Swift4 KVO data binding

example.swift

@objcMembers class ViewModel: NSObject {
    private (set) dynamic var count = 0
    private (set) dynamic var title: String? = ""
}

final class ViewController: UIViewController {
    
    private var bag = DisposableBag()
    private let viewModel = ViewModel()
    private let countLabel = UILabel()
    private let titleLabel = UILabel()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        viewModel.subscribe(\.count) { [weak self] count in
            self?.countLabel.text = "count: \(count)"
            }.dispose(by: bag)
        
        viewModel
            .bind(\.title, to: titleLabel, at: \.text)
            .dispose(by: bag)
    }
}

NSObject+Bindable.swift

protocol Bindable { }

extension NSObject: Bindable { }

extension Bindable where Self: NSObject {
    
    func subscribe<Value>(_ keyPath: KeyPath<Self, Value>, changeHandler: @escaping (Value) -> ()) -> Disposable {
        return observe(keyPath, options: [.initial, .new]) { _, change in
            guard let newValue = change.newValue else { return }
            changeHandler(newValue)
        }
    }
    
    func bind<Target, Value>(_ fromKeyPath: KeyPath<Self, Value>,
                                        to target: Target,
                                        at targetKeyPath: ReferenceWritableKeyPath<Target, Value>
        ) -> Disposable {
        
        return subscribe(fromKeyPath) { newValue in
            target[keyPath: targetKeyPath] = newValue
        }
    }
}

protocol Disposable {
    func dispose()
}

extension Disposable {
    func dispose(by bag: DisposableBag) {
        bag.append(self)
    }
}

extension NSKeyValueObservation: Disposable {}

extension Disposable where Self: NSKeyValueObservation {
    
    func dispose() {
        invalidate()
    }
}

final class DisposableBag {
    
    private var disposables: [Disposable] = []
    
    func append(_ disposable: Disposable) {
        disposables.append(disposable)
    }
    
    deinit {
        disposables.forEach { $0.dispose() }
    }
}