MVCtest

calcBrain.swift

struct CaluculatorBrain {
    private var accumulator:Double?
    
    mutating func performOperation(_ symbol:String){
        switch symbol {
        case "π":
            accumulator = Double.pi
        case "√":
            if let operand = accumulator{
            accumulator = sqrt(operand)
            }
        default:
            break
        }
    }
    
    mutating func setOperand(_ operand:Double){
        accumulator = operand
    }
    
    var result:Double?{
        get{
            return accumulator
        }
    }

}

viewController(calc).swift

var displayValue:Double{
        get {
           return Double(display.text!)!
        }
        set {
            display.text = String(newValue)
        }
    }
    
    private var brain = CaluculatorBrain()
    
    
    @IBAction func performOperation(_ sender: UIButton) {
        if userIsMiddleOfTyping{
            brain.setOperand(displayValue)
            userIsMiddleOfTyping = false
        }
        
        if let mathematicalSymbol = sender.currentTitle{
           brain.performOperation(mathematicalSymbol)
        }
        if let result = brain.result{
        displayValue = result
        }
    }