generic_enum 0.0.2

The library provides a generic enumerations base class. It includes examples on how to create enumeration classes with a generic value type. Generic enumeration classes can be used in switch statement [...]

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 generic classes with a fixed set of static constant instances. 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, follow these steps:

1. Extend GenericEnum<T> where T is a Dart built-in type or a class with a const constructor.
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.
4. Optionally: Create a static final field of type BuiltMap<T,EnumClass>to provide easy access to all values and instances.

import 'package:built_collection/generic_enum.dart';
import 'package:generic_enum/generic_enum.dart';


//   1. Extend GenericEnum<T>
class DpiResolution extends GeneralizedEnum<int> {

  // 2. Define a private const constructor that calls the super constructor
  //    and passes on the value of type int.
  const DpiResolution._(int) : 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);

  // 4. Define a private static field mapping each value to its instance.
  static final valueMap = BuiltMap<String,DpiResolution>({
    90: LOW,
    300: MEDIUM,
    600: HIGH,
  });


}

In principle, the creation of the valueMap, and additional methods for serialization could be done with a source code generator (see package built_value). At least for simple value types (the most common use-case) it is straightforward to serialize the value and retrieve the instance via the valueMap.

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. See example.dart.

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;

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

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.

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

import 'package:generic_enum/generic_enum.dart';

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 final  valueMap = BuiltMap<String, Constraint>({
    'NOT NULL': NOT_NULL,
    'PRIMARY KEY': PRIMARY_KEY,
    'UNIQUE': UNIQUE,
  });

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

The Constraint class could 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

Using the package analyzer, the data model User can be traversed with the help of a SimpleElementVisitor.

When processing annotations (for example during source code generation: see method _addConstraint below), the recommended way of retrieving an annotation of type GenericEnum is via source_gen's ConstantReader.

This is because the generic enum value field is located in the parent class and ConstantReader searches parent classes if a field is not found in the current class.

import 'package:analyzer/dart/element/element.dart';
import 'package:analyzer/dart/element/type.dart';
import 'package:analyzer/dart/element/visitor.dart';
import 'package:sqlite_entity/sqlite_entity.dart';
import 'package:source_gen/source_gen.dart' show TypeChecker, ConstantReader, ;
import 'package:sqlite_generator/src/type_utils.dart';

/// Used to traverse classed annotated with @Table
class TableVisitor extends SimpleElementVisitor {

  static var constraintChecker = TypeChecker.fromRuntime(Constraint);

  List<FieldElement> fields = [];

  /// Mapping field name to constraint.
  Map<String,List<Constraint>> constraints = {};

  @override
  visitFieldElement(FieldElement element) {
    fields.add(element);
    _addConstraint(element);
  }

  _addConstraint(FieldElement element){
    var annotation = element.metadata;

    if (annotation == null) return;

    // Check if annotation is of type Constraint.
    if (!_constraintChecker.isAssignableFromType(annotation.type)) return;

    // Read value of generic enum.
    // Note: ConstantReader searches for the field name 'value' in a
    // super class if it is not found in the annotation class.
    String annotationValue = ConstantReader(annotation.computeConstantValue).read('value').stringValue;

    // Retrieve the Constraint instance.
    var constraintInstance = Constraint.valueMap[annotationValue];
    this.constraints.add({element.name: constraintInstance});
  }
}

For more information about the topic of source code generation see the packages: analyzer and source_gen.

Examples

For a simple example on how to create and use a generic enum see: example.dart.

Features and bugs

Please file feature requests and bugs at the issue tracker.