The following code is to share with you that swift 3.0 realizes the countdown function of SMS verification code. The specific example code is as follows:

class TCCountDown {
private var countdownTimer: Timer?
var codeBtn = UIButton()
private var remainingSeconds: Int = 0 {
  willSet {
    codeBtn.setTitle(" To obtain \(newValue) seconds ", for: .normal)
    if newValue <= 0 {
      codeBtn.setTitle(" Get verification code ", for: .normal)
      isCounting = false
    }
  }
}
var isCounting = false {
  willSet {
    if newValue {
      countdownTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(self.updateTime), userInfo: nil, repeats: true)
      remainingSeconds = 60
      codeBtn.setTitleColor(BtnCodeColor, for: .normal)
    } else {
      countdownTimer?.invalidate()
      countdownTimer = nil
      codeBtn.setTitleColor(MainColor, for: .normal)
    }
    codeBtn.isEnabled = !newValue
  }
}
@objc private func updateTime() {
  remainingSeconds -= 1
}
 }
 // A method is called
 var countDown = TCCountDown()// instantiation
 countDown.isCounting = true// Open countdown