generic_enum 0.2.4

Enumeration classes with generic value type. Can be used in switch statements, as annotations, or as constant parameters in constructors and functions.

example/README.md

Generic Enumeration Example

Introduction

GenericEnum is a base class for creating enumeration classes with generic value type. These classes appear to the user of the library much like a Dart enum would. For example, generic enums can be used in switch statements, to initialize variables, or as default parameters in functions and constructors.

Build Process

The main section contains a step-by-step guide on how to define and build generic enumeration classes.

Generic Enums as Annotations

GenericEnum classes have a constant constructor and as such can be used as annotations. Annotations are commonly found in source code generating libraries.

Since generic enums are normal classes they can contain methods and final fields in addition to the value field. The example below includes the getters isPrimary, isUnique, isNotNull.

As an example, we could generate an annotation class, say Constraint, that enables users to select a supported Sqlite constraint.

import 'package:generic_enum/generic_enum.dart';

part 'constraint.g.dart';

@GenerateFromJson()
class Constraint extends GenericEnum<String> {
  const Constraint._(String value) : super(value);

  static const Constraint NOT_NULL = Constraint._('NOT NULL');
  static const Constraint PRIMARY_KEY = Constraint._('PRIMARY KEY');
  static const Constraint UNIQUE = Constraint._('UNIQUE');

  static Map<String, Constraint> get valueMap => _$ConstraintValueMap;
  factory Constraint.fromJson(Map<String,dynamic> json) => _$ConstraintFromJson(json);

  bool get isPrimary => (this == PRIMARY_KEY);
  bool get isUnique => (this == UNIQUE);
  bool get isNotNull => (this == NOT_NULL);
}

The Constraint class can be used to annotate a field in the data class User.

import 'constraint.dart';
import 'table.dart';

@Table
class User{
  const User({@required this.id, @required this.userName});

  // Use generic enum as annotation
  @Constraint.PRIMARY_KEY
  final int id;

  @Constraint.NOT_NULL
  final String userName;
}

Retrieving Annotations of Type Generic Enum

The recommended way of retrieving the value of an annotation of type GenericEnum by using the class ConstantReader provided by the package source_gen. ConstantReader is a representation of a compile-time constant expression.

The following program demonstrates how to retrieve a constant of type Constraint. In the simple example below, the function initializeLibraryReaderForDirectory provided by source_gen_test is used to load the source code and initialize an object of type LibraryReader. In a standard setting this task is delegated to a builder that reads a builder configuration and loads the relevant assets.

import 'package:example/src/sqlite_type.dart' show Constraint;
import 'package:generic_reader/generic_reader.dart';
import 'package:source_gen/source_gen.dart' show ConstantReader;
import 'package:source_gen_test/src/init_library_reader.dart';


/// Demonstrates how to use [GenericReader] to read constants
/// with parameterized type from a static representation
/// of a compile-time constant expression
/// represented by a [ConstantReader].
Future<void> main() async {
  /// Reading library.
  final userLib = await initializeLibraryReaderForDirectory(
    'lib/src',
    'user.dart',
  );

  /// Get constantReader representing the annotation:
  final idCR = ConstantReader(userLib.classes.first.fields[0].
    metadata[0].computeConstantValue(),
  );

  /// Reading the annotation value of type [String].
  final value = cr.peek('value').stringValue;

  /// Retrieving the [GenericEnum] instance.
  final constraint = Constraint.valueMap[value];

  print('Retrieving an annotation of type [Constraint].');
  print(constraint);
  // Prints:
  // Retrieving an annotation of type [Constraint].
  // PRIMARY_KEY
}

For more information about source code generation see: analyzer and source_gen.

Examples

For examples on how to create generic enums see: generic_enum_example.

Features and bugs

Please file feature requests and bugs at the issue tracker.