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Translation and Internationalization (i18n) for Flutter. Easy to use for both large and small projects. Uses Dart extensions to reduce boilerplate.

pub package

Translate your app! #

"Thank you for making the i18n_extension plugin. It has helped me a lot in my latest project and I will surely use it again in my next Flutter project. It is so easy to set up and use and the code boilerplate is indeed very minimal."

— Tomáš Jeřábek, Consultant/Developer

This is a Flutter package. For a Dart-only package, see i18n_extension_core

This package was mentioned by Google during the Dart 2.7 announcement

Read the Medium article

Start with a widget containing some text:

Text('How are you?')

Translate it by simply adding .i18n to the string:

Text('How are you?'.i18n)

If the current locale is 'pt' (the language code for Portuguese) or pt_BR (language code for Brazilian Portuguese), then the text shown in the screen will be 'Como vai?', the Portuguese translation to the above text. And so on for any other locales you want to support:

// Shows 'How are you?' when current locale is en_US.
// Shows '¿Cómo estás?' when current locale is es.
// Shows 'Comment ça va?' when current locale is fr.
Text('How are you?'.i18n)

As shown above, the original English text is itself the "translation key" that's used to look up the translation.

But you can actually use objects of any type as translation keys. By adding .i18n they will turn into translated strings in the current locale:

// Const values  
const greetings = UniqueKey();
greetings.i18n // Turns into 'How are you?' in en, 'Como vai?' in pt  

// Final variables  
final faq = 'faq';
faq.i18n // 'FAQ' in en, 'Perguntas frequentes' in pt

// Enums  
enum MyColors { red, blue }
MyColors.red.i18n // 'Red' in en, 'Vermelho' in pt
MyColors.blue.i18n // 'Blue' in en, 'Azul' in pt

// Numbers, booleans, Dates  
12.i18n // 'Twelve' in en, 'Doze' in pt
true.i18n // 'Yes' in en, 'Sim' in pt
false.i18n // 'No' in en, 'Não'
DateTime(2021, 1, 1).i18n // 'New Year' in en, 'Ano Novo' in pt

// Your own object types  
class User { ... }
User('John').i18n // 'Mr. John' in en, 'Sr. John' in pt

You can also provide different translations depending on modifiers, such as plural quantities:

print('There is 1 item'.plural(0)); // Prints 'There are no items'
print('There is 1 item'.plural(1)); // Prints 'There is 1 item'
print('There is 1 item'.plural(2)); // Prints 'There are 2 items'

And you can invent your own modifiers according to any conditions. For example, some languages have different translations for different genders. So you could create gender versions for Gender modifiers:

print('There is a person'.gender(Gender.male)); // Prints 'There is a man'
print('There is a person'.gender(Gender.female)); // Prints 'There is a woman'
print('There is a person'.gender(Gender.they)); // Prints 'There is a person'

Also, interpolating strings is easy, with the fill method:

// Prints 'Hello John, this is Mary' in English.
// Prints 'Olá John, aqui é Mary' in Portuguese.
// Prints 'Olá John, aqui é Mary' in Portuguese.
print('Hello %s, this is %s'.i18n.fill(['John', 'Mary']));

See it working #

Try running the example.

Good for simple or complex apps #

I'm always interested in creating packages to reduce boilerplate. For example, async_redux is about Redux without boilerplate, align_positioned is about creating layouts using fewer widgets, and themed is about simplifying the usage of colors and fonts. Likewise, the current package is about reducing boilerplate for translations. It does everything the plain old Localizations.of(context) does, but much easier.

It's meant for both the one-person app developer and the big company team. It has you covered in all stages of your translation efforts:

  1. When you create your widgets, it makes it easy for you to define which strings (or other objects serving as translation keys) should be translated by simply adding .i18n to them. These are called "translatable strings" or "translatable identifiers".

  2. When you want to start your translation efforts, it can automatically list for you all strings that need translation. If you miss any of them, or if you later add more strings or modify some of them, it will let you know what changed and how to fix it.

  3. You can then provide your translations manually in a very easy-to-use format.

  4. Or you can easily integrate it with professional translation services, importing it from or exporting it to any format you want.

