@protocol BaseClassDelegate;

@interface BaseClass : NSObject

@property (nonatomic, weak) id <BaseClassDelegate> delegate;

@end

@protocol BaseClassDelegate <NSObject>

-(void)baseDelegateMethod;

@end

protocol SubClassDelegate : BaseClassDelegate {

    func additionalSubClassDelegateMethod();
}

class SubClass: BaseClass {

    @IBAction func onDoSomething(sender: AnyObject) {

        delegate?.additionalSubClassDelegateMethod();  <--- No such method in 'delegate'
    }
}

class SubClass: BaseClass {
    var myDelegate: SubClassDelegate? {
        get { return delegate as? SubClassDelegate }
        set { delegate = newValue }
    }
    @IBAction func onDoSomething(sender: AnyObject) {
        myDelegate?.additionalSubClassDelegateMethod();
    }
}

(delegate as? SubClassDelegate)?.additionalSubClassDelegateMethod();

protocol SubclassDelegate: ClassDelegate {
    func subclassDelegateMethod()
}

class Subclass: Class {
    // here we assume that super.delegate property exists
    @IBAction func buttonPressedOrSomeOtherTrigger() {
        if let delegate: SubclassDelegate = self.delegate as? SubclassDelegate {
            delegate.subclassDelegateMethod()
        }
    }
}

extension SomeOtherClass: SubclassDelegate {
    let someObject = Subclass()
    someObject.delegate = self

    func subclassDelegateMethod() {
        // yay!
    }
}