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Internationalizing Flutter Apps


What you’ll learn

  • How to track the device's locale (the user's preferred language).
  • How to manage locale-specific app values.
  • How define the locales an app supports. {{site.alert.end}}

If your app might be deployed to users who speak another language then you'll need to "internationalize" it. That means you'll need to write the app in a way that makes it possible to "localize" values like text and layouts for each language or "locale" that the app supports. Flutter provides widgets and classes that help with internationalization and the Flutter libraries themselves are internationalized.

The tutorial that follows is largely written in terms of the Flutter MaterialApp class, since most applications are written that way. Applications written in terms of the lower level WidgetsApp class can also be internationalized using the same classes and logic.

Sample internationalized apps

If you'd like to start out by reading the code for an internationalized Flutter app, here are two small examples. The first one is intended to be as simple as possible, and the second one uses the APIs and tools provided by the intl package. If Dart's intl package is new to you, see Using the Dart intl tools.

Setting up an internationalized app: the flutter_localizations package {#setting-up}

By default Flutter only provides US English localizations. To add support for other languages, an application must specify additional MaterialApp properties, and include a separate package called flutter_localizations. As of May 2018, this package supports 24 languages.

To use flutter_localizations, add the package as a dependency to your pubspec.yaml file:

{% prettify yaml %} dependencies: flutter: sdk: flutter flutter_localizations: sdk: flutter {% endprettify %}

Next, import the flutter_localizations library and specify localizationsDelegates and supportedLocales for MaterialApp:

{% prettify dart %} import 'package:flutter_localizations/flutter_localizations.dart';

MaterialApp( localizationsDelegates: [ // ... app-specific localization delegate[s] here GlobalMaterialLocalizations.delegate, GlobalWidgetsLocalizations.delegate, ], supportedLocales: [ const Locale('en', 'US'), // English const Locale('he', 'IL'), // Hebrew // ... other locales the app supports ], // ... ) {% endprettify %}

Apps based on WidgetsApp are similar except that the GlobalMaterialLocalizations.delegate isn't needed.

The elements of the localizationsDelegates list are factories that produce collections of localized values. GlobalMaterialLocalizations.delegate provides localized strings and other values for the Material Components library. GlobalWidgetsLocalizations.delegate defines the default text direction, either left to right or right to left, for the widgets library.

More information about these app properties, the types they depend on, and how internationalized Flutter apps are typically structured, can be found below.

Tracking the locale: The Locale class and the Localizations widget

The Locale class is used to identify the user's language. Mobile devices support setting the locale for all applications, usually via a system settings menu. Internationalized apps respond by displaying values that are locale-specific. For example, if the user switches the device's locale from English to French then a Text widget that displayed "Hello World" would be rebuilt with "Bonjour le monde".

The Localizations widget defines the locale for its child and the localized resources that the child depends on. The WidgetsApp widget creates a Localizations widget and rebuilds it if the system's locale changes.

You can always lookup an app's current locale with Localizations.localeOf():

{% prettify dart %} Locale myLocale = Localizations.localeOf(context); {% endprettify %}

Loading and retrieving localized values

The Localizations widget is used to load and lookup objects that contain collections of localized values. Apps refer to these objects with Localizations.of(context,type). If the device's locale changes, the Localizations widget automatically loads values for the new locale and then rebuilds widgets that used it. This happens because Localizations works like an InheritedWidget. When a build function refers to an inherited widget an implicit dependency on the inherited widget is created. When an inherited widget changes (when the Localizations widget's locale changes), its dependent contexts are rebuilt.

Localized values are loaded by the Localizations widget's list of LocalizationsDelegates. Each delegate must define an asynchronous load() method that produces an object which encapsulates a collection of localized values. Typically these objects define one method per localized value.

In a large app, different modules or packages might be bundled with their own localizations. That's why the Localizations widget manages a table of objects, one per LocalizationsDelegate. To retrieve the object produced by one of the LocalizationsDelegate's load methods, you specify a BuildContext and the object's type.

