在Swift中制作计算器

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var displayLabel: UILabel!
    
    private var isFinishedTypingNumber: Bool = true
    private var currentValue: Double {
        get {
            guard let number = Double(displayLabel.text!) else {
                fatalError("Cannot convert display label text to a Double.")
            }
            return number
        }
        set {
            displayLabel.text = String(newValue)
        }
    }
    
    private var firstNumber: Double = 0
    private var operation: String = ""
    
    @IBAction func numberPressed(_ sender: UIButton) {
        if let numberValue = sender.currentTitle {
            if isFinishedTypingNumber {
                displayLabel.text = numberValue
                isFinishedTypingNumber = false
            } else {
                displayLabel.text = displayLabel.text! + numberValue
            }
        }
    }
    
    @IBAction func operationPressed(_ sender: UIButton) {
        isFinishedTypingNumber = true
        firstNumber = currentValue
        if let operationValue = sender.currentTitle {
            operation = operationValue
        }
    }
    
    @IBAction func equalsPressed(_ sender: UIButton) {
        isFinishedTypingNumber = true
        let secondNumber = currentValue
        
        var result: Double = 0
        
        switch operation {
        case "+":
            result = firstNumber + secondNumber
        case "-":
            result = firstNumber - secondNumber
        case "*":
            result = firstNumber * secondNumber
        case "/":
            result = firstNumber / secondNumber
        default:
            fatalError("The operation passed in does not match any of the cases.")
        }
        
        currentValue = result
    }
    
    @IBAction func clearPressed(_ sender: UIButton) {
        displayLabel.text = "0"
        isFinishedTypingNumber = true
        firstNumber = 0
        operation = ""
    }
}