同じタブを二度タップして titleView を first responder にする

MakeTitleViewFirstResponderByTouchingTabTwice.swift

import UIKit

import RxCocoa
import RxSwift

final class TabBarController: UITabBarController {
    private let disposeBag = DisposeBag()

    override func viewDidLoad() {
        super.viewDidLoad()

        rx.didSelect
            .withPrevious()
            .subscribe(onNext: { previous, next in
                guard previous == next, let topViewController = (next as? UINavigationController)?.topViewController else { return }
                topViewController.navigationItem.titleView?.becomeFirstResponder()
            })
            .disposed(by: disposeBag)
    }
}

extension ObservableType {
    public func withPrevious() -> Observable<(E?, E?)> {
        return self
           .scan((nil, nil)) { state, next in let (_, previous) = state; return (previous, next) }
           .skip(1)
    }
}