Setup #

Wrap your widget tree with the I18n widget, below the MaterialApp, together with the localizationsDelegates and the supportedLocales:

import 'package:i18n_extension/i18n_widget.dart';
import 'package:flutter_localizations/flutter_localizations.dart';


Widget build(BuildContext context) {
  return MaterialApp(
      localizationsDelegates: [
      supportedLocales: [
        const Locale('en', 'US'),
        const Locale('pt', 'BR'),
      home: I18n(child: ...)

Note: To be able to import flutter_localizations.dart you must add this to your pubspec.yaml:

    sdk: flutter

  i18n_extension: ...

The code home: I18n(child: ...) shown above will translate your strings to the current system locale. Or you can override it with your own locale, like this:

  initialLocale: Locale('pt', 'BR'),
  child: ...

Note: Don't ever put translatable strings in the same widget where you declared the I18n widget, since they may not respond to future locale changes. For example, this is a mistake:

Widget build(BuildContext context) {
  return I18n(
    child: Scaffold(
      appBar: AppBar(title: Text('Hello there'.i18n)),
      body: MyScreen(),

You may put translatable strings in any widgets down the tree.

A quick recap of Dart locales #

The correct way to create a Locale is to provide a language code (usually 2 or 3 lowercase letters) and a country code (usually 2 uppercase letters), as two separate Strings.

For example:

var locale = Locale('en', 'US');

print(locale); // Prints `en_US`.
print(locale.languageCode); // Prints `en`.

You can, if you want, omit the country code:

var locale = Locale('en');

print(locale); // Prints `en`.
print(locale.languageCode); // Prints `en`.

But you cannot provide language code and country code as a single String. This is wrong:

// This will create a language called en_US and no country code.
var locale = Locale('en_US');

print(locale); // Prints `en_US`.
print(locale.languageCode); // Also prints `en_US`.

To help avoiding this mistake, the i18n_extension may throw an error if your language code contains underscores.

Translating a widget #

When you create a widget that has translatable strings, add this default import to the widget's file:

import 'package:i18n_extension/default.i18n.dart';

This will allow you to add .i18n and .plural() to your strings, but won't translate anything.

When you are ready to create your translations, you must create a dart file to hold them. This file can have any name, but I suggest you give it the same name as your widget and change the termination to .i18n.dart.

For example, if your widget is in file my_widget.dart, the translations could be in file my_widget.i18n.dart

You must then remove the previous default import, and instead import your own translation file:

import 'my_widget.i18n.dart';

Your translation file itself will be something like this:

import 'package:i18n_extension/i18n_extension.dart';

extension Localization on String {

  static var _t = Translations.byText('en_us') +
      'en_us': 'Hello, how are you?',
      'pt_br': 'Olá, como vai você?',
      'es': '¿Hola! Cómo estás?',
      'fr': 'Salut, comment ca va?',
      'de': 'Hallo, wie geht es dir?',

  String get i18n => localize(this, _t);

The above example shows a single translatable string, translated to American English, Brazilian Portuguese, general Spanish, French and German.

You can, however, translate as many strings as you want, by simply adding more translation maps:

import 'package:i18n_extension/i18n_extension.dart';

extension Localization on String {

    static var _t = Translations.byText('en_us') +
          'en_us': 'Hello, how are you?',
          'pt_br': 'Olá, como vai você?',
        } +
          'en_us': 'Hi',
          'pt_br': 'Olá',
        } +
          'en_us': 'Goodbye',
          'pt_br': 'Adeus',

  String get i18n => localize(this, _t);

Strings themselves are the translation keys #

The locale you pass in the Translations factory is called the default locale. For example, in Translations.byText('en_us') the default locale is en_us. The strings inside your Text widget should be in the language of that locale.

The strings themselves are used as keys when searching for translations to the other locales. For example, in the Text below, 'Hello, how are you?' is both the translation to English, and the key to use when searching for its other translations:

Text('Hello, how are you?'.i18n)

If any translation key is missing from the translation maps, the key itself will be used, so the text will still appear in the screen, untranslated.

If the translation key is found, it will choose the language according to the following rules:

  1. It will use the translation to the exact current locale, for example en_us.

  2. If this is absent, it will use the translation to the general language of the current locale, for example en.

  3. If this is absent, it will use the translation to any other locale with the same language, for example en_uk.

  4. If this is absent, it will use the value of the key in the default language.

  5. If this is absent, it will use the key itself as the translation.

Try running the example using strings as translation keys.

