Ios 将数据传递给TabBar控制器

我希望在两个ViewController之间传递数据，并抛出TabBarController。 在第一个ViewController中，我找到了文本字段和按钮。 在我找到的第二个标签里面。 当我在textField和push button中写入一些文本时，我希望这些文本出现在第二个ViewController的标签中。但什么也没发生

我的代码是：

第一个视图控制器：

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

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


}

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

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

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

第二视图控制器：

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

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


}

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

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

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

---

我认为问题出在

tabBarController.viewControllers?.first as? SecondViewController

您可能希望这样做：

tabBarController.viewControllers?[1] as? SecondViewController

---

不要忘记数组是从0索引的，因此

viewControllers？[1]

实际上返回数组中的第二个元素。

我认为问题出在

tabBarController.viewControllers?.first as? SecondViewController

您可能希望这样做：

tabBarController.viewControllers?[1] as? SecondViewController

---

不要忘记数组是从0索引的，因此

viewControllers？[1]

实际上返回数组中的第二个元素。

首先，我认为表示选项卡的视图控制器应该嵌入导航控制器，然后链接到TabBarController

其次，在控制器之间发送数据的首选和推荐方式是通过协议（委托）。下面是一个很好的示例，您可以一步一步地查看：

然而，如果你正在寻找解决方案的快速解决方案，我认为布鲁诺·菲利普的回答在某种程度上是正确的，但不是完全正确的。我们无法确定视图控制器列表中的哪个索引中有哪个控制器。我认为这应该奏效：

@IBAction func enter(_ sender: Any) {
    if textField.text != "" {
        if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

            //check if there are view controllers in the tabBarController
            guard let vcList = tabBarController.viewControllers else {

                return
            }

            for controller in vcList {

                if let second = controller as? SecondViewController {
                    //this will be executed only when a controller is SeconfViewController
                    second.label.text = textField.text
                    tabBarController.selectedIndex = 0
                }
            }

        }
    }
}

编辑：

我自己尝试过，问题是您试图设置label.text，而事实上label组件从未初始化过。我认为，如果您只是将textField值存储到SecondViewController中的myString变量中，它就会工作（不确定）

然而，这里有一个使用协议（委托）的解决方案，这是在控制器之间发送数据的正确方式。问你可能有的任何问题。这应该起作用：

@IBAction func enter(_ sender: Any) {
    if textField.text != "" {
        if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

            //check if there are view controllers in the tabBarController
            guard let vcList = tabBarController.viewControllers else {

                return
            }

            for controller in vcList {

                if let second = controller as? SecondViewController {
                    //this will be executed only when a controller is SeconfViewController
                    second.label.text = textField.text
                    tabBarController.selectedIndex = 0
                }
            }

        }
    }
}

FirstViewController：

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

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


}

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

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

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

第二视图控制器：

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

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


}

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

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

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

---

首先，我认为表示选项卡的视图控制器应该嵌入到导航控制器中，然后链接到TabBarController

其次，在控制器之间发送数据的首选和推荐方式是通过协议（委托）。下面是一个很好的示例，您可以一步一步地查看：

然而，如果你正在寻找解决方案的快速解决方案，我认为布鲁诺·菲利普的回答在某种程度上是正确的，但不是完全正确的。我们无法确定视图控制器列表中的哪个索引中有哪个控制器。我认为这应该奏效：

@IBAction func enter(_ sender: Any) {
    if textField.text != "" {
        if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

            //check if there are view controllers in the tabBarController
            guard let vcList = tabBarController.viewControllers else {

                return
            }

            for controller in vcList {

                if let second = controller as? SecondViewController {
                    //this will be executed only when a controller is SeconfViewController
                    second.label.text = textField.text
                    tabBarController.selectedIndex = 0
                }
            }

        }
    }
}

编辑：

我自己尝试过，问题是您试图设置label.text，而事实上label组件从未初始化过。我认为，如果您只是将textField值存储到SecondViewController中的myString变量中，它就会工作（不确定）

然而，这里有一个使用协议（委托）的解决方案，这是在控制器之间发送数据的正确方式。问你可能有的任何问题。这应该起作用：

@IBAction func enter(_ sender: Any) {
    if textField.text != "" {
        if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

