如何在viewdidload（）中使用回调函数？

Question

我得到了一个带有函数（requestSitesData）的类（Client），它可以从服务器获取数据，并且它有@escaping：

func requestSitesData(response: @escaping (_ data: Data?, _ error: Error?) -> Void) {
        DispatchQueue.global(qos: .default).asyncAfter(deadline: .now() + randomTimeDelay()) {
            guard self.isRequestSuccessful() else {
                response(nil, ClientError.requestFailed)

                return
            }
            guard let fileURL = Bundle.main.url(forResource: self.responseFilename, withExtension: "json") else {
                response(nil, ClientError.noFile)
                return 
            }
            do {
                let data = try Data(contentsOf: fileURL)
                let _ = try JSONSerialization.jsonObject(with: data)
                response(data, nil)
            } catch let error {
                response(nil, error)
            }
        }
    }

所以我认为响应函数是一个回调函数。在我的项目中，我有一个全局变量来存储来自服务器的数据，并想在我的viewcontroller上显示它。但似乎无论我在callcontroller类中放置call_back函数，都会在所有函数之后调用回调函数（应该如此），因此我无法在viewdidload（）中获取数据。

func call_back(data: Data?, error: Error?) -> Void {
        print("call_back called")
        if let file_data = data {
            if let sites_map = try? JSONSerialization.jsonObject(with: file_data, options: .mutableLeaves) as! [String: [Dictionary<String, Any>]] {
                if let sites = sites_map["sites"] {
                    for site in sites {
                        if let name = site["name"] as? String,
                        let slogan = site["slogan"] as? String,
                        let url = site["url"] as? String,
                        let description = site["description"] as? String,
                        let color = site["color"] as? String {
                            let single_site: Site = Site(name: name, slogan: slogan, url: url, description: description, color: color)
                            sites_res.append(single_site)
                        }
                    }
                }
            }
        }
    }

override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let c = Client()
        c.requestSitesData { (response_data, response_error) in
            self.call_back(data: response_data, error: response_error)
            print("ddd")
        }

    }

我不知道如何在viewdidload（）中获取数据，是不是因为我调用这个call_back函数的方式是错误的？有人可以帮忙吗？

Answer 1

修改功能如下：

func call_back(data: Data?, error: Error?) -> [String: [Dictionary<String, Any>]]? {
            print("call_back called")
            if let file_data = data {
                if let sites_map = try? JSONSerialization.jsonObject(with: file_data, options: .mutableLeaves) as? [String: [Dictionary<String, Any>]] {
                    return sites_map
                }
            }
            return nil
        }


override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
        let c = Client()
        c.requestSitesData { (response_data, response_error) in
            if let sites_map = self.call_back(data: response_data, error: response_error){
        if let sites = sites_map["sites"] {
            for site in sites {
                if let name = site["name"] as? String,
                    let slogan = site["slogan"] as? String,
                    let url = site["url"] as? String,
                    let description = site["description"] as? String,
                    let color = site["color"] as? String {
                    let single_site: Site = Site(name: name, slogan: slogan, url: url, description: description, color: color)
                    sites_res.append(single_site)
                }
            }
        }
    }
  }
}

Answer 2

您将需要重新加载更新回调内部视图的数据，转义闭包比它调用的函数更长，因此其中的回调只会在函数完成执行后发生，并将期望值返回给回调。

问题是您不知道返回该数据需要多长时间，您要么创建一个更新的函数来重新加载视图并使回调函数调用它，要么您可以在打开之前获取数据呈现ViewController和将数据传递给它。

编辑1

尝试按照下面显示的方式进行操作，以便更新View Controller中的数据或仅更新View中的数据，但更改它以符合您的需要。

func call_back(data: Data?, error: Error?) -> Void {
    print("call_back called")
    if let file_data = data {
        if let sites_map = try? JSONSerialization.jsonObject(with: file_data, options: .mutableLeaves) as! [String: [Dictionary<String, Any>]] {
            if let sites = sites_map["sites"] {
                for site in sites {
                    if let name = site["name"] as? String,
                    let slogan = site["slogan"] as? String,
                    let url = site["url"] as? String,
                    let description = site["description"] as? String,
                    let color = site["color"] as? String {
                        let single_site: Site = Site(name: name, slogan: slogan, url: url, description: description, color: color)
                        sites_res.append(single_site)
                    }
                }
            }
        }
    }
}
override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.
    let c = Client()
    c.requestSitesData { (response_data, response_error) in
        self.call_back(data: response_data, error: response_error)
        updateData()
    }

}

func updateData(){
//either update your data here or just update the data in the subviews.
    mylabel.text = data.text
}