import Foundation

class C: NSObject {
    @objc dynamic var foo: String = "Foo"

    var observer: NSKeyValueObservation?

    override init() {
        super.init()

        self.observer = self.observe(\.foo) { s, _ in
            print("foo changed: \(s.foo)")
        }
    }

    deinit {
        print("deinit was called")
        self.observer?.invalidate()
        self.observer = nil
    }
}

do {
    let c = C()

    c.foo = "Bar"
}

foo changed: Bar
deinit was called
2017-08-23 12:22:01.950 Untitled[85112:12623133] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'An instance 0x7fff2cc09840 of class Untitled.C was deallocated while key value observers were still registered with it. Current observation info: <NSKeyValueObservationInfo 0x7fff2cc0d8f0> (
<NSKeyValueObservance 0x7fff2cc0f7b0: Observer: 0x7fff2cc0bdc0, Key path: foo, Options: <New: NO, Old: NO, Prior: NO> Context: 0x0, Property: 0x7fff2cc0d820>
)'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff9ffde2cb __exceptionPreprocess + 171
    1   libobjc.A.dylib                     0x00007fffb4e1d48d objc_exception_throw + 48
    2   CoreFoundation                      0x00007fffa005cc3d +[NSException raise:format:] + 205
    3   Foundation                          0x00007fffa19bc384 NSKVODeallocate + 293
    4   Untitled                            0x000000010225bc35 main + 133
    5   libdyld.dylib                       0x00007fffb5703235 start + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
Abort trap: 6