KVO.swift

    // MARK: - KVO
    
    var observedPaths: [String] = []
    
    // Usage - observeKVO(keyPath: #keyPath(camera.inputCamera.whiteBalanceMode))
    func observeKVO(keyPath: String) -> Bool {
        if shouldObserveKVO {
            if self.classForCoder.automaticallyNotifiesObservers(forKey: keyPath) {
                observedPaths.append(keyPath)
                addObserver(self, forKeyPath: keyPath, options: [.old, .new], context: nil)
                return true
            }
        }
        return false
    }
    
    func unObserveKVO(keyPath: String) {
        if let index = observedPaths.index(of: keyPath) {
            observedPaths.remove(at: index)
        }
        removeObserver(self, forKeyPath: keyPath)
    }
    
    func unObserveAllKVO() {
        for keyPath in observedPaths {
            removeObserver(self, forKeyPath: keyPath)
        }
    }
    
    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if let keyPath = keyPath {
            switch keyPath {
            case #keyPath(camera.inputCamera.iso):
                slider.value = camera.inputCamera.iso
            default:
                break
            }
        }
    }