generic_enum_example 0.0.3

Demonstrates how to define and build generic enumeration classes using the library generic_enum.

Generic Enum Example

Introduction

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.

This library demonstrates how to define and built generic enumeration classes.

Starting Point

This package 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. Build instructions are provided below.

Boilerplate

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. 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.

4. Annotate the class with @GenerateBuiltMap and @GenerateFromJson.

5. Define an accessor for the private variable _valueMap.

6. Define a name factory constructor named .fromJson pointing to the function fromJson.

   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()   //         <----------- 4. Annotate class
   @GenerateFromJson()   //
   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 BuiltMap<int, DpiResolution> get valueMap => _$DpiResolutionValueMap;
   
     // 6. Define the named factory constructor .fromJson:
     factory DpiResolution.fromJson(Map<String,dynamic> json) => _$DpiResolutionFromJson(json);
   
   }
   

7. 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
   

8. Build the project by running the command

   $ 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
      // **************************************************************************
   
      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
      });
   

Features and bugs

Please file feature requests and bugs at the issue tracker.