For example, the localized strings for the Material Components widgets are defined by the MaterialLocalizations class. Instances of this class are created by a LocalizationDelegate provided by the MaterialApp class. They can be retrieved with Localizations.of:

{% prettify dart %} Localizations.of(context, MaterialLocalizations); {% endprettify %}

This particular Localizations.of() expression is used frequently, so the MaterialLocalizations class provides a convenient shorthand:

{% prettify dart %} static MaterialLocalizations of(BuildContext context) { return Localizations.of(context, MaterialLocalizations); }

/// References to the localized values defined by MaterialLocalizations /// are typically written like this:

tooltip: MaterialLocalizations.of(context).backButtonTooltip, {% endprettify %}

## Using the bundled LocalizationsDelegates

To keep things as small and uncomplicated as possible, the flutter package includes implementations of the MaterialLocalizations and WidgetsLocalizations interfaces that only provide US English values. These implementation classes are called DefaultMaterialLocalizations and DefaultWidgetsLocalizations, respectively. They're included automatically unless a different delegate of the same base type is specified with the app's localizationsDelegates parameter.

The flutter_localizations package includes multi-language implementations of the localizations interfaces called GlobalMaterialLocalizations and GlobalWidgetsLocalizations. International apps must specify localization delegates for these classes as described in Setting up an internationalized app.

{% prettify dart %} import 'package:flutter_localizations/flutter_localizations.dart';

MaterialApp( localizationsDelegates: [ // ... app-specific localization delegate[s] here GlobalMaterialLocalizations.delegate, GlobalWidgetsLocalizations.delegate, ], supportedLocales: [ const Locale('en', 'US'), // English const Locale('he', 'IL'), // Hebrew // ... other locales the app supports ], // ... ) {% endprettify %}

The global localization delegates construct locale-specific instances of the corresponding classes. For example, GlobalMaterialLocalizations.delegate is a LocalizationsDelegate that produces an instance of GlobalMaterialLocalizations.

As of May 2018, the global localization classes support about 24 languages.

Defining a class for the app's localized resources

Putting all of this together for an internationalized app usually starts with the class that encapsulates the app's localized values. The example that follows is typical of such classes.

Complete source code for this example app.

This example is based on the APIs and tools provided by the intl package. An alternative class for the app's localized resources describes an example that doesn't depend on the intl package.

The DemoLocalizations class contains the app's strings (just one for the example) translated into the locales that the app supports. It uses the initializeMessages() function generated by Dart's intl package to load the translated strings, and Intl.message() to look them up.

{% prettify dart %} class DemoLocalizations { static Future load(Locale locale) { final String name = locale.countryCode.isEmpty ? locale.languageCode : locale.toString(); final String localeName = Intl.canonicalizedLocale(name); return initializeMessages(localeName).then((_) { Intl.defaultLocale = localeName; return DemoLocalizations(); }); }

static DemoLocalizations of(BuildContext context) { return Localizations.of(context, DemoLocalizations); }

String get title { return Intl.message( 'Hello World', name: 'title', desc: 'Title for the Demo application', ); } } {% endprettify %}

A class based on the intl package imports a generated message catalog that provides the initializeMessages() function and the per-locale backing store for Intl.message(). The message catalog is produced by an intl tool that analyzes the source code for classes that contain Intl.message() calls. In this case that would just be the DemoLocalizations class.

Specifying the app's supportedLocales parameter

Although Flutter's Material Components library includes support for about 16 languages, only English language translations are available by default. It's up to the developer to decide exactly which languages to support, since it wouldn't make sense for the toolkit libraries to support a different set of locales than the app does.

The MaterialApp supportedLocales parameter limits locale changes. When the user changes the locale setting on their device, the app's Localizations widget only follows suit if the new locale is a member of the this list. If an exact match for the device locale isn't found, then the first supported locale with a matching languageCode is used. If that fails, then the first element of the supportedLocales list is used.

