CupertinoTabBar requires Localizations parent

Summary

Instances of CupertinoTabBar must have a Localizationsparent in order to provide a localized Semantics hint. Trying to instantiate a CupertinoTabBar without localizations will result in an assertion such as the following:

CupertinoTabBar requires a Localizations parent in order to provide an appropriate Semantics hint
for tab indexing. A CupertinoApp will provide the DefaultCupertinoLocalizations, or you can
instantiate your own Localizations.
'package:flutter/src/cupertino/bottom_tab_bar.dart':
Failed assertion: line 213 pos 7: 'localizations != null'

Context

To support localized semantics information, the CupertinoTabBar requires localizations.

Before this change, the Semantics hint provided to the CupertinoTabBar was a hard-coded String, ‘tab, $index of $total’. The content of the semantics hint was also updated from this original String to ‘Tab $index of $total’ in English.

If your CupertinoTabBar is within the scope of a CupertinoApp, the DefaultCupertinoLocalizations will already be instantiated and may suit your needs without having to make a change to your existing code.

If your CupertinoTabBar is not within a CupertinoApp, you may provide the localizations of your choosing using the Localizations widget.

Migration guide

If you are seeing a 'localizations != null' assertion error, make sure locale information is being provided to your CupertinoTabBar.

Code before migration:

import 'package:flutter/cupertino.dart';

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

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MediaQuery(
      data: const MediaQueryData(),
      child: CupertinoTabBar(
        items: const <BottomNavigationBarItem>[
          BottomNavigationBarItem(
            icon: Icon(CupertinoIcons.add_circled),
            title: Text('Tab 1'),
          ),
          BottomNavigationBarItem(
            icon: Icon(CupertinoIcons.add_circled_solid),
            title: Text('Tab 2'),
          ),
        ],
        currentIndex: 1,
      ),
    );
  }
}

Code after migration (Providing localizations via the CupertinoApp):

import 'package:flutter/cupertino.dart';

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

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CupertinoApp(
      home: CupertinoTabBar(
        items: const <BottomNavigationBarItem>[
          BottomNavigationBarItem(
            icon: Icon(CupertinoIcons.add_circled),
            title: Text('Tab 1'),
          ),
          BottomNavigationBarItem(
            icon: Icon(CupertinoIcons.add_circled_solid),
            title: Text('Tab 2'),
          ),
        ],
        currentIndex: 1,
      ),
    );
  }
}

Code after migration (Providing localizations via the Localizations widget):

import 'package:flutter/cupertino.dart';

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

class Foo extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Localizations(
      locale: const Locale('en', 'US'),
      delegates: <LocalizationsDelegate<dynamic>>[
        DefaultWidgetsLocalizations.delegate,
        DefaultCupertinoLocalizations.delegate,
      ],
      child: MediaQuery(
        data: const MediaQueryData(),
        child: CupertinoTabBar(
          items: const <BottomNavigationBarItem>[
            BottomNavigationBarItem(
              icon: Icon(CupertinoIcons.add_circled),
              title: Text('Tab 1'),
            ),
            BottomNavigationBarItem(
              icon: Icon(CupertinoIcons.add_circled_solid),
              title: Text('Tab 2'),
            ),
          ],
          currentIndex: 1,
        ),
      ),
    );
  }
}

Timeline

This change was introduced in May 2020, in 1.18.0-9.0.pre.

References

API documentation:

Relevant PR: