Question

I have a mixin class that bundles functionality for different types that do not share a common heritage. The mixing is applied using the @Mixin annotation, so it is handled at compile time.

Some of the mixin methods return this as the result of a method call. The problem is that the this is of the mixing type and not the type of the base class. When I want to work typed in the rest of the application a ClassCastException is thrown saying that the mixing type can not be cast to the base type.

In the example code below return this returns an object of type AMixin instead of an Object of type BaseClass.

How can I have return this return an object of type BaseClass instead of an object of type AMixin?

class AMixin {

  def getWhatIWant(){
    if(isWhatIwant){
      return this
    } else {
      getChildWhatIWant()
    }
  }

  def getChildWhatIWant(){
    for (def child in childred) {
        def whatIWant = child.getWhatIWant()
        if (whatIWant) {
            return whatIWant
        }
    }
    return null
  }
}


@Mixin(AMixin)
class BaseClass {
  boolean isWhatiWant
  List<baseClass> children
}

Answer 1

I just ran into this same situation. I solved it by setting ‘this’ from the concrete class into a private variable ‘me’ inside the concrete class and return ‘me’ in the Mixin classes. For example:

class MyMixin {
    def mixinMethod() {
        // do stuff
        return me
    }
}

@Mixin(MyMixin)
class MyConcreteClass {
    private MyConcreteClass me
    MyConcreteClass() {
        me = this
    }
}

I feel like it’s a bit kludgy, but I think it’s a lot simpler than this other solution. I personally need the ability to use the same Mixin in multiple classes, and it sounds like this other proposed solution would not allow for that if you cannot assign multiple Categories to a single Mixin class.