NSRangeException - cannot remove observer

Question

I am trying to capture images and record video from camera but whenever I dismiss my view with camera I get this error:

Terminating app due to uncaught exception 'NSRangeException', reason: 'Cannot remove an observer App.CameraViewController 0x16ea7f70 for the key path "movieFileOutput.recording" from App.CameraViewController 0x16ea7f70 because it is not registered as an observer.' *** First throw call stack: (0x2a9e6fef 0x3908ac8b 0x2a9e6f35 0x2b6850f5 0x2b684b07 0x2b6849ab 0xbeb40 0xbca88 0x85c173 0x864d67 0x85ea61 0x866b09 0x867e19 0x39782dc1 0x39782b14) libc++abi.dylib: terminating with uncaught exception of type NSException

My code:

class CameraViewController: UIViewController, CameraDelegate, AVCaptureFileOutputRecordingDelegate {

    // MARK:
    // MARK: - Property
    @IBOutlet weak var captureButton: UIButton!
    @IBOutlet var previewView: CameraPreviewView!

    weak var cameraDelegate: CameraDelegate?

    var sessionQueue: dispatch_queue_t?
    var captureSession: AVCaptureSession?
    var stillImageOutput: AVCaptureStillImageOutput?
    var videoDeviceInput: AVCaptureDeviceInput?
    var movieFileOutput: AVCaptureMovieFileOutput?

    var previewLayer: AVCaptureVideoPreviewLayer?

    var deviceAuthorized: Bool  = false
    var backgroundRecordId: UIBackgroundTaskIdentifier = UIBackgroundTaskInvalid
    var sessionRuningAndDeviceAuthorized: Bool {
        get {
            return (self.captureSession?.running != nil && self.deviceAuthorized )
        }
    }
    var runtimeErrorHandlingObserver: AnyObject?
...
}

Adding observers:

    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)

        dispatch_async(self.sessionQueue!, {
            // TODO: Make constants
            self.addObserver(self, forKeyPath: "sessionRunningAndDeviceAuthorized", options: NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New, context: &SessionRunningAndDeviceAuthorizedContext)
            self.addObserver(self, forKeyPath: "stillImageOutput.capturingStillImage", options: NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New, context: &CapturingStillImageContext)
            self.addObserver(self, forKeyPath: "movieFileOutput.recording", options: NSKeyValueObservingOptions.Old | NSKeyValueObservingOptions.New, context: &RecordingContext)


            NSNotificationCenter.defaultCenter().addObserver(self, selector: "subjectAreaDidChange:", name: AVCaptureDeviceSubjectAreaDidChangeNotification, object: self.videoDeviceInput?.device)

            weak var weakSelf = self

            self.runtimeErrorHandlingObserver = NSNotificationCenter.defaultCenter().addObserverForName(AVCaptureSessionRuntimeErrorNotification, object: self.captureSession, queue: nil, usingBlock: {
                (note: NSNotification?) in
                var stronSelf: CameraViewController = weakSelf!
                dispatch_async(stronSelf.sessionQueue!, {
                    if let sess = stronSelf.captureSession {
                        sess.startRunning()
                    }
                })

            })

            self.captureSession?.startRunning()
        })                                    
    }

And code with removing observers:

override func viewWillDisappear(animated: Bool) {
        super.viewWillDisappear(animated)

        dispatch_async(self.sessionQueue!, {

            if let sess = self.captureSession {
                sess.stopRunning()

                NSNotificationCenter.defaultCenter().removeObserver(self, name: AVCaptureDeviceSubjectAreaDidChangeNotification, object: self.videoDeviceInput?.device)
                NSNotificationCenter.defaultCenter().removeObserver(self.runtimeErrorHandlingObserver!)

                // TODO: Make constants

                self.removeObserver(self, forKeyPath: "sessionRunningAndDeviceAuthorized", context: &SessionRunningAndDeviceAuthorizedContext)
                self.removeObserver(self, forKeyPath: "stillImageOutput.capturingStillImage", context: &CapturingStillImageContext)
                self.removeObserver(self, forKeyPath: "movieFileOutput.recording", context: &SessionRunningAndDeviceAuthorizedContext)

            }
        })
    }

Answer 1

You are using the wrong context in removeObserver(...)

Also, why are you removing the observers in an asynchronous block?