DynamicMemberLookupable.swift

import RxSwift
import RxCocoa

@dynamicMemberLookup
protocol DynamicMemberLookupableObservableType: AssociateObservable {
  subscript<T>(dynamicMember keyPath: KeyPath<Value, T>) -> Observable<T> { get }
}

extension DynamicMemberLookupableObservableType {
  subscript<T>(dynamicMember keyPath: KeyPath<Value, T>) -> Observable<T> {
    self.myself.map({ $0[keyPath: keyPath] })
  }
}

extension Observable: DynamicMemberLookupableObservableType { }

@dynamicMemberLookup
protocol DynamicMemberLookupableBinder {
  associatedtype Element
  subscript<T>(dynamicMember keyPath: ReferenceWritableKeyPath<Element, T>) -> Binder<T> { get }
}

extension Reactive: DynamicMemberLookupableBinder where Base: AnyObject {
  typealias Element = Base
  
  subscript<T>(dynamicMember keyPath: ReferenceWritableKeyPath<Element, T>) -> Binder<T> {
    Binder(self.base) { base, value in
      base[keyPath: keyPath] = value
    }
  }
}

// usage
Observable.just("ABC").first // Observable<Character?>
SomeButton.rx.isHighlighted // Binder<Bool>