Stack-based navigation

Now that you understand slivers and scrolling, you can implement navigation between screens. In this lesson, you'll update the small-screen view such that when a contact group is tapped, it navigates to the contact list for that group.

First, revert changes in the adaptive layout widget so that it displays the ContactGroupsPage by default on small screens.

class _AdaptiveLayoutState extends State<AdaptiveLayout> {
  int selectedListId = 0;
​
  void _onContactListSelected(int listId) {
    setState(() {
      selectedListId = listId;
    });
  }
​
  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (context, constraints) {
        final isLargeScreen = constraints.maxWidth > largeScreenMinWidth;
​
        if (isLargeScreen) {
          return _buildLargeScreenLayout();
        } else {
          return const ContactGroupsPage(); // Reverted
        }
      },
    );
  }
}

The ContactGroupsPage already uses a _ContactGroupsView and provides it with a callback. That callback needs to be updated to navigate when a group is tapped, rather than printing the group to the console.

Ensure that the onListSelected callback in lib/screens/contact_groups.dart is implemented as follows:

class ContactGroupsPage extends StatelessWidget {
  const ContactGroupsPage({super.key});
​
  @override
  Widget build(BuildContext context) {
    return _ContactGroupsView(
      onListSelected: (list) => Navigator.of(context).push(
        CupertinoPageRoute(
          title: list.title,
          builder: (context) => ContactListsPage(listId: list.id),
        ),
      ),
    );
  }
}

This small code block contains the most important new information on this page.

Navigator.of(context) retrieves the nearest Navigator widget from the widget tree. The push method adds a new route to the navigator's stack, and displays the widget returned from the builder property.

This is the most basic implementation of using stack-based navigation, where new screens are pushed on top of the current screen. To navigate back to the previous screen, you'd use the Navigator.pop method.

CupertinoPageRoute creates iOS-style page transitions with the following features:

• A slide-in animation from the right.
• Automatic back button support.
• Proper title handling.
• Swipe-to-go-back gesture support.

For large screens, you need a sidebar that doesn't navigate but instead updates the main content area. Thanks to the refactoring in the previous step, creating this component is more straightforward. Add this widget to the bottom of lib/screens/contact_groups.dart:

// ...
​
/// A sidebar component for selecting contact groups, designed for large screens.
class ContactGroupsSidebar extends StatelessWidget {
  const ContactGroupsSidebar({
    super.key,
    required this.selectedListId,
    required this.onListSelected,
  });
​
  final int selectedListId;
  final Function(int) onListSelected;
​
  @override
  Widget build(BuildContext context) {
    return _ContactGroupsView(
      selectedListId: selectedListId,
      onListSelected: (list) => onListSelected(list.id),
    );
  }
}

This sidebar component reuses the _ContactGroupsView and provides a different callback. Instead of navigating, it calls onListSelected with the ID of the tapped list. It also passes the selectedListId to _ContactGroupsView so that the selected item can be highlighted.

For the large screen layout, you need a detail view that doesn't show navigation controls. Just like the sidebar, this can be recreated by reusing the _ContactListView. Add this widget to the bottom of your contacts.dart file:

// ...
​
/// A detail view component for showing contacts in a specific list.
class ContactListDetail extends StatelessWidget {
  const ContactListDetail({super.key, required this.listId});
​
  final int listId;
​
  @override
  Widget build(BuildContext context) {
    return _ContactListView(
      listId: listId,
      automaticallyImplyLeading: false,
    );
  }
}

The detail view reuses _ContactListView and sets the automaticallyImplyLeading parameter to false to hide the back button, as navigation is handled by the sidebar.

Now, connect the sidebar to your adaptive layout. Update your adaptive_layout.dart file to import the necessary files and update the large screen layout:

import 'package:flutter/cupertino.dart';
import 'package:rolodex/screens/contact_groups.dart';
import 'package:rolodex/screens/contacts.dart';

Then update the _buildLargeScreenLayout method:

Widget _buildLargeScreenLayout() {
  return CupertinoPageScaffold(
    backgroundColor: CupertinoColors.extraLightBackgroundGray,
    child: SafeArea(
      child: Row(
        children: [
          SizedBox(
            width: 320,
            child: ContactGroupsSidebar(
              selectedListId: selectedListId,
              onListSelected: _onContactListSelected,
            ),
          ),
          Container(
            width: 1,
            color: CupertinoColors.separator,
          ),
          Expanded(
            child: ContactListDetail(listId: selectedListId),
          ),
        ],
      ),
    ),
  );
}

This code creates the classic menu-detail layout where the sidebar controls the content of the detail area.

Hot reload your app and test the navigation:

Small screens (<600px width):

• Tap contact groups to navigate to contact details.
• Use the back button or a swipe gesture to return.
• This is a classic stack-based navigation flow.

Large screens (>600px width):

• Click contact groups in the sidebar to update the detail view.
• There is no navigation stack. The selection updates the content area.
• This is a master-detail interface pattern.

The app automatically chooses the appropriate navigation pattern based on screen size. This provides an optimal experience on both phones and tablets.