packages/flutter/lib/src/widgets/localizations.dart

// Copyright 2014 The Flutter Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/// @docImport 'package:flutter/material.dart';
/// @docImport 'package:flutter_localizations/flutter_localizations.dart';
///
/// @docImport 'app.dart';
/// @docImport 'reorderable_list.dart';
library;

import 'package:flutter/foundation.dart';
import 'package:flutter/rendering.dart';

import 'app.dart';
import 'basic.dart';
import 'binding.dart';
import 'debug.dart';
import 'framework.dart';

// Examples can assume:
// class Intl { Intl._(); static String message(String s, { String? name, String? locale }) => ''; }
// Future<void> initializeMessages(String locale) => Future<void>.value();
// late BuildContext context;
// class Foo { }
// const Widget myWidget = Placeholder();

// Used by loadAll() to record LocalizationsDelegate.load() futures we're
// waiting for.
class _Pending {
  _Pending(this.delegate, this.futureValue);
  final LocalizationsDelegate<dynamic> delegate;
  final Future<dynamic> futureValue;
}

// A utility function used by Localizations to generate one future
// that completes when all of the LocalizationsDelegate.load() futures
// complete. The returned map is indexed by each delegate's type.
//
// The input future values must have distinct types.
//
// The returned Future<Map> will resolve when all of the input map's
// future values have resolved. If all of the input map's values are
// SynchronousFutures then a SynchronousFuture will be returned
// immediately.
//
// This is more complicated than just applying Future.wait to input
// because some of the input.values may be SynchronousFutures. We don't want
// to Future.wait for the synchronous futures.
Future<Map<Type, dynamic>> _loadAll(
  Locale locale,
  Iterable<LocalizationsDelegate<dynamic>> allDelegates,
) {
  final output = <Type, dynamic>{};
  List<_Pending>? pendingList;

  // Only load the first delegate for each delegate type that supports
  // locale.languageCode.
  final types = <Type>{};
  final delegates = <LocalizationsDelegate<dynamic>>[];
  for (final delegate in allDelegates) {
    if (!types.contains(delegate.type) && delegate.isSupported(locale)) {
      types.add(delegate.type);
      delegates.add(delegate);
    }
  }

  for (final delegate in delegates) {
    final Future<dynamic> inputValue = delegate.load(locale);
    dynamic completedValue;
    final Future<dynamic> futureValue = inputValue.then<dynamic>((dynamic value) {
      return completedValue = value;
    });
    if (completedValue != null) {
      // inputValue was a SynchronousFuture
      final Type type = delegate.type;
      assert(!output.containsKey(type));
      output[type] = completedValue;
    } else {
      pendingList ??= <_Pending>[];
      pendingList.add(_Pending(delegate, futureValue));
    }
  }

  // All of the delegate.load() values were synchronous futures, we're done.
  if (pendingList == null) {
    return SynchronousFuture<Map<Type, dynamic>>(output);
  }

  // Some of delegate.load() values were asynchronous futures. Wait for them.
  return Future.wait<dynamic>(
    pendingList.map<Future<dynamic>>((_Pending p) => p.futureValue),
  ).then<Map<Type, dynamic>>((List<dynamic> values) {
    assert(values.length == pendingList!.length);
    for (var i = 0; i < values.length; i += 1) {
      final Type type = pendingList![i].delegate.type;
      assert(!output.containsKey(type));
      output[type] = values[i];
    }
    return output;
  });
}

/// A factory for a set of localized resources of type `T`, to be loaded by a
/// [Localizations] widget.
///
/// Typical applications have one [Localizations] widget which is created by the
/// [WidgetsApp] and configured with the app's `localizationsDelegates`
/// parameter (a list of delegates). The delegate's [type] is used to identify
/// the object created by an individual delegate's [load] method.
///
/// An example of a class used as the value of `T` here would be
/// [MaterialLocalizations].
abstract class LocalizationsDelegate<T> {
  /// Abstract const constructor. This constructor enables subclasses to provide
  /// const constructors so that they can be used in const expressions.
  const LocalizationsDelegate();

  /// Whether resources for the given locale can be loaded by this delegate.
  ///
  /// Return true if the instance of `T` loaded by this delegate's [load]
  /// method supports the given `locale`'s language.
  bool isSupported(Locale locale);

