Crash when calling Obj-C property from Swift extension - objective-c

Question

Using this Objective-C property:
#interface TSOnboardingPersonalizeViewController
#property (nonatomic, weak) NSObject<PersonalizeContentCoordinatorDelegate> * _Nullable delegate;
#end
crashes in a Swift extension like this:
extension TSOnboardingPersonalizeViewController {
func next() {
self.delegate?.performAction(.Forward)
}
}
POing self.delegate shows:
(lldb) po self.delegate
▿ Optional<protocol<PersonalizeContentCoordinatorDelegate>>
▿ Some : <MyApp.PersonalizeContentCoordinator: 0x8e964cf58140>
But it doesn't crash if I cast the property to the protocol type:
extension TSOnboardingPersonalizeViewController {
func next() {
if let delegate = self.delegate as? PersonalizeContentCoordinatorDelegate {
delegate.performAction(.Forward)
}
}
}
POing delegate shows:
(lldb) po delegate
<MyApp.PersonalizeContentCoordinator: 0x7f86bcf59930>
Why do I need to explicity cast the property to the PersonalizeContentCoordinatorDelegate protocol type?
I'm using Swift 2.2 and Xcode 7.3

Answer 1

I think it could be due to the fact that the delegate is an NSObject, which probably doesn't have a performAction method. So, even though the delegate conforms to the protocol in Objective-C, Swift probably sees an NSObject (which doesn't include that method).
This seems like a "Lost in Translation" sort of thing, where Objective-C is telling Swift "This object conforms to this protocol." Then when executed, Swift tries calling that method on an NSObject which obviously doesn't include that method and crashes. This would explain why casting would work, because until you tell Swift you have an object of a certain type, it can't execute any of that type's specific methods (even if the underlying object actually is that type).
This is just a guess, though and should definitely be taken with a grain of salt (and maybe a better answer if one comes along).