Root case:

all ObjC protocol bindings are interfaces in RoboVM. As result only marshalable types can be passed to native side. Anonymous classes/lambdas passed as implementation of NSObjectProtocol will cause ClassCastException in ObjCObject.Marshaller

sample code that fails

UIToolbar toolbar = new UIToolbar();
toolbar.setDelegate(bar -> UIBarPosition.Any);

Fix:

Idea is to force classes that implement NSObjectProtocol and extends from java.lang.Object to extend from NSObject.
RoboVM compiler was extended with ObjCProtocolToObjCObjectPlugin that responsible for:

• changing super class to NSObject;
• modifying initializer (constructor) code to call NSObject.<init>() instead of Object.<init>()