Swift委托模式：委托=Nil-有解無憂

我知道這個問題在這里被問了很多，我已經閱讀了所有這些問題，但我的問題仍然存在。我想在我的專案中包含委托模式，以便從一個類呼叫另一個類中的函式。但它在那里也不起作用。所以我創建了一個全新的專案來再次練習該模式。只有3個小時，我就是做不到。我的委托仍然留在我的主 ViewController nil 中。

這是我的 ViewController 的代碼。它是初始化的 ViewController 也是委托模式的主人，而 buttonViewController 是仆人：


import UIKit

protocol ViewControllerDelegate {
    func printTest(message: String)
}

class ViewController: UIViewController {
    
    var delegate: ViewControllerDelegate?
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
    }
    

    @IBAction func perform(_ sender: UIButton) {
        let vc = storyboard?.instantiateViewController(withIdentifier: "vc") as! ButtonViewController
        if delegate == nil {
            print("Ist nil")
        }
        delegate?.printTest(message: "Test")
        present(vc, animated: true, completion: nil)
    }
    
}

如果用戶按下執行按鈕，我想在我的 buttonViewController 中呼叫一個函式，而不是我想對我的 buttonViewController 執行 segue。

這是我的 ButtonViewController：

import UIKit


class ButtonViewController: UIViewController, ViewControllerDelegate {


    override func viewDidLoad() {
        super.viewDidLoad()

        ViewController().delegate = self
        print("test")
        // Do any additional setup after loading the view.
    }

    /*
    // MARK: - Navigation

    // In a storyboard-based application, you will often want to do a little preparation before navigation
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
        // Get the new view controller using segue.destination.
        // Pass the selected object to the new view controller.
    }
    */
    
    @IBAction func performsegue(_ sender: UIButton) {
        
    }
    
    func printTest(message: String) {
        print(message   "\(self)")
    }
}

I've already tried to perform a segue that first viewDidLoad is called by my ButtonViewController, then to go back to the ViewController and then call the perform method again via a button to then see whether delegate is still nil, and yes it was unfortunately still nil.

Can somebody help me with this Problem?

Best regards!

uj5u.com熱心網友回復：

委托模式一開始讓很多人感到困惑。我發現我的大多數學生一開始都傾向于嘗試倒退，我認為這也是這里發生的事情。通常在委托模式中，有一個視圖控制器 (A) 呈現另一個 (B)。通常，B 上的某些操作應觸發 A 上的某些功能（在您的情況下，可能按下 B 上的按鈕會觸發 A 列印某些內容）。

在這種情況下，您將有兩個子類UIViewController：AViewController和BViewController和協議BViewControllerDelegate。它將設定如下：

該協議BViewControllerDelegate將具有您想要AViewController在BViewController按下按鈕時呼叫的功能。
AViewController 將符合此協議并實作此功能。
在BViewController你有你的委托屬性定義：weak var delegate: BViewControllerDelegate?。
在的呈現期間，此屬性將在的實體上BViewController由AViewController自身的實體（的實體AViewController）設定BViewController。
的實體BViewController將呼叫其委托屬性上的函式以回應按鈕按下。

class AViewController: UIViewController, BViewControllerDelegate {
    // This is 4, this segue action is invoked by a storyboard segue in the storyboard and is responsible for setting up the destination view controller and configuring it as needed (i.e., setting its delegate property)
    @IBSegueAction func makeBViewController(_ coder: NSCoder) -> BViewController {
        let bViewController = BViewController(coder: coder)!
        bViewController.delegate = self
        
        return bViewController
    }

    // Here we accomplish 2 (and also above where we declare conformance to the protocol)    
    func bViewControllerDidPerformAction(viewController: BViewController, message: String) {
        print(message)
    }
}

protocol BViewControllerDelegate: AnyObject {
    // Here we accomplish 1
    func bViewControllerDidPerformAction(viewController: BViewController, message: String)
}

class BViewController: UIViewController {
    // Here is 5
    @IBAction func buttonPressed(_ sender: Any) {
        delegate?.bViewControllerDidPerformAction(viewController: self, message: "Test Message")
    }
    
    // This is 3
    weak var delegate: BViewControllerDelegate?
}

我不是你想在你的代碼做什么明確的100％，但我相信你的ViewController喜歡應該是建立AViewController和你ButtonViewController應該設定類似BViewController。此外，這行代碼ViewController().delegate = self什么也不做，因為它創建了一個新實體ViewController并設定了它的委托，但是這個新實體立即被釋放，因為它實際上并不是在其他任何地方使用的那個。

uj5u.com熱心網友回復：

協議/委托模式用于允許實體化的控制器（或其他物件）與創建它的類進行通信。

因此，在您的場景中，您希望ViewController符合ViewControllerDelegate... 當您實體化ButtonViewController（具有ViewControllerDelegate委托變數）時，您將其ButtonViewController委托給 self ( ViewController)。

