如何在 Swift 中正确实现不同视图控制器之间的协议和委托？

Posted 2023-02-23

【中文标题】如何在 Swift 中正确实现不同视图控制器之间的协议和委托？

【英文标题】：How to properly implement protocols and delegates between different view controllers in Swift?

【发布时间】：2020-03-23 06:04:51

【问题描述】：

所以我是协议和委托的新手。但是，在了解了它之后，我尝试在我的项目中使用它。我不确定我是否做错了什么。但我无法更新标签。

---

import UIKit

class CountryVC: UIViewController 
    //MARK:-Properties   
    let deathLabel: UILabel = 
        let label = UILabel()
        label.textColor = .black
        label.textAlignment = .center
        label.text = "I am Death"
        return label
    ()
    let recoveredLabel: UILabel = 
        let label = UILabel()
        label.textColor = .black
        label.textAlignment = .center
        return label
    ()
    let infectedLabel: UILabel = 
        let label = UILabel()
        label.textColor = .black
        label.textAlignment = .center
        return label
    ()
    override func viewDidLoad() 
        super.viewDidLoad()
        view.backgroundColor = .white
        view.addSubview(deathsView)
        view.addSubview(infectedView)
        view.addSubview(recoveredView)

        deathsView.anchor(top: view.topAnchor, left: view.leftAnchor, right: view.rightAnchor, paddingTop: 100, paddingLeft: 20, paddingRight: 20, height: 100)
        deathsView.addSubview(deathLabel)
        deathLabel.anchor()
        deathLabel.centerXAnchor.constraint(equalTo: deathsView.centerXAnchor).isActive = true
        deathLabel.centerYAnchor.constraint(equalTo: deathsView.centerYAnchor).isActive = true

        infectedView.anchor(top: deathsView.bottomAnchor, left: view.leftAnchor, right: view.rightAnchor, paddingTop: 20, paddingLeft: 20, paddingRight: 20, height: 100)
        deathsView.addSubview(infectedLabel)
        infectedLabel.anchor()
        infectedLabel.centerXAnchor.constraint(equalTo: infectedView.centerXAnchor).isActive = true
        infectedLabel.centerYAnchor.constraint(equalTo: infectedView.centerYAnchor).isActive = true       
        recoveredView.anchor(top: infectedView.bottomAnchor, left: view.leftAnchor, right: view.rightAnchor, paddingTop: 20, paddingLeft: 20, paddingRight: 20, height: 100)
        recoveredView.addSubview(recoveredLabel)
        recoveredLabel.anchor()
        recoveredLabel.centerXAnchor.constraint(equalTo: recoveredView.centerXAnchor).isActive = true
        recoveredLabel.centerYAnchor.constraint(equalTo: recoveredView.centerYAnchor).isActive = true
    

---

extension CountryVC: UpdateCountry 
    func update(allCases: Int, recovered: Int, deaths: Int) 
        DispatchQueue.main.async 
            self.deathLabel.text = String(deaths)
            self.recoveredLabel.text = String(recovered)
            self.infectedLabel.text = String(allCases)
        
        print (String(deaths))
        print(String(recovered))
        print(String(allCases))
    

---

我可以看到数据被传递，因为我可以打印数据。我在我的另一个表视图控制器中调用了更新函数来获取这样的数据。

---

class CountryTableControllerTableViewController: UITableViewController 
    //MARK:- Properties
    var delegate: UpdateCountry!
    //MARK:- Initialization
    override func viewDidLoad() 
        super.viewDidLoad()
        delegate = CountryVC()
        tableView.register(UITableViewCell.self, forCellReuseIdentifier: "reuseIdentifier")
        self.navigationItem.title = "Select Your Country"
    
    // MARK: - Table view data source
    override func numberOfSections(in tableView: UITableView) -> Int  1 
    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int  countries.count 
    override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell 
        let cell = tableView.dequeueReusableCell(withIdentifier: "reuseIdentifier", for: indexPath)
        cell.textLabel?.text = countries[indexPath.row]
        return cell
    
