I have class B which has object of class A as a member field. And I want to access the new mixin-ed method of class A in the mixin-ed method for class B. I tried the following but it doesn't work.

https://dartpad.dev/?id=2af929e713c46e9b853aee84f4407007&null_safety=true

class A {}

class B {
  final A objOfA;
  const B(this.objOfA);
}

mixin MixedA on A {
  void someNewMethod() {
    // do something
  }
}

mixin MixedB on B {
  void someMethod() {
    objOfA.someNewMethod(); // <-- error
  }
}

CodePudding user response：

mixin MixedB on B means that classes that use MixedB are known only to derive from B. Class B provides no guarantee that its objOfA member is an instance of MixedA. Some of your options:

• Make B require that objOfA is a MixedA by declaring it as: final MixedA objOfA;

• If you can't unconditionally declare objOfA to be a MixedA, perhaps you could make B generic and make MixedB require a specialized base type:

  class B<DerivedA extends A> {
    final DerivedA objOfA;
    const B(this.objOfA);
  }
  
  mixin MixedB on B<MixedA> {
    // ...
  }
  

• Make MixedB perform a runtime check:

  mixin MixedB on B {
    void someMethod() {
      final objOfA = this.objOfA;
      if (objOfA is MixedA) {
        objOfA.someNewMethod();
      }
    }
  }
  

• If you can personally guarantee that objOfA will always be a MixedA when MixedB is used (but you can't provide a static, compile-time guarantee), you could make MixedB override objOfA and cast it to a MixedB:

  mixin MixedB on B {
    @override
    MixedA get objOfA => super.objOfA as MixedA;
  
    void someMethod() {
      objOfA.someNewMethod();
    }
  }