Rx filter for text fields, doesn't really work with other observers

UITextField+Rx+Extensions.swift

//
//  UITextField+Rx+Extensions.swift
//  ddxp-kiosk
//
//  Created by Richard Adem on 6/20/17.
//  Copyright © 2017 Richard Adem. All rights reserved.
//

import RxSwift
import RxCocoa

extension Reactive where Base: UITextField {
    func filterInput(withMapFilter mapFilter: @escaping (String?) -> String?) -> Disposable {
        let textValue = Variable<String?>("")
        
        let bindToUIDisposable = textValue.asObservable()
            .map(mapFilter)
            .bind(to: self.text)
        
        let bindToVariable = self.text
            .subscribe(onNext: { str in
                textValue.value = str
            }, onCompleted: {
                bindToUIDisposable.dispose()
            })
        
        return Disposables.create(bindToUIDisposable, bindToVariable)
    }
}