What is Annotations in flutter ? example it is resemble by “@” symbol

Overview

Annotations in Flutter/Dart are metadata markers prefixed with

@

Built-in Dart Annotations

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// @override -- tells compiler this overrides a parent method
class Dog extends Animal {
  @override
  void makeSound() => print('Woof');
}

// @deprecated -- marks API as outdated
@deprecated
void oldFunction() { ... }

// Use @Deprecated for message
@Deprecated('Use newFunction() instead -- removed in 3.0')
void oldFunction2() { ... }

// @protected -- should only be called by subclasses
import 'package:meta/meta.dart';

class BaseWidget extends StatelessWidget {
  @protected
  Widget buildContent(BuildContext context) => Container();
}

// @immutable -- all fields must be final
@immutable
class Config {
  final String apiUrl;
  final int timeout;
  const Config({required this.apiUrl, required this.timeout});
}

// @visibleForTesting -- public but not part of public API
@visibleForTesting
void internalSetup() { ... }

Code Generation Annotations (Most Common)

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// json_serializable -- generate fromJson/toJson
import 'package:json_annotation/json_annotation.dart';
part 'user.g.dart'; // Generated file

@JsonSerializable()
class User {
  final String name;
  final int age;

  @JsonKey(name: 'email_address') // Map different JSON key
  final String email;

  @JsonKey(defaultValue: 'guest') // Default if missing
  final String role;

  const User({required this.name, required this.age, required this.email, required this.role});

  factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
  Map<String, dynamic> toJson() => _$UserToJson(this);
}
// Run: flutter pub run build_runner build

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// freezed -- generate immutable classes + pattern matching
import 'package:freezed_annotation/freezed_annotation.dart';
part 'result.freezed.dart';

@freezed
class ApiResult<T> with _$ApiResult<T> {
  const factory ApiResult.success(T data) = Success;
  const factory ApiResult.failure(String message) = Failure;
  const factory ApiResult.loading() = Loading;
}

// Usage
final result = ApiResult.success(user);
result.when(
  success: (data) => print(data),
  failure: (msg) => print(msg),
  loading: () => print('Loading'),
);

Riverpod Annotations

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import 'package:riverpod_annotation/riverpod_annotation.dart';
part 'user_provider.g.dart';

@riverpod
Future<User> fetchUser(FetchUserRef ref, {required int id}) async {
  final api = ref.watch(apiServiceProvider);
  return api.getUser(id);
}
// Equivalent to manually writing FutureProvider.family

Annotation Categories

@override

@deprecated

@Deprecated

@immutable

@protected

@visibleForTesting

@JsonSerializable

@freezed

@riverpod

@isTest

@isTestGroup

Summary: Annotations are

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@metadata

markers. Built-in ones guide the analyzer. Code generation annotations (

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@JsonSerializable

,

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@freezed

) drive

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build_runner

to generate boilerplate code automatically.