@IBAction private func touchDigit(_ sender: UIButton) {

 let digit = sender.currentTitle! 

     if userIsInTheMiddleOfTyping { 
               let textCurrentlyInDisplay = display.text! 
                display.text = textCurrentlyInDisplay + digit 
           } else { 
                display.text = digit 
           } 
                 userIsInTheMiddleOfTyping = true 
           } 
           private var displayValue: Double { 
             get{ 
                 return Double(display.text!)!
             }
             set { 
                  display.text = String(newValue) 
                } 
             }

display를 변수로 설정해서 UILabel에 보내는 상황의 코드입니다. 

userismidle.... 을 왜 false로 보내는지 일단 모르겟습니다. *****

touchdigit는 숫자버튼이에 ui버튼으로 센더해서
         digit가 currentlytitle명령어를 통해 라벨(display)로 보내지는 것까지 알겠는데  조건문에서 처음인 텍스트커런틀리에 디지트를 더하는 것과 디지트 혼자인 상황에 대한 차이를 모르겟습니다.