Or you can, instead, use identifiers as translation keys #

Instead of:

'Hello there'.i18n

You can also do:


To that end, you can use the Translations.byId<T>() factory:

import 'package:flutter/foundation.dart';

final appbarTitle = UniqueKey();
final greetings = UniqueKey();

extension Localization on UniqueKey {
  static final _t = Translations.byId<UniqueKey>('en_us', {
    appbarTitle: {
      'en_us': 'i18n Demo',
      'pt_br': 'Demonstração i18n',
    greetings: {
      'en_us': 'Helo there',
      'pt_br': 'Olá como vai',

  String get i18n => localize(this, _t);    

Try running the example using identifiers as translation keys.

Note: The native way of doing translation in Flutter forces you to define "identifier keys" for each translation, and use those. For example, an identifier key could be helloHowAreYou or simply greetings. And then you can access the translation like this: MyLocalizations.of(context).greetings.

With i18n_extension, you can use ANY object type as translation keys. Just use Translations.byId<T>() and provide the type T of your identifier. Your T can be anything, including String, int, double, DateTime, or even your own custom object types, as long as they implement == and hashCode.

Don't forget that your extensions need to be on your type. For example, if you use int as your key type, you need to declare extension Localization on int { ... }.

If your T is Object or Object? or dynamic, then anything can be translated, and you need to write: extension Localization on Objec? { ... }

We believe having to define identifiers is a boring task, and makes it difficult for you to remember the exact text of the translations without having to look at the translation file.

For this reason we recommend you to simply type the text you want as a String inside your Text() widgets, and add .i18n to them.

The exception is when you have very large texts that you need to translate, like for example privacy policies, terms of use, long explanations etc. In those cases, you may want to use identifiers, while keeping the rest as string keys.

In the example below, privacyPolicy and termsOfUse are used as identifiers, while My Settings, Ok and Back are used as string keys:

import 'package:flutter/foundation.dart';

final privacyPolicy = UniqueKey();
final termsOfUse = UniqueKey();

extension Localization on Object {
  static final _t = Translations.byId<Object>('en_us', {
    privacyPolicy: { 'en_us': 'Very Looong text', 'pt_br': 'Very Looong text' },
    termsOfUse: { 'en_us': 'Very Looong text', 'pt_br': 'Very Looong text' },
    'My Settings': { 'en_us': 'My Settings', 'pt_br': 'Meus ajustes' },    
    'Ok': { 'en_us': 'Ok', 'pt_br': 'Salvar ajustes' },
    'Back': { 'en_us': 'Back', 'pt_br': 'Voltar' },

  String get i18n => localize(this, _t);    

You use them like this, respectively:

Text('My Settings'.i18n);

Finding missing translations #

If some string is already translated, and you later change it in the widget file, this will break the link between the key and the translation map. However, i18n_extension is smart enough to let you know when that happens, so it's easy to fix. You can even add this check to tests, as to make sure all translations are linked and complete.

When you run your app or tests, each key not found will be recorded to the static set Translations.missingKeys. And if the key is found but there is no translation to the current locale, it will be recorded to Translations.missingTranslations.

You can manually inspect those sets to see if they're empty, or create tests to do that automatically, for example:

expect(Translations.missingKeys, isEmpty);
expect(Translations.missingTranslations, isEmpty);

Note: You can disable the recording of missing keys and translations by doing:

Translations.recordMissingKeys = false;
Translations.recordMissingTranslations = false;

Another thing you may do, if you want, is to set up callbacks that the i18n_extension package will call whenever it detects a missing translation. You can then program these callbacks to throw errors if any translations are missing, or log the problem, or send emails to the person responsible for the translations.

To do that, simply inject your callbacks into Translations.missingKeyCallback and Translations.missingTranslationCallback.

For example:

Translations.missingTranslationCallback =
  (key, locale) =>
    throw TranslationsException('There are no translations in $locale for key $key.');

Defining translations by locale instead of by key #

As explained, by using the Translations.byText() constructor you define each key and then provide all its translations at the same time. This is the easiest way when you are doing translations manually, for example, when you speak English and Spanish and are creating yourself the translations to your app.

