Few cents about my commits

ClassCastException: ObjC protocol and its Java implementation



There is many API in CocoaTouch where argument is ObjC protocol type. In RoboVM protocols are mapped into interfaces. The problems begin when bro-bridge tries to marshall this protocol implementation into the native object:

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


java.lang.ClassCastException: com.mycompany.myapp.Main$$Lambda$1 cannot be cast to org.robovm.objc.ObjCObject
	at org.robovm.objc.ObjCObject$Marshaler.protocolToNative(ObjCObject.java:388)
	at org.robovm.apple.uikit.UIToolbar.$m$setDelegate$(Native Method)
	at org.robovm.apple.uikit.UIToolbar.setDelegate(UIToolbar.java)
	at com.mycompany.myapp.Main.didFinishLaunching(Main.java:26)

Root case: protocol marshaller can handle only ObjCObject. Indeed, there is no way to marshal random Java class to ObjC native world.

The existing workaround:

Simple case would be:

  • use NSObject subclass that implements interface;
  • use Adapter class if it is available;

In any case this overcomplicates the code, new coders still will face this issue and usage of simple lambda is not allowed.

The 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>()


Changes proposed as PR563