如何在Dart中保证某个命名构造函数？

Question

Let us say I have the concrete classes Cat, Dog, and Parrot, and the following interface:

class HasGuid {
  HasGuid.fromId(String id);
}

My goal is to guarantee that Cat, Dog, and Parrot all have a fromId named constructor. So, I can make calls like:

Cat.fromId("Whiskers") =returns> [A Future<Cat> object with id "Whiskers"]
Dog.fromId("Fido")     =returns> [A Future<Dog> object with id "Fido"]
Parrot.fromId("Polly") =returns> [A Future<Parrot> object with id "Poly"]

fromId is making a call across the network, for this reason I return it as a Future . I basically want a contract that states that any class that mixes/extends/implements/whatever the HasGuid class will have a named constructor of fromId . Where fromId on class T will take an identity string and will return a Future<T> .

Answer 1

There can be no assurance on constructor.

Interface(implements) assurance on instance methods. Super class(extends) or Mixins(with) also assurance on instance methods, not constructor.

Constructor return it's own type, not a Future.

So they should all have a static method or class, but no guarantee.

Answer 2

The short answer is that you cannot force a subclass to implement a specific named constructor ... all you can do is force the subclass to make sure the named constructor is called by the subclass.

Take for example the following ...

class Animal {
  String name;

  Animal.fromName(this.name);
}

class Dog extends Animal {

  Dog.withName(String name) : super.fromName(name);
}

Note the following ...

1. Animal has no zero argument constructor.
2. If you don't call the super.fromName() constructor from Dog you will get a compile error

The superclass 'Animal' doesn't have a zero argument constructor.

3. Although Dog must call the fromName() constructor ... it doesn't have to call its named constructor the same. In this case notice it is called withName() .