loginValidationExample.swift

let usr = username.rx
     .text
     .orEmpty
     .asObservable()
     .map { (str)-> Bool in 
           return str.characters.count > 4 
     }

let pwd = password.rx
     .text
     .orEmpty
     .asObservable()
     .map { (str)-> Bool in 
          return str.characters.count > 4 
     }

Observable
     .combineLatest(usr,pwd) 
     { (isValidUsr, isValidPwd) in 
         return isValidUsr && isValidPwd
     }
     .bind(to: loginBtn.rx.isEnabled)