委托方法没有被 UITabBarController 调用

Question

在FourthViewController中，我有一个 slider，其值范围为 1 到 1000。设置的值通过委托发送到PatternViewController ，它应该用于做某事（我将打印用于测试目的）。

我以前和代表一起工作过，一切都好，多次检查代码，堆栈上有多个答案，我似乎找不到问题。 任何帮助将非常感激

更新：我添加了一个按钮，以便更容易跟踪。 事实证明，第一次按下按钮，什么都没有发生。 但是如果我首先检查PatternViewController ，然后我 go 回到FourthViewController并按下按钮，代表就会被触发。 有人知道为什么会这样吗？

第四视图控制器

import UIKit

class FourthViewController: UIViewController {

//MARK: Outlets

@IBOutlet var persistenceButton: UIButton!
@IBOutlet var persistenceSlider: UISlider!
@IBOutlet var persistenceLabel: UILabel!
weak var delegate: FourthViewControllerDelegate?

//MARK: Stored Properties - Constants

let userDefaults = UserDefaults.standard
let keyName = "sliderValue"

//MARK: Initializer

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

//MARK: Actions

@IBAction func handleValueChanged(_ sender: UISlider) {
    updateLabel()
    persistSliderValue(value: persistenceSlider.value, key: keyName)
    }

//MARK: Methods

func updateLabel() {
    persistenceLabel.text = String(format: "%.2f", persistenceSlider.value)
    }

func persistSliderValue(value: Float, key: String) {
    userDefaults.set(value, forKey: key)
    }

func loadSliderValue() {
    let persistedValue = userDefaults.float(forKey: keyName)
    persistenceSlider.value = persistedValue
    updateLabel()
    }
}

func initialSetUp()  {
    persistenceButton.addTarget(self, action: #selector(handleButtonPressed), for: .touchUpInside)
    }

@objc func handleButtonPressed() {
    delegate?.valueChanged(value: persistenceSlider.value)
    }
}

模式视图控制器

import UIKit

class PatternViewController: UIViewController, FourthViewControllerDelegate {

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

    func setUp() {
        if let tabBar = self.tabBarController, let viewController = tabBar.viewControllers, let fourthViewController = viewController[3] as? FourthViewController  {
            fourthViewController.delegate = self
        }
    }

    func valueChanged(value: Float) {
        print(value)
    }
}

Answer 1

这取决于您如何实例化选项卡视图 controller。 例如，如果您使用情节提要执行此操作，则各个选项卡的视图控制器会延迟实例化，仅在用户点击它们时才实例化。 （这有助于减少因实例化所有四个选项卡的视图控制器而导致的延迟。）

While you theoretically could go ahead and have the tab bar controller instantiate the four view controllers programmatically up front, rather than just-in-time via the storyboard, I might instead consider specifying a UITabBarControllerDelegate for the tab bar controller. 让标签栏控制器的委托方法更新相关标签的视图控制器的 model。

这是一个带有两个选项卡的示例，第一个具有 slider，第二个具有显示滑块值的 label。 In this simplified example, I've moved the model object (the value associated with the slider) into the tab bar controller, and it passes it to the second view controller when you select the associated tab.

//  TabViewController.swift

import UIKit

class TabBarController: UITabBarController {
    var value: Float = 0.5
    
    override func viewDidLoad() {
        super.viewDidLoad()

        delegate = self
    }
}

// MARK: - UITabBarControllerDelegate

extension TabViewController: UITabBarControllerDelegate {
    func tabBarController(_ tabBarController: UITabBarController, didSelect viewController: UIViewController) {
        guard let viewController = viewController as? SecondViewController else { return }

        viewController.value = value
    }
}

和

//  FirstViewController.swift

import UIKit

class FirstViewController: UIViewController {

    @IBOutlet weak var slider: UISlider!

    override func viewDidLoad() {
        super.viewDidLoad()
        guard let tabBarController = tabBarController as? TabViewController else { return }
        slider.value = tabBarController.value
    }

    @IBAction func didAdjustSlider(_ sender: UISlider) {
        guard let tabBarController = tabBarController as? TabViewController else { return }
        tabBarController.value = sender.value
    }

}

和

//  SecondViewController.swift

import UIKit

class SecondViewController: UIViewController {

    @IBOutlet weak var label: UILabel!

    var value: Float = 0 { didSet { updateLabel() } }

    let formatter: NumberFormatter = {
        let formatter = NumberFormatter()
        formatter.numberStyle = .percent
        return formatter
    }()

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

    func updateLabel() {
        label?.text = formatter.string(for: value)
    }
}

Probably needless to say, I not only set the base view controller class for the two tab's view controllers, but also set the base class for the tab bar controller's storyboard scene to the above TabBarController .