  /// Start loading the resources for `locale`. The returned future completes
  /// when the resources have finished loading.
  ///
  /// It's assumed that this method will return an object that contains a
  /// collection of related string resources (typically defined with one method
  /// per resource). The object will be retrieved with [Localizations.of].
  Future<T> load(Locale locale);

  /// Returns true if the resources for this delegate should be loaded
  /// again by calling the [load] method.
  ///
  /// This method is called whenever its [Localizations] widget is
  /// rebuilt. If it returns true then dependent widgets will be rebuilt
  /// after [load] has completed.
  bool shouldReload(covariant LocalizationsDelegate<T> old);

  /// The type of the object returned by the [load] method, T by default.
  ///
  /// This type is used to retrieve the object "loaded" by this
  /// [LocalizationsDelegate] from the [Localizations] inherited widget.
  /// For example the object loaded by `LocalizationsDelegate<Foo>` would
  /// be retrieved with:
  ///
  /// ```dart
  /// Foo foo = Localizations.of<Foo>(context, Foo)!;
  /// ```
  ///
  /// It's rarely necessary to override this getter.
  Type get type => T;

  @override
  String toString() => '${objectRuntimeType(this, 'LocalizationsDelegate')}[$type]';
}

/// Interface for localized resource values for the lowest levels of the Flutter
/// framework.
///
/// This class also maps locales to a specific [Directionality] using the
/// [textDirection] property.
///
/// See also:
///
///  * [DefaultWidgetsLocalizations], which implements this interface and
///    supports a variety of locales.
abstract class WidgetsLocalizations {
  /// The reading direction for text in this locale.
  TextDirection get textDirection;

  /// The semantics label used for [SliverReorderableList] to reorder an item in the
  /// list to the start of the list.
  String get reorderItemToStart;

  /// The semantics label used for [SliverReorderableList] to reorder an item in the
  /// list to the end of the list.
  String get reorderItemToEnd;

  /// The semantics label used for [SliverReorderableList] to reorder an item in the
  /// list one space up the list.
  String get reorderItemUp;

  /// The semantics label used for [SliverReorderableList] to reorder an item in the
  /// list one space down the list.
  String get reorderItemDown;

  /// The semantics label used for [SliverReorderableList] to reorder an item in the
  /// list one space left in the list.
  String get reorderItemLeft;

  /// The semantics label used for [SliverReorderableList] to reorder an item in the
  /// list one space right in the list.
  String get reorderItemRight;

  /// The semantics label used for [RawAutocomplete] when the options list goes
  /// from empty to non-empty.
  String get searchResultsFound => 'Search results found';

  /// The semantics label used for [RawAutocomplete] when the options list goes
  /// from non-empty to empty.
  String get noResultsFound => 'No results found';

  /// Label for "copy" edit buttons and menu items.
  String get copyButtonLabel;

  /// Label for "cut" edit buttons and menu items.
  String get cutButtonLabel;

  /// Label for "paste" edit buttons and menu items.
  String get pasteButtonLabel;

  /// Label for "select all" edit buttons and menu items.
  String get selectAllButtonLabel;

  /// Label for "look up" edit buttons and menu items.
  String get lookUpButtonLabel;

  /// Label for "search web" edit buttons and menu items.
  String get searchWebButtonLabel;

  /// Label for "share" edit buttons and menu items.
  String get shareButtonLabel;

  /// The accessibility hint for an unselected radio button.
  String get radioButtonUnselectedLabel;

  /// The `WidgetsLocalizations` from the closest [Localizations] instance
  /// that encloses the given context.
  ///
  /// This method is just a convenient shorthand for:
  /// `Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations)!`.
  ///
  /// References to the localized resources defined by this class are typically
  /// written in terms of this method. For example:
  ///
  /// ```dart
  /// textDirection: WidgetsLocalizations.of(context).textDirection,
  /// ```
  static WidgetsLocalizations of(BuildContext context) {
    assert(debugCheckHasWidgetsLocalizations(context));
    return Localizations.of<WidgetsLocalizations>(context, WidgetsLocalizations)!;
  }
}

class _WidgetsLocalizationsDelegate extends LocalizationsDelegate<WidgetsLocalizations> {
  const _WidgetsLocalizationsDelegate();

