Question

这是我的协议

protocol PassDataDelegate {
    func passData(data: String)
}

我的第一个控制器

class FirstViewController: UIViewController {

    @IBOutlet weak var textField: UITextField!

    var delegate: PassDataDelegate?

    override func viewDidLoad() {
        super.viewDidLoad()
        delegate = SecondViewController()
    }

    @IBAction func sendDataButtonTapped(_ sender: Any) {
        delegate?.passData(data: textField.text!)
        performSegue(withIdentifier: "Go", sender: nil)
    }

}

第二，最后一个

class SecondViewController: UIViewController, PassDataDelegate {

    @IBOutlet weak var myLabel: UILabel!

    override func viewDidLoad() {
        super.viewDidLoad()
    }

    func passData(data: String) {
        print("This came from first: \(data). Will change UI.")
        myLabel.text = data
    }
}

应用程序在标签更改部分崩溃。在打开可选项时，它会显示为零。这有什么不对？

Answer 1

您的代码存在几个基本问题。我认为，对于代表团和UIStoryboardSegue机制，您可能也会有一些误解。您应该阅读here (Delegation)和here (Segues)。

话虽如此，让我通过内联评论解释您的问题，解释发生了什么。

//  Has to be marked as a class protocol (`: class`) so that
//  `weak var delegate: PassDataDelegate?` can be weak.
protocol PassDataDelegate: class {
  func passData(data: String)
}

class FirstViewController: UIViewController {

  @IBOutlet weak var textField: UITextField!

  //  Important!
  //  Make this a `weak` var. In your case, you would fortunately not create a retain cycle
  //  but there is a big threat of creating those when using delegation patterns with non-weak delegates.
  //
  //  In your case, if you don't make this `weak`, `SecondViewController` would never be deallocated unless you
  //  cleared this var manually (setting it to `nil`).
  //
  //  Also note that, if you're using `PassDataDelegate` solely for forwarding some data to the next view controller,
  //  you can dismiss this var entirely. There is no need to have a reference to the second view controller hanging around.
  //  In fact, as mentioned above, it can be dangerous to do so.
  //  Additionally, you don't need to make the protocol `: class` then.
  private weak var delegate: PassDataDelegate?

  override func viewDidLoad() {
    super.viewDidLoad()

    //  It doesn't make any sense to create a `SecondViewController` here.
    //  The segue mechanism will create a new instance of `SecondViewController`.
//    delegate = SecondViewController()
  }

  @IBAction func sendDataButtonTapped(_ sender: Any) {
    //  `delegate?` is `nil` here.
//    delegate?.passData(data: textField.text!)
    performSegue(withIdentifier: "Go", sender: nil)
  }

  //  This is the proper 'hook' for you to forward data or do anything with a destination
  //  view controller presented using `UIStoryboardSegue`.
  //  This method will be called by the system following your call to `performSegue`.
  override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
    super.prepare(for: segue, sender: sender)

    //  `UITextField.text` can be `nil`, so safeguard for that here.
    //  If the destination implements our protocol, we can forward data to it.
    if let text = textField.text, let delegate = segue.destination as? PassDataDelegate {

      //  This is optional. You can hang on to the destination view controller here, but
      //  review the comments above to reason about whether this makes sense or not.
      self.delegate = delegate

      //  We can safely forward the data (text) here.
      delegate.passData(data: text)
    }
  }
}

SecondViewController可以保持原样。

更新

关于Delegation

委托模式通常描述一个后退指针，它与一个发起实例进行对话。例如。使用UITableViewDataSource，UITableView与实现此协议的内容进行对话，以获取有关其数据的信息等。
您实际上是通过转发数据到SecondViewController。正如评论中所提到的，此代码甚至会中断，因为passData中SecondViewController的实施是使用尚未初始化的插座。

现在你可以在这里做三件事之一：

1

立即保留您正在使用的模式（不授权是准确的）并更改SecondViewController以使其正常工作

class SecondViewController: UIViewController, PassDataDelegate { @IBOutlet weak var myLabel: UILabel! private var data: String? override func viewDidLoad() { super.viewDidLoad() // It is safe to access `myLabel` in `viewDidLoad`. Outlets have been connected. if let data = data { myLabel.text = data } } func passData(data: String) { self.data = data // Only access `myLabel` if the view is loaded. if isViewLoaded { print("This came from first: \(data). Will change UI.") myLabel.text = data } } }

