Dart: Base class

To enforce inheritance of a class or mixin’s implementation, use the base modifier. A base class disallows implementation outside of its own library. This guarantees:

• The base class constructor is called whenever an instance of a subtype of the class is created.

• All implemented private members exist in subtypes.

• A new implemented member in a base class does not break subtypes, since 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.

// Library a.dart
base class Vehicle {
  void moveForward(int meters) {
    // ...
  }
}

// Library b.dart
import 'a.dart';

// Can be constructed
Vehicle myVehicle = Vehicle();

// Can be extended
base class Car extends Vehicle {
  int passengers = 4;
  // ...
}

// ERROR: Cannot be implemented
base class MockVehicle implements Vehicle {
  @override
  void moveForward() {
    // ...
  }
}