  // This is convenient simplification. It would be more correct test if the locale's
  // text-direction is LTR.
  @override
  bool isSupported(Locale locale) => true;

  @override
  Future<WidgetsLocalizations> load(Locale locale) => DefaultWidgetsLocalizations.load(locale);

  @override
  bool shouldReload(_WidgetsLocalizationsDelegate old) => false;

  @override
  String toString() => 'DefaultWidgetsLocalizations.delegate(en_US)';
}

/// US English localizations for the widgets library.
///
/// See also:
///
///  * [GlobalWidgetsLocalizations], which provides widgets localizations for
///    many languages.
///  * [WidgetsApp.localizationsDelegates], which automatically includes
///    [DefaultWidgetsLocalizations.delegate] by default.
class DefaultWidgetsLocalizations implements WidgetsLocalizations {
  /// Construct an object that defines the localized values for the widgets
  /// library for US English (only).
  ///
  /// [LocalizationsDelegate] implementations typically call the static [load]
  const DefaultWidgetsLocalizations();

  @override
  String get reorderItemUp => 'Move up';

  @override
  String get reorderItemDown => 'Move down';

  @override
  String get reorderItemLeft => 'Move left';

  @override
  String get reorderItemRight => 'Move right';

  @override
  String get reorderItemToEnd => 'Move to the end';

  @override
  String get reorderItemToStart => 'Move to the start';

  @override
  String get searchResultsFound => 'Search results found';

  @override
  String get noResultsFound => 'No results found';

  @override
  String get copyButtonLabel => 'Copy';

  @override
  String get cutButtonLabel => 'Cut';

  @override
  String get pasteButtonLabel => 'Paste';

  @override
  String get selectAllButtonLabel => 'Select all';

  @override
  String get lookUpButtonLabel => 'Look Up';

  @override
  String get searchWebButtonLabel => 'Search Web';

  @override
  String get shareButtonLabel => 'Share';

  @override
  String get radioButtonUnselectedLabel => 'Not selected';
  @override
  TextDirection get textDirection => TextDirection.ltr;

  /// Creates an object that provides US English resource values for the
  /// lowest levels of the widgets library.
  ///
  /// The [locale] parameter is ignored.
  ///
  /// This method is typically used to create a [LocalizationsDelegate].
  /// The [WidgetsApp] does so by default.
  static Future<WidgetsLocalizations> load(Locale locale) {
    return SynchronousFuture<WidgetsLocalizations>(const DefaultWidgetsLocalizations());
  }

  /// A [LocalizationsDelegate] that uses [DefaultWidgetsLocalizations.load]
  /// to create an instance of this class.
  ///
  /// [WidgetsApp] automatically adds this value to [WidgetsApp.localizationsDelegates].
  static const LocalizationsDelegate<WidgetsLocalizations> delegate =
      _WidgetsLocalizationsDelegate();
}

class _LocalizationsScope extends InheritedWidget {
  const _LocalizationsScope({
    super.key,
    required this.locale,
    required this.localizationsState,
    required this.typeToResources,
    required super.child,
  });

  final Locale locale;
  final _LocalizationsState localizationsState;
  final Map<Type, dynamic> typeToResources;

  @override
  bool updateShouldNotify(_LocalizationsScope old) {
    return typeToResources != old.typeToResources;
  }
}

