无法使用自定义类返回类型覆盖方法

Question

Is there any method i can use to override a method which returns custom class? When i tried to override any method with a custom class as a return type, Xcode throws me an error

Below are my codes:

class CustomClass {
  var name = "myName"
}

class Parent: UIViewController {

  override func viewDidLoad() {
    methodWithNoReturn()
    print(methodWithStringAsReturn())
    methodWithCustomClassAsReturn()
  }

}

extension Parent {

  func methodWithNoReturn() { }
  func methodWithStringAsReturn() -> String { return "Parent methodWithReturn" }
  func methodWithCustomClassAsReturn() -> CustomClass {
    return CustomClass()
  }

}


class Child: Parent {

  override func methodWithNoReturn() {
    print("Can override")
  }

  override func methodWithStringAsReturn() -> String {
    print("Cannot override")
    return "Child methodWithReturn"
  }

  override func methodWithCustomClassAsReturn() -> CustomClass {
    return CustomClass()
  }

}

The error is when overriding this method:

func methodWithCustomClassAsReturn() -> CustomClass

with error message:

Declarations from extensions cannot be overridden yet

Answer 1

No reason other than the compiler doesn't support it yet. To override a method defined in an extension to the superclass, you must declare it ObjC-compatible:

extension Parent {
    func methodWithNoReturn() { }
    func methodWithStringAsReturn() -> String { return "Parent methodWithReturn" }

    @objc func methodWithCustomClassAsReturn() -> CustomClass {
        return CustomClass()
    }   
}

// This means you must also expose CustomClass to ObjC by making it inherit from NSObject
class CustomClass: NSObject { ... }

The alternative, without involving all the ObjC snafus is to define these methods in the Parent class (not inside an extension):

class Parent: UIViewController {

  override func viewDidLoad() {
    methodWithNoReturn()
    print(methodWithStringAsReturn())
    methodWithCustomClassAsReturn()
  }

  func methodWithNoReturn() { }
  func methodWithStringAsReturn() -> String { return "Parent methodWithReturn" }
  func methodWithCustomClassAsReturn() -> CustomClass {
    return CustomClass()
  }

}