    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) 
        let urlCountry = "https://corona.lmao.ninja/countries/\(countries[indexPath.row])"
        let acturalUrl = urlCountry.lowercased()
        guard let url = URL(string: acturalUrl) else  return 
        let controler = CountryVC()
        controler.modalPresentationStyle = .fullScreen
        present(controler, animated: true, completion: nil)
        URLSession.shared.dataTask(with: url) (data, response, error) in
            guard let data = data else return
            do 
                let allCases = try JSONDecoder().decode(CountryCases.self, from: data)
                self.delegate.update(allCases: allCases.cases, recovered: allCases.recovered, deaths: allCases.deaths)  
             catch let jsonErr 
                print("Error serializing json", jsonErr)
            
        .resume()     
    

控制器显示数据，但 countryVC 中的标签未更新。

【问题讨论】：

您需要显示您的 CountryVC 类、您的 updateCountry 协议和 tableview 代码。确保设置其委托。请注意，以大写字母 UpdateCountry 开头的协议命名是 Swift 命名约定

请注意，您的代码是异步的。在完成处理程序中解码 json 之前，您应该向您展示 CountryVC 控制器。在调用 resume 后立即呈现它，它不会等待您的异步请求完成。

为什么DispatchQueue集合中标签的值

标签中的值在主线程中设置，因为更新 UI 元素总是在主队列中完成。

@LeoDabus 即使这样做了，我仍然看不到标签中的更新。

【参考方案1】：

首先按如下方式创建您的协议：

protocol CountryViewControllerDelegate: class 
    func updateLabels(_ controller: CountryViewController, country: String, allCases: Int, recovered: Int, deaths: Int)

---

然后在您的 CountryViewController 中：

class CountryViewController: UIViewController 
    weak var delegate: CountryViewControllerDelegate?
    // code ...

---

extension CountryViewController: CountryViewControllerDelegate 
    func updateLabels(_ controller: CountryViewController, country: String,  allCases: Int, recovered: Int, deaths: Int) 
        print("CountryViewController:", #function)
        DispatchQueue.main.async 

            // update your labels in the main thread 
            // self.deathLabel.text = String(deaths)
            // self.recoveredLabel.text = String(recovered)
            // self.infectedLabel.text = String(allCases)

            print("Country:", country)
            print("deaths:", deaths)
            print("recovered:", recovered)
            print("infected:", allCases)
        
    

---

终于在你的 CountryTableViewController 中：

class CountryTableViewController: UITableViewController, CountryViewControllerDelegate  

    weak var delegate: CountryViewControllerDelegate?
    // code ...

    override func viewDidLoad() 
        super.viewDidLoad()
        self.navigationItem.title = "Select Your Country"
        delegate = self
    

    // code ...

    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) 
        let country = countries[indexPath.row]
        let string = "https://corona.lmao.ninja/countries/\(country.lowercased())"
        guard let url = URL(string: link) else  return 
        let storyBoard = UIStoryboard(name: "Main", bundle: nil)
        let controller = storyBoard.instantiateViewController(withIdentifier: "CountryViewController") as! CountryViewController
        controller.delegate = self
        controller.modalPresentationStyle = .pageSheet
        controller.title = country
        let countryNavigationController = UINavigationController(rootViewController: controller)
        countryNavigationController.viewControllers = [controller]
        self.present(countryNavigationController, animated: true)

        URLSession.shared.dataTask(with: url)  data, response, error  in
            guard let data = data else  return 
            if let string = String(data: data, encoding: .utf8), string == "Country not found"  
                self.updateLabels(controller, country: country, allCases: 0, recovered: 0, deaths: 0)
                return
            
            do 
                let allCases = try JSONDecoder().decode(CountryCases.self, from: data)
                self.updateLabels(controller, country: country, allCases: allCases.cases, recovered: allCases.recovered, deaths: allCases.deaths)
             catch 
                print("JSON error:", error)
            
        .resume()
    
    func updateLabels(_ controller: CountryViewController, country: String, allCases: Int, recovered: Int, deaths: Int) 
        print("CountryTableViewController:", #function)
        controller.updateLabels(controller, country: country, allCases: allCases, recovered: recovered, deaths: deaths)
    

Sample Project