想实现这个功能：

下面的代码可以实现：

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  static const String _title = 'Flutter Code Sample';

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: _title,
      home: MyStatefulWidget(),
    );
  }
}

class MyStatefulWidget extends StatefulWidget {
  MyStatefulWidget({Key key}) : super(key: key);

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

class _MyStatefulWidgetState extends State<MyStatefulWidget> {
  int _selectedIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Row(
        children: <Widget>[
          LayoutBuilder(
            builder: (context, constraint) {
              return SingleChildScrollView(
                child: ConstrainedBox(
                  constraints: BoxConstraints(minHeight: constraint.maxHeight),
                  child: IntrinsicHeight(
                    child: NavigationRail(
                      selectedIndex: _selectedIndex,
                      onDestinationSelected: (int index) {
                        setState(() {
                          _selectedIndex = index;
                        });
                      },
                      labelType: NavigationRailLabelType.selected,
                      destinations: [
                        NavigationRailDestination(
                          icon: Icon(Icons.favorite_border),
                          selectedIcon: Icon(Icons.favorite),
                          label: Text('First'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.bookmark_border),
                          selectedIcon: Icon(Icons.book),
                          label: Text('Second'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.star_border),
                          selectedIcon: Icon(Icons.star),
                          label: Text('Third'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.favorite_border),
                          selectedIcon: Icon(Icons.favorite),
                          label: Text('First'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.bookmark_border),
                          selectedIcon: Icon(Icons.book),
                          label: Text('Second'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.star_border),
                          selectedIcon: Icon(Icons.star),
                          label: Text('Third'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.bookmark_border),
                          selectedIcon: Icon(Icons.book),
                          label: Text('Second'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.star_border),
                          selectedIcon: Icon(Icons.star),
                          label: Text('Third'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.favorite_border),
                          selectedIcon: Icon(Icons.favorite),
                          label: Text('First'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.bookmark_border),
                          selectedIcon: Icon(Icons.book),
                          label: Text('Second'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.star_border),
                          selectedIcon: Icon(Icons.star),
                          label: Text('Third'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.bookmark_border),
                          selectedIcon: Icon(Icons.book),
                          label: Text('Second'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.star_border),
                          selectedIcon: Icon(Icons.star),
                          label: Text('Third'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.favorite_border),
                          selectedIcon: Icon(Icons.favorite),
                          label: Text('First'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.bookmark_border),
                          selectedIcon: Icon(Icons.book),
                          label: Text('Second'),
                        ),
                        NavigationRailDestination(
                          icon: Icon(Icons.star_border),
                          selectedIcon: Icon(Icons.star),
                          label: Text('Third'),
                        ),
                      ],
                    ),
                  ),
                ),
              );
            },
          ),
          VerticalDivider(thickness: 1, width: 1),
          // This is the main content.
          Expanded(
            child: Center(
              child: Text('selectedIndex: $_selectedIndex'),
            ),
          )
        ],
      ),
    );
  }
}