In Flutter 3.47, what does it take to set up gen-l10n so .arb files become an AppLocalizations class wired into MaterialApp?

## The moving parts Flutter's built-in localization pipeline has three layers, and setting it up means touching each one: - **`flutter_localizations`**, a package shipped inside the Flutter SDK, which carries translated strings for Material and Cupertino widgets (date pickers, back-button tooltips, text-selection menus) in many languages, plus the text direction for each language. - **`intl`**, the Dart package whose `Intl.pluralLogic`, `Intl.selectLogic`, `NumberFormat` and `DateFormat` the generated code calls. - **gen-l10n**, the code generator inside the `flutter` tool, which reads your **App Resource Bundle** (`.arb`) files and writes the `AppLocalizations` class. ## Step by step 1. **Dependencies.** `flutter pub add flutter_localizations:"{sdk: flutter}" intl:any`, or edit `pubspec.yaml` by hand so it lists `flutter_localizations: {sdk: flutter}` and `intl`. 2. **The generate flag.** Under the `flutter:` section, set `generate: true`. The tool checks this before generating anything; without it, `flutter gen-l10n` stops with *"Attempted to generate localizations code without having the flutter: generate flag turned on."* 3. **`l10n.yaml`.** Create it next to `pubspec.yaml`. Its presence is also what makes `flutter pub get`, `flutter run` and hot reload regenerate the code automatically: the build step that runs gen-l10n is skipped when the file is missing. 4. **ARB files.** Put the template (`app_en.arb`) and one file per language (`app_de.arb`, `app_pl.arb`) in the `arb-dir`. The locale comes from the file-name suffix or from an `@@locale` key. 5. **Wire the app.** Import the generated file and pass its lists to `MaterialApp` (or `CupertinoApp`, or `WidgetsApp`). ## The `l10n.yaml` keys and their defaults | Key | Default | Meaning | |---|---|---| | `arb-dir` | `lib/l10n` | Where the template and translated `.arb` files live | | `template-arb-file` | `app_en.arb` | The file whose messages and metadata define the API | | `output-localization-file` | `app_localizations.dart` | Name of the main generated file | | `output-class` | `AppLocalizations` | Name of the generated class | | `output-dir` | same as `arb-dir` | Where the generated Dart is written | | `nullable-getter` | `true` | Whether `of(context)` returns a nullable type | | `untranslated-messages-file` | none | JSON report of messages missing per locale | When `l10n.yaml` exists, `flutter gen-l10n` ignores its command-line options and says so; delete the file if you want to drive the tool purely from flags. ## Where the output goes (and what changed in 3.32) Before Flutter 3.32 the tool generated a **synthetic package**, imported as `package:flutter_gen/gen_l10n/app_localizations.dart`, by rewriting the app's `package_config.json`. Flutter 3.32 made generation into source the default and required `generate: true`; the synthetic package has since been removed. In Flutter 3.47: - the generated files are ordinary Dart files in `arb-dir` (or `output-dir`), imported like any other file, for example `import 'l10n/app_localizations.dart';`; - `synthetic-package: true` in `l10n.yaml` is a tool error, and `synthetic-package: false` only prints a warning that the key no longer does anything. Because the files are real source, teams either commit them (reviewers see the generated API change) or ignore them and rely on `flutter pub get` regenerating them in CI. Pick one and apply it consistently. ## Wiring `MaterialApp` The generated class carries two ready-made lists: - **`AppLocalizations.localizationsDelegates`**: your `AppLocalizations.delegate` followed by `GlobalMaterialLocalizations.delegate`, `GlobalCupertinoLocalizations.delegate` and `GlobalWidgetsLocalizations.delegate`. - **`AppLocalizations.supportedLocales`**: one `Locale` per ARB file, in alphabetical order unless `preferred-supported-locales` reorders it. Passing both keeps the app's own strings and the framework's strings in step: every language you ship an ARB file for also gets translated Material widgets and the right text direction. If you need extra delegates, spread the list and append: `[...AppLocalizations.localizationsDelegates, MyExtraDelegate()]`. ## Day-to-day regeneration Once `generate: true` and `l10n.yaml` are in place, you rarely run the generator by hand: - `flutter pub get` regenerates the classes after dependency resolution. - `flutter run` regenerates them before the first build, and hot reload and hot restart run the same source generators again, so an edited ARB string appears without restarting the tool. - `flutter gen-l10n` remains useful in CI and when you want the console summary of untranslated messages without launching an app. A syntax error in an ARB file (an unbalanced brace, a plural without `other`) stops generation with the file name, message id and position, so the error appears before any Dart compiles against a stale class. ## Checking it works - Run `flutter gen-l10n` once; it reports untranslated messages per locale on the console. - Switch the device language to German and Polish and confirm both your strings and Material widgets such as a date picker change language. - On iOS, also list the supported languages in the Xcode project's localizations so the store listing shows them; Flutter's own localization does not do that for you.