KVO-driven model bindings

Bindings.swift

extension NSObjectProtocol where Self: NSObject {
	func observe<Value>(_ keyPath: KeyPath<Self, Value>, onChange: @escaping (Value) -> ()) -> NSKeyValueObservation {
		return observe(keyPath, options: [.initial, .new]) { _, change in
			// TODO: change.newValue should never be `nil`, but when observing an optional property that's set to `nil`, then change.newValue is `nil` instead of `Optional(nil)`. This is the bug report for this: https://bugs.swift.org/browse/SR-6066
			guard let newValue = change.newValue else { return }
			onChange(newValue)
		}
	}

	func bind<Value, Target>(_ sourceKeyPath: KeyPath<Self, Value>, to target: Target, at targetKeyPath: ReferenceWritableKeyPath<Target, Value>) -> NSKeyValueObservation {
		return observe(sourceKeyPath) { target[keyPath: targetKeyPath] = $0 }
	}
}

// ### Usage
class TestViewModel: NSObject {
  @dynamic var title: String = "Test"
}

class TestViewController: UIViewController {
  var observations: [NSKeyValueObservation] = []

  override func viewDidLoad() {
		super.viewDidLoad()
		observations = [
			viewModel.bind(\.title, to: navigationItem, at: \.title)
    ]
}