RxSwift extension for WebKit's WKScriptMessageHandler.

WKScriptMessageHandler+Rx.swift

import WebKit
import RxSwift
import RxCocoa

public extension Reactive where Base: WKScriptMessageHandler {
  func didReceiveMessage() -> Observable<(WKUserContentController, WKScriptMessage)> {
    return self.methodInvoked(#selector(Base.userContentController))
      .map { args in
        guard args.count == 2,
              let controller = args[0] as? WKUserContentController,
              let message = args[1] as? WKScriptMessage else {
          throw RxCocoaError.castingError(object: args,
                                          targetType: (WKUserContentController, WKScriptMessage).self)
        }

        return (controller, message)
      }
  }
}