版本：

swift 4，ios 11.2，smulater: iPhone 7

在創建project後，開始應用的製作

1.改變應用的背景顏色

2.加入素材

3.添加到應用

4.標題規格

5.添加UIlable

6.添加Textfield

7.添加Button

8.再添加一個UIlabel

9.配對輸入框

10.配對狀態欄

11.配對執行按鈕

12.隱藏狀態欄文字

13.設置鍵盤

14.代碼編寫

//  ViewController.swift
//  PrimeNumber
//
//  Created by lin on 2018/3/21.
//  Copyright © 2018年 lin. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

    @IBOutlet weak var inputTextField: UITextField!

    @IBOutlet weak var resultLabel: UILabel!
    // status bar -> white
    override var preferredStatusBarStyle: UIStatusBarStyle{
        return .lightContent
    }


    @IBAction func doThePrimeTest(_ sender: UIButton) {
        //1.Take out the number string from the inputTextField
        if let inputText = inputTextField.text{
            //2.convert the number string to Int
            if let inputNumber = Int(inputText){
                //3.Useing the checkPrime function to get result
                //4.Using the resultLabel to show result
                    /*
                        resultLabel.text = checkPrime(withNumber: inputNumber)
                        resultLbbel.isHidden = false
                        */

                //closure

                   /*
                    checkPrime(withnumber: inputNumber, andCompletionHandler: {
                    (result:String) in
                        self.resultLabel.text = result
                        self.resultLabel.isHidden = false
                    })
                   */

                //omit
                checkPrime(withnumber: inputNumber){
                    self.resultLabel.text = $0
                    self.resultLabel.isHidden = false
                }
            }
        }


        inputTextField.text = ""
    }


    override func viewDidLoad() {
        super.viewDidLoad()
        //push the keyboard up
        inputTextField.becomeFirstResponder()
    }

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

    func checkPrime(withNumber testNumber:Int) -> String{
        var isPrime:Bool? = true

        if testNumber <= 0{
            isPrime = nil
        }else if testNumber == 1{
            isPrime = false
        }else{
            for i in 2..<testNumber{
                if testNumber % i == 0{
                    //input number is not prime
                    isPrime = false
                    break
                }
            }
        }

        if isPrime == true{
            return "\(testNumber) is prime"
        }else if isPrime == false{
            return "\(testNumber) is not prime"
        }else{
            return "Please enter a number greater than 0"
        }

    }
    func checkPrime(withnumber number:Int, andCompletionHandler handler:(String) -> ()){
        handler(checkPrime(withNumber: number))
    }
}