現在，ButtonViewController可以通過它的delegate.

protocol ViewControllerDelegate {
    func printTest(message: String)
}

// make ViewController conform to ViewControllerDelegate
class ViewController: UIViewController, ViewControllerDelegate {
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    
    @IBAction func perform(_ sender: UIButton) {
        let vc = storyboard?.instantiateViewController(withIdentifier: "vc") as! ButtonViewController
        
        // set self as the delegate in ButtonViewController
        vc.delegate = self
        
        present(vc, animated: true, completion: nil)
    }

    func printTest(message: String) {
        print("printTest in delegate:", message)
    }
    
}

class ButtonViewController: UIViewController {
    
    var delegate: ViewControllerDelegate?

    override func viewDidLoad() {
        super.viewDidLoad()
        print("test")
    }
    
    @IBAction func buttonTap(_ sender: UIButton) {
        // call the delegate
        delegate?.printTest(message: "Sending to delegate!")
    }

}

編輯- 這是一個完整的運行示例。“主”視圖控制器有一個按鈕和一個標簽。點擊按鈕以顯示 ButtonVC。按鈕視圖控制器有一個文本欄位和一個“保存并關閉”按鈕。在文本欄位中輸入一些文本，點擊保存按鈕，文本將通過ViewControllerDelegate.

不需要@IBOutlet或不需要@IBAction連接——只需將一個空白視圖控制器的自定義類分配給ViewController：

// make ViewController conform to ViewControllerDelegate
class ViewController: UIViewController, ViewControllerDelegate {
    
    let btn: UIButton = {
        let v = UIButton()
        v.setTitle("Show Button VC", for: [])
        v.backgroundColor = .systemRed
        v.setTitleColor(.white, for: .normal)
        v.setTitleColor(.lightGray, for: .highlighted)
        return v
    }()
    let label: UILabel = {
        let v = UILabel()
        v.backgroundColor = .green
        v.numberOfLines = 0
        v.text = "From ButtonVC:\n"
        return v
    }()

    override func viewDidLoad() {
        super.viewDidLoad()
        
        [label, btn].forEach { v in
            v.translatesAutoresizingMaskIntoConstraints = false
            view.addSubview(v)
        }
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            btn.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            btn.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            btn.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            label.topAnchor.constraint(equalTo: btn.bottomAnchor, constant: 20.0),
            label.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            label.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
        ])
        
        btn.addTarget(self, action: #selector(showButtonsVC(_:)), for: .touchUpInside)
    }
    
    @objc func showButtonsVC(_ sender: UIButton) {
        // instantiate Button View Controller
        let vc = ButtonViewController()
        
        // set self as the delegate in ButtonViewController
        vc.delegate = self
        
        // present it
        present(vc, animated: true, completion: nil)
    }

    func printTest(message: String) {
        print("Delegate received:", message)
        label.text = "From ButtonVC:\n"   message
    }
    
}

class ButtonViewController: UIViewController {
    
    var delegate: ViewControllerDelegate?

    let textField: UITextField = {
        let v = UITextField()
        v.borderStyle = .roundedRect
        return v
    }()
    let btn: UIButton = {
        let v = UIButton()
        v.setTitle("Save and Close", for: [])
        v.backgroundColor = .systemRed
        v.setTitleColor(.white, for: .normal)
        v.setTitleColor(.lightGray, for: .highlighted)
        return v
    }()
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        view.backgroundColor = .systemYellow
        
        [textField, btn].forEach { v in
            v.translatesAutoresizingMaskIntoConstraints = false
            view.addSubview(v)
        }
        
        let g = view.safeAreaLayoutGuide
        NSLayoutConstraint.activate([
            textField.topAnchor.constraint(equalTo: g.topAnchor, constant: 20.0),
            textField.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            textField.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
            btn.topAnchor.constraint(equalTo: textField.bottomAnchor, constant: 20.0),
            btn.leadingAnchor.constraint(equalTo: g.leadingAnchor, constant: 20.0),
            btn.trailingAnchor.constraint(equalTo: g.trailingAnchor, constant: -20.0),
        ])
        
        btn.addTarget(self, action: #selector(buttonTap(_:)), for: .touchUpInside)
    }
    
    @IBAction func buttonTap(_ sender: UIButton) {
        // call the delegate
        let str = textField.text ?? "No text entered..."
        delegate?.printTest(message: str)
        dismiss(animated: true, completion: nil)
    }

}

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標籤：ios swift delegates

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