class SomeObject: NSObject {
  @objc dynamic var text: String = ""
}

let object = SomeObject()
// in RxCocoa observe, Optional type and must be typed and is a string keypath.
let textObservable = object.rx.observe(String.self, "text") // observe in RxCocoa, It's type is Observable<String?>

// RxObserve
let textObservable = object.rx.asObservable(keyPath: \.text) // It's type is Observable<String>
let textObservable = object.rx.asObservable(keyPath: \.text, options: [.new]) // default value for options is [.initial, .new]

let textObserver = object.rx.asObserver(keyPath: \.text) // It's type is AnyObserver<String>

let view = UIView()
let backgroundColorObservable = view.rx.asObservable(keyPath: \.backgroundColor) // It's type is Observable<UIColor?>