However, in other situations, such as when you are hiring professional translation services or crowdsourcing translations, it may be easier if you can provide the translations by locale/language, instead of by key. You can do that by using the Translations.byLocale() constructor.

static var _t = Translations.byLocale('en_us') +
      'en_us': {
        'Hi.': 'Hi.',
        'Goodbye.': 'Goodbye.',
      'es_es': {
        'Hi.': 'Hola.',
        'Goodbye.': 'Adiós.',

You can also add maps using the + operator:

static var _t = Translations.byLocale('en_us') +
      'en_us': {
        'Hi.': 'Hi.',
        'Goodbye.': 'Goodbye.',
    } +
      'es_es': {
        'Hi.': 'Hola.',
        'Goodbye.': 'Adiós.',

Note above, since en_us is the default locale, you could omit the translations for it.

Combining translations #

To combine translations you can use the * operator. For example:

var t1 = Translations.byText('en_us') +
      'en_us': 'Hi.',
      'pt_br': 'Olá.',

var t2 = Translations.byText('en_us') +
      'en_us': 'Goodbye.',
      'pt_br': 'Adeus.',

var translations = t1 * t2;

print(localize('Hi.', translations, locale: 'pt_br');
print(localize('Goodbye.', translations, locale: 'pt_br');

Translation modifiers #

Sometimes you have different translations that depend on a number quantity. Instead of .i18n you can use .plural() and pass it a number. For example:

int numOfItems = 3;
return Text('You clicked the button %d times'.plural(numOfItems));

This will be translated, and if the translated string contains %d it will be replaced by the number.

Then, your translations file should contain something like this:

static var _t = Translations.byText('en_us') +
    'en_us': 'You clicked the button %d times'
        .zero('You haven't clicked the button')
        .one('You clicked it once')
        .two('You clicked a couple times')
        .many('You clicked %d times')
        .times(12, 'You clicked a dozen times'),
    'pt_br': 'Você clicou o botão %d vezes'
        .zero('Você não clicou no botão')
        .one('Você clicou uma única vez')
        .two('Você clicou um par de vezes')
        .many('Você clicou %d vezes')
        .times(12, 'Você clicou uma dúzia de vezes'),

String plural(value) => localizePlural(value, this, _t);

Or, if you want to define your translations by locale:

static var _t = Translations.byLocale('en_us') +
      'en_us': {
        'You clicked the button %d times': 
          'You clicked the button %d times'
            .zero('You haven't clicked the button')
            .one('You clicked it once')
            .two('You clicked a couple times')
            .many('You clicked %d times')
            .times(12, 'You clicked a dozen times'),
      'pt_br': {
        'You clicked the button %d times': 
          'Você clicou o botão %d vezes'
            .zero('Você não clicou no botão')
            .one('Você clicou uma única vez')
            .two('Você clicou um par de vezes')
            .many('Você clicou %d vezes')
            .times(12, 'Você clicou uma dúzia de vezes'),

The plural modifiers you can use are zero, one, two, three, four, five, six, ten, times (for any number of elements, except 0, 1 and 2), many (for any number of elements, except 1, including 0), zeroOne (for 0 or 1 elements), and oneOrMore (for 1 and more elements).

Also, according to a Czech speaker, there must be a special modifier for 2, 3 and 4. To that end you can use the twoThreeFour modifier.

Note: It will use the most specific plural modifier. For example, .two is more specific than .many. If no applicable modifier can be found, it will default to the unversioned string. For example, this: 'a'.zero('b').four('c') will default to "a" for 1, 2, 3, or more than 5 elements.

Note: The .plural() method actually accepts any Object?, not only an integer number. In case it's not an integer, it will be converted into an integer. The rules are:

  1. If the modifier is an int, its absolute value will be used (meaning a negative value will become positive).
  2. If the modifier is a double, its absolute value will be used, like so: 1.0 will be 1; Values below 1.0 will become 0; Values larger than 1.0 will be rounded up.
  3. Strings will be converted to int, or if that fails to double. Conversion is done like so: First, it will discard other
 chars than numbers, dot and the minus sign, by converting them to spaces; Then it will convert
 using `int.tryParse`; Then it will convert using `double.tryParse`; If all fails, it will
 be zero.
  1. Other objects will be converted to a string (using the toString method), and then the above rules will apply.

