Generic Enum Annotation

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Introduction

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

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

  • @GenerateValueMap(): Requests the generation of an unmodifiable map _$<ClassName>ValueMap containing the enum values and const static instances.
  • @GenerateFromJson(): Requests the generation of the private function _$<ClassName>FromJson and the generation of _$<ClassName>ValueMap. Note: A ToJson function is already provided by the base class generic_enum.
  • @GenerateIdentifier(): Requests the generation of the private function _$<ClassName>Identifier. The function can be use to retrieve the identifier of each defined const generic enum instance.

Usage

To use this library include generic_enum and 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>. To use the serialization methods, T should have fromJson and toJson methods.
  2. Define a private const constructor that calls the super constructor and passes on the value of type T.
  3. Define the static const instances of DpiResolution. You may capitalize instance names to mark them as constants.

The following steps are optional. They are only required if one needs access to a list of all defined values and instances or if json-serialization is needed. In principle, a map containing values and instances and serialization functions could be maintained manually. When defining several generic enumeration classes it might be more convenient to use a builder.

  1. Annotate the class with @GenerateFromJson.

  2. Define an accessor for the private variable _$DpiResolutionValueMap.

  3. Define a factory constructor named fromJson calling the private function _$DpiResolutionFromJson.

    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>
    @GenerateFromJson()   //         <----------- 4. Annotate class
    class DpiResolution extends GenericEnum<int> {
      // 2. Define a private const constructor that calls the super constructor
      //    and passes on the value of type int.
      const DpiResolution._(int value) : super(value);
    
      // 3. 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
      static Map<int, DpiResolution> get valueMap => _$DpiResolutionValueMap;
    
      // 6. Define the named factory constructor .fromJson:
      factory DpiResolution.fromJson(Map<String,dynamic> json) => _$DpiResolutionFromJson(json);
    
    }
    
  4. 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
    
  5. Build the project by running the command

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

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.

Libraries

generic_enum_annotation