Feat: Add derived object comparability

[mrgnhnt96]

Status

IN DEVELOPMENT

Breaking Changes

NO

Description

When comparing derived (sub) classes to it's base class, the result it always false. I would expect that, if the base props matches the derived class's base props, the result would be true.

Todos

• [x] Tests
• [x] Documentation
• [ ] Examples

Steps to Test or Reproduce

Outline the steps to test or reproduce the PR here.

import 'package:equatable/equatable.dart';

class A extends Equatable {
  const A(this.value);
  final String value;

  @override
  List<Object?> get props => [value];
}

class B extends A {
  const B(String a, this.code) : super(a);
  final int code;

  @override
  List<Object?> get props => [...super.props, code];

  @override
  Set<Type> get derived => {A};
}

class C extends B {
  const C(
    String value,
    int code, {
    required this.isCool,
  }) : super(value, code);

  final bool isCool;

  @override
  Set<Type> get derived => {B, ...super.derived};

  @override
  List<Object?> get props => [...super.props, isCool];
}

void main() {
  final notEqualA = const A('not-equal');

  final instanceA = const A('a');
  final instanceB = const B('a', 1);
  final instanceC = const C('a', 1, isCool: true);

  print(notEqualA == instanceA); // false

  print(instanceA == instanceB); // true
  print(instanceA == instanceC); // true

  print(instanceB == instanceA); // true
  print(instanceB == instanceC); // true

  print(instanceC == instanceA); // true
  print(instanceC == instanceB); // true

  // reversed
  print(instanceB == instanceA); // true
  print(instanceC == instanceA); // true

  print(instanceA == instanceB); // true
  print(instanceC == instanceB); // true

  print(instanceA == instanceC); // true
  print(instanceB == instanceC); // true
}

Impact to Remaining Code Base

This PR will affect:

• This should have no impact on the Code Base

Possible Concerns

By manipulating the == to match derivatives from the base class, the hashCode will not return the same result as the == operator. This only occurs when derived is overridden, otherwise the results ARE the same.

HashCode Docs

/*
Base:        instanceA
Derivative:  instanceB
*/

print(instanceA == instanceB); // true
print(instanceA.hashCode == instanceB.hashCode); // false

[karimkod]

I think it would be a better idea to add a configuration (like we have for stringify) to include runtimeType in equality instead of "derived".

[felangel]

Closing this for now but happy to continue discussing this via an issue or on discord.