generic_enum 0.1.2

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

Generic Enumeration Classes for Dart

Introduction

Enumerations are ideal when we want to model choosing from a limited set of constant values. In Dart, the value of an enum resolves to a String.

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.

Boilerplate

To create a generic enum class, say DpiResolution, the following steps are required:

1. Extend GenericEnum<T> where T is a Dart built-in type or a class with a const constructor. (To use the serialization methods, T should have fromJson and toJson methods.)
2. Annotate the class with @GenerateMap and @GenerateJson.
3. Define a private const constructor that calls the super constructor and passes on the value of type T.
4. Define the static const instances of DpiResolution. You may capitalize instance names to mark them as constants.
5. Define accessors for the private variable _valueMap and _toJson.

   import 'package:built_collection/built_collection.dart';
   import 'package:generic_enum/generic_enum.dart';
   import 'package:generic_enum_annotation/generic_enum_annotation.dart';
   
   //   0. Add a part statement pointing to the generated file.
   part 'dpi_resolution.g.dart';
   
   //   1. Extend GenericEnum<T>
   @GenerateBuiltMap()   //         <----------- 2. Annotate class
   @GenerateToFromJson() //
   class DpiResolution extends GenericEnum<int> {
     // 3. Define a private const constructor that calls the super constructor
     //    and passes on the value of type int.
     const DpiResolution._(int value) : super(value);
   
     // 4. Define static constant instances of type DpiResolution
     static const DpiResolution LOW = DpiResolution._(90);
     static const DpiResolution MEDIUM = DpiResolution._(300);
     static const DpiResolution HIGH = DpiResolution._(600);
   
     // 5. Give access to _valueMap and _toJson.
     static BuiltMap<int, DpiResolution> get valueMap => _valueMap;
     Map<String, dynamic> toJson() => _toJson(this);
   }
   

Usage

GenericEnum instances and their value are compile-time constants and can be used in switch statements to initalize other constants, final variables, or as parameters or default parameters in constructors and functions.

The sample class ScannerSettings (defined below) illustrates the use of a generic enum.

The value of generic enums can be accessed directly using dot notation (like in the initializer statement below).

class ScannerSettings{

  const ScannerSettings({
    this.scanMode,
    this.size,
    this.dpiResolution = DpiResolution.Medium,
    },
  ):_dipRes = dpiResolution.value; // Access value using dot-notation.

  final DpiResolution dpiResolution;
  final int _dpiRes;
  final ScanMode;
  final ScanSize;
}

See section generic_enum/example for details of how to use generic enums as annotations and how to retrieve their value using Dart's static analyzer package.

Examples

For examples on how to create generic enums see: generic_enum_example. For more information about how to use generic enums as annotations see: example

Features and bugs

Please file feature requests and bugs at the issue tracker.