/// Defines the [Locale] for its `child` and the localized resources that the
/// child depends on.
///
/// ## Defining localized resources
///
/// {@tool snippet}
///
/// This following class is defined in terms of the
/// [Dart `intl` package](https://github.com/dart-lang/i18n/tree/main/pkgs/intl). Using the `intl`
/// package isn't required.
///
/// ```dart
/// class MyLocalizations {
///   MyLocalizations(this.locale);
///
///   final Locale locale;
///
///   static Future<MyLocalizations> load(Locale locale) {
///     return initializeMessages(locale.toString())
///       .then((void _) {
///         return MyLocalizations(locale);
///       });
///   }
///
///   static MyLocalizations of(BuildContext context) {
///     return Localizations.of<MyLocalizations>(context, MyLocalizations)!;
///   }
///
///   String title() => Intl.message('<title>', name: 'title', locale: locale.toString());
///   // ... more Intl.message() methods like title()
/// }
/// ```
/// {@end-tool}
/// 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
/// `MyLocalizations` class.
///
/// One could choose another approach for loading localized resources and looking them up while
/// still conforming to the structure of this example.
///
/// ## Loading localized resources
///
/// Localized resources are loaded by the list of [LocalizationsDelegate]
/// `delegates`. Each delegate is essentially a factory for a collection
/// of localized resources. There are multiple delegates because there are
/// multiple sources for localizations within an app.
///
/// Delegates are typically simple subclasses of [LocalizationsDelegate] that
/// override [LocalizationsDelegate.load]. For example a delegate for the
/// `MyLocalizations` class defined above would be:
///
/// ```dart
/// // continuing from previous example...
/// class _MyDelegate extends LocalizationsDelegate<MyLocalizations> {
///   @override
///   Future<MyLocalizations> load(Locale locale) => MyLocalizations.load(locale);
///
///   @override
///   bool isSupported(Locale locale) {
///     // in a real implementation this would only return true for
///     // locales that are definitely supported.
///     return true;
///   }
///
///   @override
///   bool shouldReload(_MyDelegate old) => false;
/// }
/// ```
///
/// Each delegate can be viewed as a factory for objects that encapsulate a set
/// of localized resources. These objects are retrieved with
/// by runtime type with [Localizations.of].
///
/// The [WidgetsApp] class creates a [Localizations] widget so most apps
/// will not need to create one. The widget app's [Localizations] delegates can
/// be initialized with [WidgetsApp.localizationsDelegates]. The [MaterialApp]
/// class also provides a `localizationsDelegates` parameter that's just
/// passed along to the [WidgetsApp].
///
/// ## Obtaining localized resources for use in user interfaces
///
/// Apps should retrieve collections of localized resources with
/// `Localizations.of<MyLocalizations>(context, MyLocalizations)`,
/// where MyLocalizations is an app specific class defines one function per
/// resource. This is conventionally done by a static `.of` method on the
/// custom localized resource class (`MyLocalizations` in the example above).
///
/// For example, using the `MyLocalizations` class defined above, one would
/// lookup a localized title string like this:
///
/// ```dart
/// // continuing from previous example...
/// MyLocalizations.of(context).title()
/// ```
///
/// If [Localizations] were to be rebuilt with a new `locale` then
/// the widget subtree that corresponds to [BuildContext] `context` would
/// be rebuilt after the corresponding resources had been loaded.
///
/// This class is effectively an [InheritedWidget]. If it's rebuilt with
/// a new `locale` or a different list of delegates or any of its
/// delegates' [LocalizationsDelegate.shouldReload()] methods returns true,
/// then widgets that have created a dependency by calling
/// `Localizations.of(context)` will be rebuilt after the resources
/// for the new locale have been loaded.
///
/// The [Localizations] widget also instantiates [Directionality] in order to
/// support the appropriate [Directionality.textDirection] of the localized
/// resources.
class Localizations extends StatefulWidget {
  /// Create a widget from which localizations (like translated strings) can be obtained.
  Localizations({
    super.key,
    required this.locale,
    required this.delegates,
    this.child,
    this.isApplicationLevel = false,
  }) : assert(
         delegates.any(
           (LocalizationsDelegate<dynamic> delegate) =>
               delegate is LocalizationsDelegate<WidgetsLocalizations>,
         ),
       );