这种方法实际上非常麻烦，因为你需要在任何时候调用passData这个事实。因此，您不知道您的网点是否已经初始化，这会导致代码膨胀和重复。坏。

2

完全剥离协议并使用更简单的方法

class FirstViewController: UIViewController { @IBOutlet weak var textField: UITextField! // This is the proper 'hook' for you to forward data or do anything with a destination // view controller presented using `UIStoryboardSegue`. // This method will be called by the system following your call to `performSegue`. override func prepare(for segue: UIStoryboardSegue, sender: Any?) { super.prepare(for: segue, sender: sender) // `UITextField.text` can be `nil`, so safeguard for that here. // If the destination is a `SecondViewController`, we know that is has `public var data: String` and we can forward data to it. if let text = textField.text, let destination = segue.destination as? SecondViewController { // We can safely forward the data (text) here. destination.data = text } } } class SecondViewController: UIViewController { @IBOutlet weak var myLabel: UILabel! // Deliberatly marking this a `public` to make clear that // you're intented to set this from the 'outside'. public var data: String? { didSet { if isViewLoaded { myLabel.text = data } } } override func viewDidLoad() { super.viewDidLoad() // It is safe to access `myLabel` in `viewDidLoad`. Outlets have been connected. if let data = data { myLabel.text = data } } }

同样，有些事情我们不喜欢他的做法：

仍然重复代码并且必须检查isViewLoaded

您特别想使用协议，我们不在这里

我们可以通过在init SecondViewController中提供数据来解决重复性代码问题。但是，由于您正在使用segue，故事板将为您实例化目标视图控制器，您无法控制它。现在你可以使用segue剥离，但这很快就会远离原始问题，并且是一种完全不同的仅代码方法。所以这也不好。

3

使用协议但正确应用委托模式。

protocol DataProvider: class { func provideData() -> String? } protocol DataHandler: class { var providerDelegate: DataProvider? { get set } } class FirstViewController: UIViewController, DataProvider { @IBOutlet weak var textField: UITextField! func provideData() -> String? { return textField.text } override func prepare(for segue: UIStoryboardSegue, sender: Any?) { super.prepare(for: segue, sender: sender) // If the destination is a `DataHandler`, we can set yourselves as its provider. if let destination = segue.destination as? DataHandler { destination.providerDelegate = self } } } class SecondViewController: UIViewController, DataHandler { @IBOutlet weak var myLabel: UILabel! weak var providerDelegate: DataProvider? override func viewDidLoad() { super.viewDidLoad() if let data = providerDelegate?.provideData() { // Safe to access `myLabel`, because we are in `viewDidLoad`. myLabel.text = data } } }

这种方法是最通用的。双方都不关心处理程序和提供程序究竟是什么。请注意，在经典委托模式中，您可能没有DataHandler协议，并在SecondViewController中检查具体类型（此处为prepareForSegue）。但是，这种方法更灵活，同时仍然将代理编入其中。从SecondViewController的角度来看，这种方法也是最强大的。它不必在任何时候处理passData，而是可以自己决定何时向其代理人（DataProvider）询问数据。通过这种方式，SecondViewController可以推断出所有出口等都已初始化并且处理数据是安全的。

Answer 2

SecondViewController() 不故事板中设计的控制器。这是一个没有连接插座的全新实例（这是崩溃的原因）。您需要对SecondViewController实例的真实引用。

假设SecondViewController实例是segue的目标视图控制器，则不需要protocol / delegate，通过segue传递数据

class FirstViewController: UIViewController {

    @IBOutlet weak var textField: UITextField!

    @IBAction func sendDataButtonTapped(_ sender: Any) {
        performSegue(withIdentifier: "Go", sender: nil)
    }

    func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        if segue.identifier == "Go" {
           let secondController = segue.destination as! SecondViewController 
           controller.passedData = textField.text!
        }
    }
}

class SecondViewController: UIViewController, PassDataDelegate {

    @IBOutlet weak var myLabel: UILabel!

    var passedData = ""

    override func viewDidLoad() {
        super.viewDidLoad()
        print("This came from first: \(passedData). Will change UI.")
        myLabel.text = passedData
    }
}

为什么使用委托传递数据在Swift 4中失败

2 个答案:

更新

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