Custom modifiers #

You can actually create any modifiers you want. For example, some languages have different translations for different genders. So you could create .gender() that accepts Gender modifiers:

enum Gender {they, female, male}

int gnd = Gender.female;
return Text('There is a person'.gender(gnd));

Then, your translations file should use .modifier() and localizeVersion() like this:

static var _t = Translations.byText('en_us') +
    'en_us': 'There is a person'
        .modifier(Gender.male, 'There is a man')
        .modifier(Gender.female, 'There is a woman')
        .modifier(Gender.they, 'There is a person'),
    'pt_br': 'Há uma pessoa'
        .modifier(Gender.male, 'Há um homem')
        .modifier(Gender.female, 'Há uma mulher')
        .modifier(Gender.they, 'Há uma pessoa'),

String gender(Gender gnd) => localizeVersion(gnd, this, _t);

Interpolation #

Your translations file may declare a fill method to do interpolations:

static var _t = Translations.byText('en_us') +
    'en_us': 'Hello %s, this is %s',
    'pt_br': 'Olá %s, aqui é %s',

String get i18n => localize(this, _t);

String fill(List<Object> params) => localizeFill(this, params);

Then you may use it like this:

print('Hello %s, this is %s'.i18n.fill(['John', 'Mary']));

The above code will print Hello John, this is Mary if the locale is English, or Olá John, aqui é Mary if it's Portuguese.

It uses the sprintf package internally. I don't know how closely it follows the C sprintf specification, but here it is.

Direct use of translation objects #

If you have a translation object you can use the localize function directly to perform translations:

var translations = Translations.byText('en_us') +
      'en_us': 'Hi',
      'pt_br': 'Olá',

// Prints 'Hi'.
print(localize('Hi', translations, locale: 'en_us');

// Prints 'Olá'.
print(localize('Hi', translations, locale: 'pt_br');

// Prints 'Hi'.
print(localize('Hi', translations, locale: 'not valid');

Changing the current locale #

To change the current locale, do this:

I18n.of(context).locale = Locale('pt', 'BR');

To return the current locale to the system default, do this:

I18n.of(context).locale = null;

Note: The above will change the current locale only for the i18n_extension, and not for Flutter as a whole.

Reading the current locale #

To read the current locale, do this:

// Both ways work:
Locale locale = I18n.of(context).locale;
Locale locale = I18n.locale;

// Or get the locale as a lowercase string. Example: 'en_us'.
String localeStr = I18n.localeStr;

// Or get the language of the locale, lowercase. Example: 'en'.
String language = I18n.language;

Observing locale changes #

There is a global static callback you can use to observe locale changes:

I18n.observeLocale = 
  ({required Locale oldLocale, required Locale newLocale}) 
      => print('Changed from $oldLocale to $newLocale.');

Const Translations #

The ConstTranslations class allows you to define the translations as a const object, all at once. This not only is a little bit more efficient, but it's also better for "hot reload", since a const variable will respond to hot reloads, while final variables will not. Here you provide all locale translations of the first translatable string, then all locale translations of the second one, and so on:

static const _t = ConstTranslations('en_us',
     'i18n Demo': {
       'en_us': 'i18n Demo',
       'pt_br': 'Demonstração i18n',
     'Some text': {
       'en_us': 'Some text',
       'pt_br': 'Algum texto',

IMPORTANT: Make sure the locales you provide are correct (no spaces, lowercase etc). Since this constructor is const, the package can't normalize the locales for you. If you are not sure, call ConstTranslations.normalizeLocale(locale) on the locale before using it.

Unfortunately, the ConstTranslations class is not as flexible as the Translations class, as you can't define modifiers like plural() etc with it. This limits its usefulness.

Dart-only package #

This i18n_extension Flutter package depends on the Dart-only package i18n_extension_core.

If you are creating code for a Dart server (backend) like Celest, or developing some Dart-only package yourself that does not depend on Flutter, then you can use the i18n_extension_core package directly:

import 'package:i18n_extension_core/i18n_extension_core.dart';

extension Localization on String {
   static var t = Translations.byText('en_us') + {'en_us':'Hello', 'pt_br':'Olá'};
   String get i18n => localize(this, t);

expect('Hello'.i18n, 'Hola');

The only important difference is that you must use DefaultLocale.set() instead of I18n.of(context).locale = ... to set the locale. And you won't have access and won't need to use the i18n widget, obviously.

