If you init method of timer
let timer = Timer(timeInterval: 3, target: self, selector: #selector(update(_:)), userInfo: [key : value], repeats: false)
func update(_ timer : Timer) {
}
then add it to loop using method other selector will not be called
RunLoop.main.add(timer!, forMode: .defaultRunLoopMode)
NOTE : If you are want this to repeat make repeats true and keep the reference of timer otherwise update method will not be called.
If you are using this method.
Timer.scheduledTimer(timeInterval: seconds, target: self, selector: #selector(update(_:)), userInfo: nil, repeats: true)
keep a reference for later use if repeats is true.