  /// Overrides the inherited [Locale] or [LocalizationsDelegate]s for `child`.
  ///
  /// This factory constructor is used for the (usually rare) situation where part
  /// of an app should be localized for a different locale than the one defined
  /// for the device, or if its localizations should come from a different list
  /// of [LocalizationsDelegate]s than the list defined by
  /// [WidgetsApp.localizationsDelegates].
  ///
  /// For example you could specify that `myWidget` was only to be localized for
  /// the US English locale:
  ///
  /// ```dart
  /// Widget build(BuildContext context) {
  ///   return Localizations.override(
  ///     context: context,
  ///     locale: const Locale('en', 'US'),
  ///     child: myWidget,
  ///   );
  /// }
  /// ```
  ///
  /// The `locale` and `delegates` parameters default to the [Localizations.locale]
  /// and [Localizations.delegates] values from the nearest [Localizations] ancestor.
  ///
  /// To override the [Localizations.locale] or [Localizations.delegates] for an
  /// entire app, specify [WidgetsApp.locale] or [WidgetsApp.localizationsDelegates]
  /// (or specify the same parameters for [MaterialApp]).
  factory Localizations.override({
    Key? key,
    required BuildContext context,
    Locale? locale,
    List<LocalizationsDelegate<dynamic>>? delegates,
    Widget? child,
  }) {
    final List<LocalizationsDelegate<dynamic>> mergedDelegates = Localizations._delegatesOf(
      context,
    );
    if (delegates != null) {
      mergedDelegates.insertAll(0, delegates);
    }
    return Localizations(
      key: key,
      locale: locale ?? Localizations.localeOf(context),
      delegates: mergedDelegates,
      child: child,
    );
  }

  /// The resources returned by [Localizations.of] will be specific to this locale.
  final Locale locale;

  /// This list collectively defines the localized resources objects that can
  /// be retrieved with [Localizations.of].
  final List<LocalizationsDelegate<dynamic>> delegates;

  /// The widget below this widget in the tree.
  ///
  /// {@macro flutter.widgets.ProxyWidget.child}
  final Widget? child;

  /// Whether this is the main localizations widget that represents the app's
  /// locale.
  final bool isApplicationLevel;

  /// The locale of the Localizations widget for the widget tree that
  /// corresponds to [BuildContext] `context`.
  ///
  /// If no [Localizations] widget is in scope then the [Localizations.localeOf]
  /// method will throw an exception.
  static Locale localeOf(BuildContext context) {
    final _LocalizationsScope? scope = context
        .dependOnInheritedWidgetOfExactType<_LocalizationsScope>();
    assert(() {
      if (scope == null) {
        throw FlutterError(
          'Requested the Locale of a context that does not include a Localizations ancestor.\n'
          'To request the Locale, the context used to retrieve the Localizations widget must '
          'be that of a widget that is a descendant of a Localizations widget.',
        );
      }
      if (scope.localizationsState.locale == null) {
        throw FlutterError(
          'Localizations.localeOf found a Localizations widget that had a unexpected null locale.\n',
        );
      }
      return true;
    }());
    return scope!.localizationsState.locale!;
  }

  /// The locale of the Localizations widget for the widget tree that
  /// corresponds to [BuildContext] `context`.
  ///
  /// If no [Localizations] widget is in scope then this function will return
  /// null.
  static Locale? maybeLocaleOf(BuildContext context) {
    final _LocalizationsScope? scope = context
        .dependOnInheritedWidgetOfExactType<_LocalizationsScope>();
    return scope?.localizationsState.locale;
  }

  // There doesn't appear to be a need to make this public. See the
  // Localizations.override factory constructor.
  static List<LocalizationsDelegate<dynamic>> _delegatesOf(BuildContext context) {
    final _LocalizationsScope? scope = context
        .dependOnInheritedWidgetOfExactType<_LocalizationsScope>();
    assert(scope != null, 'a Localizations ancestor was not found');
    return List<LocalizationsDelegate<dynamic>>.of(scope!.localizationsState.widget.delegates);
  }

  /// Returns the localized resources object of the given `type` for the widget
  /// tree that corresponds to the given `context`.
  ///
  /// Returns null if no resources object of the given `type` exists within
  /// the given `context`.
  ///
  /// This method is typically used by a static factory method on the `type`
  /// class. For example Flutter's MaterialLocalizations class looks up Material
  /// resources with a method defined like this:
  ///
  /// ```dart
  /// static MaterialLocalizations of(BuildContext context) {
  ///   return Localizations.of<MaterialLocalizations>(context, MaterialLocalizations)!;
  /// }
  /// ```
  static T? of<T>(BuildContext context, Type type) {
    final _LocalizationsScope? scope = context
        .dependOnInheritedWidgetOfExactType<_LocalizationsScope>();
    return scope?.localizationsState.resourcesFor<T?>(type);
  }

  @override
  State<Localizations> createState() => _LocalizationsState();

