Simplify Equality Comparisons

Overview

Being able to compare objects in Dart often involves having to override the == operator as well as hashCode.

Not only is it verbose and tedious, but failure to do so can lead to inefficient code which does not behave as we expect.

By default, == returns true if two objects are the same instance.

Let’s say we have the following class:

class Person {
  final String name;

  const Person(this.name);
}

We can create create instances of Person like so:

void main() {
  final Person bob = Person("Bob");
}

Later if we try to compare two instances of Person either in our production code or in our tests we will run into a problem.

print(bob == Person("Bob")); // false

For more information about this, you can check out the official Dart Documentation.

In order to be able to compare two instances of Person we need to change our class to override == and hashCode like so:

class Person {
  final String name;

  const Person(this.name);

  @override
  bool operator ==(Object other) =>
    identical(this, other) ||
    other is Person &&
    runtimeType == other.runtimeType &&
    name == other.name;

  @override
  int get hashCode => name.hashCode;
}

Now if we run the following code again:

print(bob == Person("Bob")); // true

it will be able to compare different instances of Person.

You can see how this can quickly become a hassle when dealing with complex classes. This is where Equatable comes in!

What does Equatable do?

Equatable overrides == and hashCode for you so you don’t have to waste your time writing lots of boilerplate code.

There are other packages that will actually generate the boilerplate for you; however, you still have to run the code generation step which is not ideal.

With Equatable there is no code generation needed and we can focus more on writing amazing applications and less on mundane tasks.

Usage

First, we need to do add equatable to the dependencies of the pubspec.yaml

dependencies:
  equatable: ^1.1.0

Next, we need to install it:

# Dart
pub get

# Flutter
flutter packages get

Lastly, we need to extend Equatable

import 'package:equatable/equatable.dart';

class Person extends Equatable {
  final String name;

  Person(this.name);

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

When working with json:

import 'package:equatable/equatable.dart';

class Person extends Equatable {
  final String name;

  Person(this.name);

  @override
  List<Object> get props => [name];

  factory Person.fromJson(Map<String, dynamic> json) {
    return Person(json['name']);
  }
}

We can now compare instances of Person just like before without the pain of having to write all of that boilerplate. Note:Equatable is designed to only work with immutable objects so all member variables must be final (This is not just a feature of Equatable – overriding a hashCode with a mutable value can break hash-based collections).

Equatable also supports const constructors:

import 'package:equatable/equatable.dart';

class Person extends Equatable {
  final String name;

  const Person(this.name);

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

toString Implementation

Equatable can implement toString method including all the given props. If you want that behaviour, just include the following:

  @override
  bool get stringify => true;

For instance:

import 'package:equatable/equatable.dart';

class Person extends Equatable {
  final String name;

  const Person(this.name);

  @override
  List<Object> get props => [name];
  
  @override
  bool get stringify => true;
}

For the name Bob, the outuput will be:

Person(Bob)

This flag by default is false and toString will return just the type:

Person

Recap

Without Equatable

class Person {
  final String name;

  const Person(this.name);

  @override
  bool operator ==(Object other) =>
    identical(this, other) ||
    other is Person &&
    runtimeType == other.runtimeType &&
    name == other.name;

  @override
  int get hashCode => name.hashCode;
}

With Equatable

import 'package:equatable/equatable.dart';

class Person extends Equatable {
  final String name;

  Person(this.name);

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

EquatableMixin

Sometimes it isn’t possible to extend Equatable because your class already has a superclass. In this case, you can still get the benefits of Equatable by using the EquatableMixin.

Usage

Let’s say we want to make an EquatableDateTime class, we can use EquatableMixin like so:

class EquatableDateTime extends DateTime with EquatableMixin {
  EquatableDateTime(
    int year, [
    int month = 1,
    int day = 1,
    int hour = 0,
    int minute = 0,
    int second = 0,
    int millisecond = 0,
    int microsecond = 0,
  ]) : super(year, month, day, hour, minute, second, millisecond, microsecond);

  @override
  List<Object> get props {
    return [year, month, day, hour, minute, second, millisecond, microsecond];
  }
}

Now if we want to create a subclass of EquatableDateTime, we can just override props.

class EquatableDateTimeSubclass extends EquatableDateTime {
  final int century;

  EquatableDateTime(
    this.century,
    int year,[
    int month = 1,
    int day = 1,
    int hour = 0,
    int minute = 0,
    int second = 0,
    int millisecond = 0,
    int microsecond = 0,
  ]) : super(year, month, day, hour, minute, second, millisecond, microsecond);

  @override
  List<Object> get props => super.props..addAll([century]);
}

Performance

You might be wondering what the performance impact will be if you use Equatable.

Performance Tests have been written to test how Equatable stacks up to manually overriding == and hashCode in terms of class instantiation as well as equality comparison.

Results (average over 10 runs)

Equality Comparison A == A

Instantiation A()

*Performance Tests run using: Dart VM version: 2.4.0

Download Flutter Equality Comparisons source code on GitHub

https://github.com/felangel/equatable

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