ViewController.swift

import UIKit

enum CalculationEnum: Int {
    case PLUS
    case MINUS
    case TIMES
    case DIVIDE
    
    func calculate(target1: Double, target2: Double) -> Double {
        
        switch self {
        case .PLUS:
            return target1 + target2
        case .MINUS:
            return target1 - target2
        case .TIMES:
            return target1 * target2
        case .DIVIDE:
            return target1 / target2
        }
    }
}

class ViewController: UIViewController {

    @IBOutlet weak var numberTextField1: UITextField!
    @IBOutlet weak var numberTextField2: UITextField!
    
    @IBOutlet weak var calculationType: UISegmentedControl!
    
    @IBOutlet weak var resultLabel: UILabel!
    
    @IBOutlet var tapGesture: UITapGestureRecognizer!
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // ジェスチャー追加
        self.view.addGestureRecognizer(self.tapGesture)
    }
    
    @IBAction func calculate(sender: AnyObject) {

        let selectedType: Int = self.calculationType.selectedSegmentIndex
        
        // 0除算チェック
        // ２番目のテキストフィールドがブランクでもエラーが出るのでそこも考慮に入れたチェック
        if selectedType == CalculationEnum.fromRaw(3) && isZeroDivide() {
            self.resultLabel.text = "割る数には0以外を入力してください"
            return
        }
        
        let calEnum = CalculationEnum(rawValue: selectedType)
        
        let target1 = (self.numberTextField1.text as NSString).doubleValue
        let target2 = (self.numberTextField2.text as NSString).doubleValue

        // 計算処理
        var result = calEnum!.calculate(target1, target2: target2)
        
        self.resultLabel.text = "\(result)"
    }

    // 0除算になっているか
    func isZeroDivide() -> Bool {
        if self.numberTextField2.text.isEmpty || self.numberTextField2.text == "0" {
            return true
        }
        return false
    }
    
    // キーボード外をタップしてナンバーパッドを閉じる
    @IBAction func closeNumberPad(sender: UITapGestureRecognizer) {
        self.view.endEditing(true)
    }
    
    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }
}