In terms of the previous DemoApp example, the app only accepts the US English or French Canadian locales, and it substitutes US English (the first locale in the list) for anything else.

An app that wants to use a different "locale resolution" method can provide a localeResolutionCallback. For example, to have your app unconditionally accept whatever locale the user selects:

{% prettify dart %} class DemoApp extends StatelessWidget { @override Widget build(BuildContext context) { return MaterialApp( localeResolutionCallback(Locale locale, Iterable supportedLocales) { return locale; } // ... ); } } {% endprettify %}

An alternative class for the app's localized resources

The previous DemoApp example was defined in terms of the Dart intl package. Developers can choose their own approach for managing localized values for the sake of simplicity or perhaps to integrate with a different i18n framework.

Complete source code for this example app.

In this version of DemoApp the class that contains the app's localizations, DemoLocalizations, includes all of its translations directly in per language Maps.

{% prettify dart %} class DemoLocalizations { DemoLocalizations(this.locale);

final Locale locale;

static DemoLocalizations of(BuildContext context) { return Localizations.of(context, DemoLocalizations); }

static Map<String, Map<String, String>> _localizedValues = { 'en': { 'title': 'Hello World', }, 'es': { 'title': 'Hola Mundo', }, };

String get title { return _localizedValues[locale.languageCode]['title']; } } {% endprettify %}

In the minimal app the DemoLocalizationsDelegate is slightly different. Its load method returns a SynchronousFuture because no asynchronous loading needs to take place.

{% prettify dart %} class DemoLocalizationsDelegate extends LocalizationsDelegate { const DemoLocalizationsDelegate();

@override bool isSupported(Locale locale) => ['en', 'es'].contains(locale.languageCode);

@override Future load(Locale locale) { return SynchronousFuture(DemoLocalizations(locale)); }

@override bool shouldReload(DemoLocalizationsDelegate old) => false; } {% endprettify %}

Appendix: Using the Dart intl tools

Before building an API using the Dart intl package you'll want to review the intl package's documentation. Here's a summary of the process for localizing an app that depends on the intl package.

The demo app depends on a generated source file called l10n/messages_all.dart which defines all of the localizable strings used by the app.

Rebuilding l10n/messages_all.dart requires two steps.

  1. With the app's root directory as the current directory, generate `l10n/intl_messages.arb` from `lib/main.dart`:

    {% prettify sh %} $ flutter pub pub run intl_translation:extract_to_arb --output-dir=lib/l10n lib/main.dart {% endprettify %}

    The intl_messages.arb file is a JSON format map with one entry for each Intl.message() function defined in main.dart. This file serves as a template for the English and Spanish translations, intl_en.arb and intl_es.arb. These translations are created by you, the developer.

  2. With the app's root directory as the current directory, generate `intl_messages_.dart` for each `intl_.arb` file and `intl_messages_all.dart`, which imports all of the messages files:

    {% prettify sh %} $ flutter pub pub run intl_translation:generate_from_arb --output-dir=lib/l10n
    --no-use-deferred-loading lib/main.dart lib/l10n/intl_*.arb {% endprettify %}

    The DemoLocalizations class uses the generated initializeMessages() function (defined in intl_messages_all.dart) to load the localized messages and Intl.message() to look them up.

Appendix: Updating the iOS app bundle

iOS applications define key application metadata, including supported locales, in an Info.plist file that is built into the application bundle. To configure the locales supported by your app, you'll need to edit this file.

First, open your project's ios/Runner.xcworkspace Xcode workspace file then, in the Project Navigator, open the Info.plist file under the Runner project's Runner folder.

Next, select the Information Property List item, select Add Item from the Editor menu, then select Localizations from the pop-up menu.

Select and expand the newly-created Localizations item then, for each locale your application supports, add a new item and select the locale you wish to add from the pop-up menu in the Value field. This list should be consistent with the languages listed in the supportedLocales parameter.

Once all supported locales have been added, save the file.