写一个简单的加法计算器，代码如下：

class ViewController: UIViewController {

var numText1: UITextField?
var numText2: UITextField?
var resultLabel: UILabel?



override func viewDidLoad() {
    super.viewDidLoad()
    setupUI()
    // Do any additional setup after loading the view, typically from a nib.
}

///button的点击方法
func calc() -> () {
    print(#function)
    print("\(String(describing: numText1?.text)) \(String(describing: numText2?.text))")
    
    
    //将文本框内容转换为数值
    guard let num1 = Int(numText1?.text ?? "") ,let num2 = Int(numText2?.text ?? "")  else{
        print("必须输入数字")
        return
    }
    resultLabel?.text = "\(num2+num1)"
}

func setupUI() -> (){
    //1.两个textfield
    let tf1 = UITextField(frame: CGRect(x: 20, y: 20, width: 100, height: 30))
    tf1.borderStyle = .line
    tf1.text = "0"
    
    view.addSubview(tf1)
    
    
    let tf2 = UITextField(frame: CGRect(x: 140, y: 20, width: 100, height: 30))
    tf2.borderStyle = .line
    tf2.text = "0"
    
    view.addSubview(tf2)
    
    
    numText1 = tf1
    numText2 = tf2
    
    //2.三个label
    let lb1 = UILabel(frame: CGRect(x: 130, y: 20, width: 10, height: 30))
    lb1.text = "+"
    lb1.textAlignment = .center
    
    view.addSubview(lb1)
    
    let lb2 = UILabel(frame: CGRect(x: 240, y: 20, width: 10, height: 30))
    lb2.text = "="
    lb2.textAlignment = .center
    
    view.addSubview(lb2)
    
    
    let lb3 = UILabel(frame: CGRect(x: 240, y: 20, width: 60, height: 30))
    lb3.text = "0"
    lb3.textAlignment = .right
    
    view.addSubview(lb3)
    resultLabel = lb3
    
    //3.一个button
    let btn = UIButton()
    
    btn.setTitle("计算", for: UIControlState(rawValue: 0))
    btn.setTitleColor(UIColor.black, for: UIControlState(rawValue: 0))
    btn.sizeToFit()
    btn.center = view.center
    view.addSubview(btn)
    
    btn.addTarget(self, action: #selector(calc), for: .touchUpInside)
    
}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}
}

几行简单的代码。