Dart：隐式接口和抽象 class 之间的区别

Question

I am unable to understand the use of 'abstract' keyword in dart. If any class when implemented can act implicitly as an abstract class, what does the abstract keyword has to offer?

Answer 1

Non-abstract classes cannot have abstract methods. So, if you define an interface without implementation, you have to use an abstract class.

From thedocumentation :

// This class is declared abstract and thus
// can't be instantiated.
abstract class AbstractContainer {
  // Define constructors, fields, methods...

  void updateChildren(); // Abstract method.
}

In other words, an abstract class doesn't necessarily provide a default implementation of its interface.

Example

A pure abstract class can be useful as a specification (when the user of this class doesn't know or is not interested in specific implementation). One of the common examples can be a repository:

abstract class UserRepository {
  Future<void> save(User user);
  Future<User> load(String id);
}

class UseCase {
  UseCase(this.repository);

  final UserRepository repository;
}

In this example, UseCase doesn't care about the specific implementation of the repository (and it shouldn't if we speak about Clean Architecture or another layered architecture approach). It just defines that, for it to work, it needs a repository class implementing this specific interface.

Somewhere in the infrastructure layer, you can define the implementation of this repository, eg using SharedPreferences :

class SharedPreferencesUserRepository implements UserRepository {
  @override
  Future<void> save(User user) {
    // Implementation
  }

  @override
  Future<User> load(String id) {
    // Implementation
  }
}

And when you create a UseCase instance, you pass the concrete class implementing the interface.