Importing and exporting #

The i18n_extension package is optimized so that you can easily create and manage all of your translations yourself, by hand.

However, for large projects with big teams you probably need to follow a more involved process:

  • Export all your translatable strings to files in some external format your professional translator, or your crowdsourcing tool uses (see formats below).

  • Continue developing your app while waiting for the translations.

  • Import the translation files into the project and test the app in each language you added.

  • Repeat the process as needed, translating just the changes between each app revision. As necessary, perform additional localization steps yourself.

Formats #

The following formats may be used with translations:

Importing #

Up to version 8.0.0, the i18n_extension package contained the importer library. It has now been separated and is now independently available as a standalone package. You can find it at: https://pub.dev/packages/i18n_extension_importer.

Note: Those importers were contributed by Johann Bauer, and were separated into their own package by Xiang Li.

Currently, only .PO and .JSON importers are supported out-of-the-box. If you want to help creating importers for any of the other formats above, please PR there.

It also includes the GetStrings exporting utility, which is a useful script designed to automate the export of all translatable strings from your project.

Add your translation files as assets to your app in a directory structure like this:

 \_ assets
    \_ locales
       \_ de.po
       \_ fr.po

Then you can import them using GettextImporter or JSONImporter:

import 'package:i18n_extension_importer/io/import.dart';
import 'package:i18n_extension/i18n_extension.dart';

class MyI18n {
  static var translations = Translations.byLocale('en');

  static Future<void> loadTranslations() async {
    translations +=
        await GettextImporter().fromAssetDirectory('assets/locales');

extension Localization on String {
  String get i18n => localize(this, MyI18n.translations);
  String plural(value) => localizePlural(value, this, MyI18n.translations);
  String fill(List<Object> params) => localizeFill(this, params);

For usage in main.dart, see here.

Note: When using .po files, make sure not to include the country code, because the locales are generated from the filenames which don't contain the country code and if you'd include the country codes, you'll get errors like this: There are no translations in 'en_us' for 'Hello there'.

Note: If you need to import any other custom format, remember importing is easy to do because the Translation constructors use maps as input. If you can generate a map from your file format, you can then use the Translation() or Translation.byLocale() constructors to create the translation objects.

The GetStrings exporting utility #

A utility script to automatically export all translatable strings from your project was also contributed by Johann Bauer.

Simply run flutter pub run i18n_extension_importer:getstrings in your project root directory, and you will get a list of strings to translate in strings.json. This file can then be sent to your translators or be imported in translation services like Crowdin, Transifex or Lokalise. You can use it as part of your CI pipeline in order to always have your translation templates up to date.

Note the tool simply searches the source code for strings to which getters like .i18n are applied. Since it is not very smart, you should not make it too hard:

print('Hello World!'.i18n); // This would work.

// But the tool would not be able to spot this 
// since it doesn't execute the code.
var x = 'Hello World';

Other ways to export #

As previously discussed, i18n_extension will automatically list all keys into a map if you use some unknown locale, run the app, and manually or automatically go through all the screens. For example, create a Greek locale if your app doesn't have Greek translations, and it will list all keys into Translations.missingTranslationCallback.

Then you can read from this map and create your exported file. There is also this package that goes through all screens automatically.


Q: Do I need to maintain the translation files as Dart files?

A: Not really. You do have a Dart file that creates a Translation object, yes, and this object is optimized for easily creating translations by hand. But it creates them from maps. So if you can create maps from some file you can use that file. For example, a simple code generator that reads .json und outputs Dart maps would do the job: var _t = Translations.byText('en_us') + readFromJson('myfile.json').

Q: How do you handle changing the locale? Does the I18n class pick up changes to the locale automatically or would you have to restart the app?

A: It should pick changes to the locale automatically. Also, you can change the locale manually at any time by doing I18n.of(context).locale = Locale('pt', 'BR');.

Q: What's the point of importing 'default.i18n.dart'?

A: This is the default file to import from your widgets. It lets the developer add .i18n to any strings they want to mark as being a "translatable string". Later, someone will have to remove this default file and add another one with the translations. You basically just change the import later. The point of importing 'default.i18n.dart' before you create the translations for that widget is that it will record them as missing translations, so that you don't forget to add those translations later.

Q: Can I do translations outside of widgets?

