AppDelegate:

class AppDelegate: UIResponder, UIApplicationDelegate {

var window: UIWindow?

//头一个的label与第二个tenfield之间传值，第二个texfild与第二个label传值

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    // Override point for customization after application launch.
    self.window = UIWindow(frame: UIScreen.main.bounds)
    self.window?.backgroundColor = #colorLiteral(red: 0.2588235438, green: 0.7568627596, blue: 0.9686274529, alpha: 1)
    self.window?.makeKeyAndVisible()
    //创建导航视图控制器的根视图
    let vc = ViewController()
    //2.创建导航视图控制器，并为她制定根视图控制器
    let navigation = UINavigationController(rootViewController: vc)
    
    //3.将导航视图控制器设置为window的根视图控制器
    self.window?.rootViewController = navigation

    
    
    return true
}

ViewController:
import UIKit
//代理传值第四步
class ViewController: UIViewController,SencondViewControllerDelegate {
//两个属性
var label:UILabel!
var textfield:UITextField!
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
self.setupViews()
self.view.backgroundColor = #colorLiteral(red: 0.8549019694, green: 0.250980407, blue: 0.4784313738, alpha: 1)
self.title = "FirstVC"

}
func setupViews()   {
    self.label = UILabel(frame: CGRect(x: 107, y: 150, width: 200, height: 40))
    self.label.backgroundColor = UIColor.blue
    self.view.addSubview(label)
    
    
    self.textfield = UITextField(frame: CGRect(x: 107, y: 220, width: 200, height: 40))
    self.textfield.backgroundColor = UIColor.green
    self.view.addSubview(textfield)
    
    let button = UIButton(frame: CGRect(x: 107, y: 280, width: 200, height: 40))
    button.setTitle("跳转", for: .normal)
    button.backgroundColor = UIColor.cyan
    self.view.addSubview(button)
    button.addTarget(self, action: #selector(buttonAction), for: .touchUpInside)
}

传值：
func buttonAction()   {
        let secondVC = SencondViewController()
        //属性赋值第二步
        secondVC.content = self.textfield.text
        //闭包传值第二步
        secondVC.passvalue = {
            (text:String) in
            //取出参数值为前一个界面的控件赋值
            self.label.text = text
        }
        //代理传值的第三步
        secondVC.delegate = self
       
        
        self.navigationController?.pushViewController(secondVC, animated: true)
    }
    
    //代理传值的第五部
    func sendValue(text: String) {
        self.label.text = text
        
    }
    

SencondViewController.swift：
//从前往后
//属性传值：分为1.定义一个要与要传数据类型相同的属性
// 2.给属性赋值
// 3.取出属性中的值使用
/*从后往前传值
闭包传值：
1.在后一个界面定义一个闭包属性
2.在前一个界面为闭包属性赋值
3.在pop回去之前调用闭包

代理传值
1.定义协议
2.定义代理属性
3.在外界指定代理对象
4.代理对象所在类遵循协议
5.代理对象所在类实现协议中的方法
6.通知代理对象干活

*/

import UIKit
//代理第一步
protocol SencondViewControllerDelegate {
func sendValue(text:String)

}

class SencondViewController: UIViewController {

    //两个属性
    var label:UILabel!
    var textfield:UITextField!
    //属性传值第一步：
    var content: String!
    //闭包传值第一步
    var passvalue:((String)->())!
    
    //代理传值第二步
    var delegate:SencondViewControllerDelegate!
    
       override func viewDidLoad() {
        super.viewDidLoad()
           self.setupViews()
        //属性传值第三部
        self.label.text = self.content
        
           
       self.view.backgroundColor = #colorLiteral(red: 0.721568644, green: 0.8862745166, blue: 0.5921568871, alpha: 1)
        self.title = "sencondVC"
       
        
    }
    func setupViews()   {
        self.label = UILabel(frame: CGRect(x: 107, y: 150, width: 200, height: 40))
        self.label.backgroundColor = UIColor.blue
        self.view.addSubview(label)
        
        
        self.textfield = UITextField(frame: CGRect(x: 107, y: 220, width: 200, height: 40))
        self.textfield.backgroundColor = UIColor.green
        self.view.addSubview(textfield)
        
        let button = UIButton(frame: CGRect(x: 107, y: 280, width: 200, height: 40))
        button.setTitle("返回", for: .normal)
        button.backgroundColor = UIColor.cyan
        self.view.addSubview(button)
        button.addTarget(self, action: #selector(buttonAction), for: .touchUpInside)
    }
    
    func buttonAction()   {
        
        //闭包传值的第三部
//        self.passvalue(self.textfield.text!)
        
         //代理传值第六步
        self.delegate.sendValue(text: self.textfield.text!)
        
        self.navigationController?.popViewController(animated: true)
    }