            //check if there are view controllers in the tabBarController
            guard let vcList = tabBarController.viewControllers else {

                return
            }

            for controller in vcList {

                if let second = controller as? SecondViewController {
                    //this will be executed only when a controller is SeconfViewController
                    second.label.text = textField.text
                    tabBarController.selectedIndex = 0
                }
            }

        }
    }
}

FirstViewController：

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

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


}

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

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

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

第二视图控制器：

import UIKit

class FirstViewController: UIViewController {
    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: Any) {
        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController, let second = tabBarController.viewControllers?.first as? SecondViewController {
                second.label.text = textField.text
                tabBarController.selectedIndex = 0
            }
        }
    }

    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!
    var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = myString
        // Do any additional setup after loading the view, typically from a nib.
    }

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


}

import Foundation
import UIKit

protocol LabelChangeDelegate: class {

    func changeLabelWithText(_ text: String?)
}

class FirstViewController: UIViewController {

    weak var delegate: LabelChangeDelegate?

    @IBOutlet weak var textField: UITextField!

    @IBAction func enter(_ sender: UIButton) {

        if textField.text != "" {
            if let window = UIApplication.shared.delegate?.window, let tabBarController = window?.rootViewController as? UITabBarController {

                //check if there are view controllers in the tabBarController
                guard let vcList = tabBarController.viewControllers else {

                    return
                }

                for controller in vcList {

                    if let second = controller as? SecondViewController {
                        //this will be executed only when a controller is SeconfViewController

                        //set the delegate - who needs the data
                        delegate = second
                        //call the delegate function which will commmunicate with the delegate
                        delegate?.changeLabelWithText(textField.text!)
                        //don't know why you need this
                        tabBarController.selectedIndex = 0
                    }
                }

            }
        }
    }
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    }

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

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


}

import Foundation
import UIKit

class SecondViewController: UIViewController, LabelChangeDelegate {

    @IBOutlet weak var label: UILabel!

    //lazy init
    lazy var myString = String()

    override func viewDidLoad() {
        super.viewDidLoad()

        //set label when the view loads, not in the first controller
        label.text = myString

        // Do any additional setup after loading the view, typically from a nib.
    }

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

    //delegate function
    func changeLabelWithText(_ text: String?) {
        guard let sentText = text else {

            //no text sent
            return
        }

        myString = sentText
    }

}

---

我不建议你这样做。这是对理解的一种解释

tabBarController中的ViewController知道它们的

tabBarController

。您可以使用

self.tabBarController

访问它

secondViewController

是

viewControllers

so

let second=tabBarController.viewControllers？[1]as？SecondViewController

如果您尚未访问

secondViewController

，其视图将不会加载，因此插座仍将为

nil

。您可以使用

强制加载视图

如果要切换到第二个选项卡，则需要使用

tabBarController.selectedIndex=1

---

---

更好的方法…

您应该将字符串传递给

SecondViewController

的属性，而不是直接设置出口：

second.myString = textField.text ?? ""

然后将该字符串指定给覆盖

视图的
标签

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    label.text = myString
}

在
viewwillbeen
中设置它的原因是
viewwillbeen
将在每次显示视图之前运行
viewDidLoad
仅在首次加载视图时运行一次。由于您希望该功能可以多次运行，
视图将显示
是正确的覆盖方式。
我不建议您这样做。这是对理解的一种解释

tabBarController中的ViewController知道它们的
tabBarController
。您可以使用
self.tabBarController
访问它
secondViewController
是
viewControllers
so
let second=tabBarController.viewControllers？[1]as？SecondViewController
如果您尚未访问
secondViewController
，其视图将不会加载，因此插座仍将为
nil
。您可以使用
强制加载视图

如果要切换到第二个选项卡，则需要使用
tabBarController.selectedIndex=1


更好的方法…

您应该将字符串传递给
SecondViewController
的属性，而不是直接设置出口：

second.myString = textField.text ?? ""

然后将该字符串指定给覆盖
视图的
标签

override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    label.text = myString
}

在
viewwillbeen
中设置它的原因是
viewwillbeen
将在每次显示视图之前运行
viewDidLoad
仅在首次加载视图时运行一次。由于您希望该功能能够多次工作，
viewwillbeen
是正确的覆盖。
致命错误：在展开可选值时意外发现nil否，不工作=（f