A: Yes, since you don't need access to context. It actually reads the current locale from I18n.locale, which is static, and all the rest is done with pure Dart code. So you can translate anything you want, from any code you want. You can also define a locale on the fly if you want to do translations to a locale different from the current one.

Q: By using identifier keys like howAreYou, I know that there's a localization key named howAreYou because otherwise my code wouldn't compile. There is no way to statically verify that 'How are you?'.i18n will do what I want it to do.

A: i18n_extension lets you decide if you want to use identifier keys like howAreYou or not. Not having to use those was one thing I was trying to achieve. I hate having to come up with these keys. I found that the developer should just type the text they want and be done with it. In other words, in i18n_extension you don't need to type a key; you may type the text itself (in your default language). So there is no need to statically verify anything. Your code will always compile when you type a String, and that exact string will be used for your default language. It will never break.

Q: But how can I statically verify that a string has translations? Just showing the translatable string as defined in the source code will not hide that some translations are missing?

A: You can statically verify that a string should have translations because it has .i18n attached to it. What you can't do is statically verify that those translations were actually provided for all supported languages. But this is also the case when you use older methods. With the older methods you also just know it should have translations, because it has a translation key, but the translation itself may be missing, or worse yet, outdated. With i18n_extension at least you know that the translation to the default language exists and is not outdated.

Q: What happens if a developer tries to call i18n on a string without translations, wouldn't that be harder to catch?

A: With i18n_extension you can generate a report with all missing translations, and you can even add those checks to tests. In other words, you can just freely modify any translatable string, and before your launch you get the reports and fix all the translations.

Q: There are a lot of valid usages for String that don't deal with user-facing messages. I like to use auto-complete to see what methods are available (by typing someString.), and seeing loads of unrelated extension methods in there could be annoying.

A: The translation extension is contained in your widget file. You won't have this extension in scope for your business classes, for example. So .i18n will only appear in your auto-complete inside of your widget classes, where it makes sense.

Q: Do I actually need one .i18n.dart (a translations file) per widget?

A: No you don't. It's suggested that you create a translation file per widget if you are doing translations by hand, but that's not a requirement. The reason I think separate files is a good idea is that sometimes internationalization is not only translations. You may need to format dates in specific ways, or make complex functions to create specific strings that depend on variables etc. So in these cases you will probably need somewhere to put this code. In any case, to make translations work all you need a Translation object which you can create in many ways, by adding maps to it using the + operator, or by adding other translation objects together using the * operator. You can create this Translation objects anywhere you want, in a single file per widget, in a single file for many widgets, or in a single file for the whole app. Also, if you are not doing translations by hand but importing strings from translation files, then you don't even need a separate file. You can just add extension Localization on String { String get i18n => localize(this, Translations.byText('en_us') + load('file.json')); } to your own widget file.

Q: Won't having multiple files with extension Localization lead to people importing the wrong file and have translations missing?

A: The package records all your missing translations, and you can also easily log or throw an exception if they are missing. So you will know if you import the wrong file. You can also add this reports to your unit tests. It will let you know even if you import the right file and translations are missing in some language, and it will let you know even if you import from .arb files and translations are missing in some language.

Q: Are there importers for X?

A: Currently, only .PO and .JSON importers are supported out-of-the-box. Keep in mind this lib development is still new, and I hope the community will help writing more importers/exporters. We hope to have those for .arb .icu .xliff .csv and .yaml, but we're not there yet. However, since the Translations object use maps as input/output, you can use whatever file you want if you convert them to a map yourself.

Q: How does it report missing translations?

A: _At the moment you should just print Translations.missingKeys and Translations.missingTranslations. We'll later create a Translations.printReport() function that correlates these two pieces of information and outputs a more readable report.

Q: The package says it's "Non-boilerplate", but doesn't .i18n.dart contain boilerplate?

A: The only necessary boilerplate for .i18n.dart files is static var _t = Translations.byText('...') + and String get i18n => localize(this, _t);. The rest are the translations themselves. So, yeah, it's not completely without boilerplate, but saying "Less-boilerplate" is not that catchy.

The Flutter packages I've authored:

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Marcelo Glasberg:

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Translation and Internationalization (i18n) for Flutter. Easy to use for both large and small projects. Uses Dart extensions to reduce boilerplate.

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#internationalization #localization #translation #i18n #extension


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flutter, i18n_extension_core, intl


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