  @override
  void debugFillProperties(DiagnosticPropertiesBuilder properties) {
    super.debugFillProperties(properties);
    properties.add(DiagnosticsProperty<Locale>('locale', locale));
    properties.add(IterableProperty<LocalizationsDelegate<dynamic>>('delegates', delegates));
  }
}

class _LocalizationsState extends State<Localizations> {
  final GlobalKey _localizedResourcesScopeKey = GlobalKey();
  Map<Type, dynamic> _typeToResources = <Type, dynamic>{};

  Locale? get locale => _locale;
  Locale? _locale;
  set locale(Locale? locale) {
    assert(locale != null);
    if (_locale == locale) {
      return;
    }
    WidgetsBinding.instance.platformDispatcher.setApplicationLocale(locale!);
    _locale = locale;
  }

  @override
  void initState() {
    super.initState();
    load(widget.locale);
  }

  bool _anyDelegatesShouldReload(Localizations old) {
    if (widget.delegates.length != old.delegates.length) {
      return true;
    }
    final List<LocalizationsDelegate<dynamic>> delegates = widget.delegates.toList();
    final List<LocalizationsDelegate<dynamic>> oldDelegates = old.delegates.toList();
    for (var i = 0; i < delegates.length; i += 1) {
      final LocalizationsDelegate<dynamic> delegate = delegates[i];
      final LocalizationsDelegate<dynamic> oldDelegate = oldDelegates[i];
      if (delegate.runtimeType != oldDelegate.runtimeType || delegate.shouldReload(oldDelegate)) {
        return true;
      }
    }
    return false;
  }

  @override
  void didUpdateWidget(Localizations old) {
    super.didUpdateWidget(old);
    if (widget.locale != old.locale || (_anyDelegatesShouldReload(old))) {
      load(widget.locale);
    }
  }

  void load(Locale locale) {
    final Iterable<LocalizationsDelegate<dynamic>> delegates = widget.delegates;
    if (delegates.isEmpty) {
      this.locale = locale;
      return;
    }

    Map<Type, dynamic>? typeToResources;
    final Future<Map<Type, dynamic>> typeToResourcesFuture = _loadAll(locale, delegates)
        .then<Map<Type, dynamic>>((Map<Type, dynamic> value) {
          return typeToResources = value;
        });

    if (typeToResources != null) {
      // All of the delegates' resources loaded synchronously.
      _typeToResources = typeToResources!;
      this.locale = locale;
    } else {
      // - Don't rebuild the dependent widgets until the resources for the new locale
      // have finished loading. Until then the old locale will continue to be used.
      // - If we're running at app startup time then defer reporting the first
      // "useful" frame until after the async load has completed.
      RendererBinding.instance.deferFirstFrame();
      typeToResourcesFuture.then<void>((Map<Type, dynamic> value) {
        if (mounted) {
          setState(() {
            _typeToResources = value;
            this.locale = locale;
          });
        }
        RendererBinding.instance.allowFirstFrame();
      });
    }
  }

  T resourcesFor<T>(Type type) {
    final resources = _typeToResources[type] as T;
    return resources;
  }

  TextDirection get _textDirection {
    final resources = _typeToResources[WidgetsLocalizations] as WidgetsLocalizations;
    return resources.textDirection;
  }

  @override
  Widget build(BuildContext context) {
    // TODO(chunhtai): notify engine about application locale if this is
    // application level locale.
    if (_locale == null) {
      return const SizedBox.shrink();
    }
    return Semantics(
      // If this is not application level, we need to explicit mark the
      // semantics subtree with the locale.
      localeForSubtree: widget.isApplicationLevel ? null : widget.locale,
      container: !widget.isApplicationLevel,
      textDirection: _textDirection,
      child: _LocalizationsScope(
        key: _localizedResourcesScopeKey,
        locale: _locale!,
        localizationsState: this,
        typeToResources: _typeToResources,
        child: Directionality(textDirection: _textDirection, child: widget.child!),
      ),
    );
  }
}

