderDelegate(
            (BuildContext context, int index) {
              return Container(
                color: index % 2 == 0 ? Colors.amber[200] : Colors.blue[200],
              );
            },
            childCount: childCount,
          ),
        ),
        SliverFillRemaining(
          hasScrollBody: false,
          child: Container(
            child: Align(
              alignment: Alignment.bottomCenter,
              child: Padding(
                padding: const EdgeInsets.all(32.0),
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
                  children: <Widget>[
                    FloatingActionButton(
                      onPressed: () => _removeRows(),
                      child: Icon(Icons.arrow_upward),
                    ),
                    FloatingActionButton(
                      onPressed: () => _addRows(),
                      child: Icon(Icons.arrow_downward),
                    ),
                  ],
                ),
              ),
            ),
          ),
        ),
      ],
    );
  }
}