Reactive extension for Realm 2.4 object property observing

gistfile1.txt

extension Reactive where Base: Object {

    func observe<T>(_ property: String, type: T) -> Observable<T> {

        return Observable.create { observer in
            let token = self.base.addNotificationBlock { change in

                switch change {

                case .change(let properties):
                    if let change = properties.first(where: { $0.name == property }) {
                        guard let newValue = change.newValue as? T else {
                            fatalError("Could not cast observed value \(change.newValue) to \(T.self)")
                        }
                        observer.onNext(newValue)
                    }
                case .deleted:
                    break
                case .error(let error):
                    observer.onError(error)
                }
            }

            return Disposables.create {
                observer.onCompleted()
                token.stop()
            }
        }
    }
}

Doesn't quite work.. Having trouble casting the observed value to T, which is weird. E.g. lldb says change.newValue is Optional<String> but change.newValue as! T crashes with EXC_BAD_INSTRUCTION