/// A helper class used to manage localization resolution.
///
/// See also:
///   * [WidgetsApp], which utilizes [LocalizationsResolver] to handle locales.
class LocalizationsResolver extends ChangeNotifier with WidgetsBindingObserver {
  /// Creates a [LocalizationsResolver] that determines the best-fit locale from the set of
  /// [supportedLocales].
  ///
  /// If provided, locale resolution will attempt to use [locale] as the current locale rather
  /// than the system locale.
  ///
  /// Locale resolution behavior can be overridden by providing [localeListResolutionCallback]
  /// or [localeResolutionCallback].
  ///
  /// The delegates set via [localizationsDelegates] collectively define all of the localized
  /// resources for a [Localizations] widget.
  ///
  /// See also:
  ///
  ///  * [LocalizationsResolver.localeListResolutionCallback] and
  ///    [LocalizationsResolver.localeResolutionCallback] for more details on locale resolution
  ///    behavior.
  ///  * [LocalizationsDelegate] for more details about providing localized resources to a
  ///    [Localizations] widget.
  LocalizationsResolver({
    required Iterable<Locale> supportedLocales,
    Locale? locale,
    LocaleListResolutionCallback? localeListResolutionCallback,
    LocaleResolutionCallback? localeResolutionCallback,
    Iterable<LocalizationsDelegate<Object?>>? localizationsDelegates,
  }) : _locale = locale,
       _localeListResolutionCallback = localeListResolutionCallback,
       _localeResolutionCallback = localeResolutionCallback,
       _localizationsDelegates = localizationsDelegates,
       _supportedLocales = supportedLocales {
    _resolvedLocale = _resolveLocales(
      WidgetsBinding.instance.platformDispatcher.locales,
      supportedLocales,
    );
    WidgetsBinding.instance.addObserver(this);
  }

  @override
  void dispose() {
    WidgetsBinding.instance.removeObserver(this);
    super.dispose();
  }

  /// Replace one or more of the properties used for localization resolution and re-resolve the
  /// locale.
  void update({
    required Locale? locale,
    required LocaleListResolutionCallback? localeListResolutionCallback,
    required LocaleResolutionCallback? localeResolutionCallback,
    required Iterable<LocalizationsDelegate<Object?>>? localizationsDelegates,
    required Iterable<Locale> supportedLocales,
  }) {
    _locale = locale;
    _localeListResolutionCallback = localeListResolutionCallback;
    _localeResolutionCallback = localeResolutionCallback;
    _localizationsDelegates = localizationsDelegates;
    if (_supportedLocales != supportedLocales) {
      _supportedLocales = supportedLocales;
      _updateResolvedLocale(WidgetsBinding.instance.platformDispatcher.locales);
    }
  }

  /// The currently resolved [Locale] based on the current platform locale and
  /// the provided set of [supportedLocales].
  Locale get locale {
    final Locale appLocale = _locale != null
        ? _resolveLocales(<Locale>[_locale!], supportedLocales)
        : _resolvedLocale!;
    assert(_debugCheckLocalizations(appLocale));
    return appLocale;
  }

  /// {@macro flutter.widgets.widgetsApp.localizationsDelegates}
  Iterable<LocalizationsDelegate<Object?>> get localizationsDelegates {
    // Combine the Localizations for Widgets with the ones contributed
    // by the localizationsDelegates parameter, if any. Only the first delegate
    // of a particular LocalizationsDelegate.type is loaded so the
    // localizationsDelegate parameter can be used to override
    // WidgetsLocalizations.delegate.
    return <LocalizationsDelegate<Object?>>[
      ...?_localizationsDelegates,
      DefaultWidgetsLocalizations.delegate,
    ];
  }

  Iterable<LocalizationsDelegate<Object?>>? _localizationsDelegates;

  /// {@macro flutter.widgets.widgetsApp.localeListResolutionCallback}
  ///
  /// See also:
  ///
  ///  * [basicLocaleListResolution], the default locale resolution algorithm.
  LocaleListResolutionCallback? get localeListResolutionCallback => _localeListResolutionCallback;
  LocaleListResolutionCallback? _localeListResolutionCallback;

  /// {@macro flutter.widgets.LocaleResolutionCallback}
  LocaleResolutionCallback? get localeResolutionCallback => _localeResolutionCallback;
  LocaleResolutionCallback? _localeResolutionCallback;

