generic_enum_annotation 0.0.6

Defines annotations processed by source code generating library generic_enum_builder.

Generic Enum Annotation

Introduction

This library defines annotation classes (with const constructor). These are used to annotate classes that extend GenericEnum.

GenericEnum is a base class for creating generic classes with a fixed set of static constant instances. These classes appear to the user 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.

The annotations defined in this library are:

• @GenerateBuiltMap(): Requests the generation of a BuiltMap containing the enum values and const static instances.
• @GenerateToFromJson(): Requests the generation of the methods _toJson and fromJson

Important: @GenerateToFromJson() requires @GenerateBuiltMap() since the fromJson function uses the BuiltMap to retrieve generic enum instances based on their value.

Usage

To use this library include generic_enum_annotation as dependency in your pubspec.yaml file. Include generic_enum_builder, source_gen, build_runner as dev_dependencies.

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 @GenerateBuiltMap and @GenerateToFromJson.

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);
   }
   

6. Configure the build targets (and amend the generate_for entry). In your local build.yaml file add the following targets:

   targets:
     $default:
       builders:
         # Configure the builder `pkg_name|builder_name`
         generic_enum_builder|map_builder:
           # Only run this builder on the specified input.
           enabled: true
           generate_for:
             - lib/*.dart
         # Configure the builder `pkg_name|builder_name`
         generic_enum_builder|json_builder:
           # Only run this builder on the specified input.
           enabled: true
           generate_for:
             - lib/*.dart
   

7. Build the project by running the command

   $ flutter packages pub run build_runner build --delete-conflicting-outputs
   

   The generated file dpi_resolution.g.dart should have the following content:

      // GENERATED CODE - DO NOT MODIFY BY HAND
   
      part of 'dpi_resolution.dart';
   
      // **************************************************************************
      // JsonGenerator
      // **************************************************************************
   
      Map<String, dynamic> _toJson(DpiResolution instance) => instance.toJson();
   
      DpiResolution fromJson(Map<String, dynamic> json) {
        int value = GenericEnum.fromJson(json).value;
        return DpiResolution.valueMap[value];
      }
   
      // **************************************************************************
      // MapGenerator
      // **************************************************************************
   
      final _valueMap = BuiltMap<int, DpiResolution>({
        DpiResolution.LOW.value: DpiResolution.LOW,
        DpiResolution.MEDIUM.value: DpiResolution.MEDIUM,
        DpiResolution.HIGH.value: DpiResolution.HIGH
      });
   
   

Examples

The package generic_enum_example provides a complete example on how to define and build generic enumeration classes. As a starting point users could clone this repository, add their own generic enum classes to the folder lib and build the library.

Features and bugs

Please file feature requests and bugs at the issue tracker.