Understanding the base Keyword in Dart 3.10

20 May 2023 - 2 mins read time
Tags: Flutter Dart base class Dart 3.10

Introduction

Dart 3.10 introduced a new keyword base as part of its language enhancements. The base keyword is used to enforce inheritance of a class or mixin’s implementation and disallows implementation outside of its own library. This ensures several guarantees which are key to maintaining the integrity of the class.

Benefits of Using the base Keyword

Constructor Guarantee: The base class constructor is always called whenever an instance of a subtype of the class is created.
Private Members Guarantee: All implemented private members exist in subtypes.
Member Inheritance Guarantee: A new implemented member in a base class does not break subtypes, as all subtypes inherit the new member. This is true unless the subtype already declares a member with the same name and an incompatible signature.

You must mark any class which implements or extends a base class as base, final, or sealed. This prevents outside libraries from breaking the base class guarantees.

Let’s explore this concept with a new example:

// Library shapes.dart
base class Shape {
  void draw() {
    // ...
  }
}

// Library circles.dart
import 'shapes.dart';

// Can be constructed
Shape myShape = Shape();

// Can be extended
base class Circle extends Shape {
  double radius = 5.0;
  // ...
}

// ERROR: Cannot be implemented
base class MockShape implements Shape {
  @override
  void draw() {
    // ...
  }
}

In the above example, Shape is a base class defined in the shapes.dart library. It has a method draw(). We then define a Circle class in the circles.dart library, which extends the Shape base class, and this is perfectly fine.

However, when we try to implement the Shape class with MockShape, we get an error because we cannot implement a base class outside its library.

The base keyword is a powerful tool to ensure your class hierarchy remains intact and safe from external modifications. It encourages best practices in object-oriented programming and strengthens Dart’s capabilities in creating scalable and maintainable applications.

Thank you for reading. Happy coding!