  /// {@macro flutter.widgets.widgetsApp.supportedLocales}
  ///
  /// See also:
  ///
  ///  * [localeResolutionCallback], an app callback that resolves the app's locale
  ///    when the device's locale changes.
  ///  * [localizationsDelegates], which collectively define all of the localized
  ///    resources used by this app.
  ///  * [basicLocaleListResolution], the default locale resolution algorithm.
  Iterable<Locale> get supportedLocales => _supportedLocales;
  Iterable<Locale> _supportedLocales;

  Locale? _locale;

  /// This is the resolved locale, and is one of the supportedLocales.
  Locale? _resolvedLocale;

  @override
  void didChangeLocales(List<Locale>? locales) {
    _updateResolvedLocale(locales);
  }

  /// Recompute the resolved locale based on [preferredLocales] and
  /// [supportedLocales], update the resolved locale, and notify listeners
  /// only if the resolved locale changed.
  ///
  /// This method is called from both [update] (when locale resolution
  /// parameters change) and [didChangeLocales] (when system locales change).
  void _updateResolvedLocale(List<Locale>? preferredLocales) {
    final Locale newLocale = _resolveLocales(preferredLocales, supportedLocales);
    if (newLocale != _resolvedLocale) {
      _resolvedLocale = newLocale;
      notifyListeners();
    }
  }

  Locale _resolveLocales(List<Locale>? preferredLocales, Iterable<Locale> supportedLocales) {
    // Attempt to use localeListResolutionCallback.
    if (localeListResolutionCallback != null) {
      final Locale? locale = localeListResolutionCallback!(preferredLocales, supportedLocales);
      if (locale != null) {
        return locale;
      }
    }
    // localeListResolutionCallback failed, falling back to localeResolutionCallback.
    if (localeResolutionCallback != null) {
      final Locale? locale = localeResolutionCallback!(
        preferredLocales != null && preferredLocales.isNotEmpty ? preferredLocales.first : null,
        supportedLocales,
      );
      if (locale != null) {
        return locale;
      }
    }
    // Both callbacks failed, falling back to default algorithm.
    return basicLocaleListResolution(preferredLocales, supportedLocales);
  }

  @override
  String toString() => '$LocalizationsResolver';

  bool _debugCheckLocalizations(Locale locale) {
    assert(() {
      final Set<Type> unsupportedTypes = localizationsDelegates
          .map<Type>((LocalizationsDelegate<dynamic> delegate) => delegate.type)
          .toSet();
      for (final LocalizationsDelegate<dynamic> delegate in localizationsDelegates) {
        if (!unsupportedTypes.contains(delegate.type)) {
          continue;
        }
        if (delegate.isSupported(locale)) {
          unsupportedTypes.remove(delegate.type);
        }
      }
      if (unsupportedTypes.isEmpty) {
        return true;
      }

      FlutterError.reportError(
        FlutterErrorDetails(
          exception:
              "Warning: This application's locale, $locale, is not supported by all of its localization delegates.",
          library: 'widgets',
          informationCollector: () => <DiagnosticsNode>[
            for (final Type unsupportedType in unsupportedTypes)
              ErrorDescription(
                '• A $unsupportedType delegate that supports the $locale locale was not found.',
              ),
            ErrorSpacer(),
            if (unsupportedTypes.length == 1 &&
                unsupportedTypes.single.toString() == 'CupertinoLocalizations')
            // We previously explicitly avoided checking for this class so it's not uncommon for applications
            // to have omitted importing the required delegate.
            ...<DiagnosticsNode>[
              ErrorHint(
                'If the application is built using GlobalMaterialLocalizations.delegate, consider using '
                'GlobalMaterialLocalizations.delegates (plural) instead, as that will automatically declare '
                'the appropriate Cupertino localizations.',
              ),
              ErrorSpacer(),
            ],
            ErrorHint(
              'The declared supported locales for this app are: ${supportedLocales.join(", ")}',
            ),
            ErrorSpacer(),
            ErrorDescription(
              'See https://flutter.dev/to/internationalization/ for more '
              "information about configuring an app's locale, supportedLocales, "
              'and localizationsDelegates parameters.',
            ),
          ],
        ),
      );
      return true;
    }());
    return true;
  }
}