滚动时不隐藏BottomAppBar

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('My App'),
        ),
        body: MyScrollableContent(),
        bottomNavigationBar: BottomAppBar(
          child: Row(
            mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: <Widget>[
              IconButton(icon: Icon(Icons.home), onPressed: () {}),
              IconButton(icon: Icon(Icons.search), onPressed: () {}),
              IconButton(icon: Icon(Icons.settings), onPressed: () {}),
            ],
          ),
        ),
      ),
    );
  }
}

class MyScrollableContent extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ListView.builder(
      itemCount: 50,
      itemBuilder: (context, index) {
        return ListTile(title: Text('Item $index'));
      },
    );
  }
}