执行 segue swift 4

Posted 2023-02-23

【中文标题】执行 segue swift 4

【英文标题】：perform segue swift 4

【发布时间】：2018-05-18 05:40:07

【问题描述】：

我正在尝试使用我的数据库验证用户的登录。当按下登录按钮时，我想根据从数据库返回的信息来判断是否会发生转场。我从最初设置为 true 的变量决策开始，如果我无法验证用户，我想将其设置为 false 并防止 segue。这是我的代码，但它有问题。最后的 return 语句始终为真。基本上发生的事情是在从数据库返回响应之前首先调用 .resume() 之后最后的 return 语句。有人可以澄清为什么会发生这种情况

 override func shouldPerformSegue(withIdentifier identifier: String,sender: 
 Any?) -> Bool
 
             var decision = true
            let url = URL(string:"http://192.23.25.98/login/php")
            var request = URLRequest(url: url!)
            request.httpMethod = "POST"
            let body = "UserNameLogIn=\(userName.text!.lowercased())&PasswordLogIn=\(passWord.text!.lowercased())"
            request.httpBody=body.data(using: String.Encoding.utf8)
            URLSession.shared.dataTask(with: request)  (data:Data?, response:URLResponse?, error:Error?) in
                if (error == nil)
               
                    DispatchQueue.main.async(execute: 
                        do
                        
                            let json = try JSONSerialization.jsonObject(with: data!, options: .mutableContainers) as? Dictionary<AnyHashable,AnyObject>
                            guard let parseJson = json else
                                print ("error parsing")
                                return
                            
                            let status = parseJson["status"]
                            if (status != nil)
                            
                                if (parseJson.count>3)
                                
                                    decision = true

                                
                                else
                                
                                    decision = false


                                
                            
                        
                        catch
                        

                            print("error: \(error)")
                        
                    )
                
                else
                
                    decision = false
                
                .resume()
    return decision

【问题讨论】：

您不能执行异步操作来从shouldPerformSegue返回值。

我试过 DispatchQueue.main.sync 还是不行

没关系，数据任务还是异步的。

抱歉，您能否进一步详细说明。你的意思是 URLSession.shared.dataTask 总是异步的？

是的。您需要在不进行任何网络访问的情况下在瞬间做出决定。

【参考方案1】：

与其在点击按钮时启动 segue 并尝试在 shouldPerformSegue(withIdentifier:sender:) 中捕获异步过程，不如让按钮定义一个 @IBAction 来执行验证请求并以编程方式启动 segue，如果它成功了。

因此：

从按钮中移除 segue。

通过control在视图控制器之间添加segue - 从第一个场景上方栏中的视图控制器图标拖动到第二个场景：

选择 segue 并在属性检查器中为其命名：

从按钮中移除 segue 后，您现在可以将按钮连接到 @IBAction 方法：

当您完成验证逻辑时，您的登录操作应该performSegue(withIdentifier:sender:)：

performSegue(withIdentifier: "NextSceneSegue", sender: self)

例如

@IBAction func didTapLoginButton(_ sender: Any) 
    let url = URL(string:"http://192.23.25.98/login/php")
    var request = URLRequest(url: url!)
    request.httpMethod = "POST"
    request.httpBody = ["UserNameLogIn": useridTextField.text!, "PasswordLogIn": passwordTextField.text!]
        .map  $0.key + "=" + $0.value.addingPercentEncoding(withAllowedCharacters: .urlQueryValueAllowed)! 
        .joined(separator: "&")
        .data(using: .utf8)

    URLSession.shared.dataTask(with: request)  data, response, error in
        DispatchQueue.main.async 
            guard let data = data, let httpResponse = response as? HTTPURLResponse, (200 ..< 300) ~= httpResponse.statusCode, error == nil else 
                // handle basic network errors here
                return
            

            guard let json = (try? JSONSerialization.jsonObject(with: data)) as? [String: Any] else 
                // handle json parsing errors here
                return
            

            if json["status"] == nil  
                // handle missing status here
                return
            

            guard json.count > 3 else 
                // handle incorrect count here
                return
            

            self.performSegue(withIdentifier: "NextSceneSegue", sender: self)
        
    .resume()

注意，我对请求正文中的值进行百分比编码。尤其是密码，可能包含保留字符。而且我没有弄错使用.urlQueryAllowed，而是使用了.urlQueryValueAllowed，其中我抽出了几个保留字符：

extension CharacterSet 

    /// Returns the character set for characters allowed in the individual parameters within a query URL component.
    ///
    /// The query component of a URL is the component immediately following a question mark (?).
    /// For example, in the URL `http://www.example.com/index.php?key1=value1#jumpLink`, the query
    /// component is `key1=value1`. The individual parameters of that query would be the key `key1`
    /// and its associated value `value1`.
    ///
    /// According to RFC 3986, the set of unreserved characters includes
    ///
    /// `ALPHA / DIGIT / "-" / "." / "_" / "~"`
    ///
    /// In section 3.4 of the RFC, it further recommends adding `/` and `?` to the list of unescaped characters
    /// for the sake of compatibility with some erroneous implementations, so this routine also allows those
    /// to pass unescaped.

    static var urlQueryValueAllowed: CharacterSet = 
        let generalDelimitersToEncode = ":#[]@"    // does not include "?" or "/" due to RFC 3986 - Section 3.4
        let subDelimitersToEncode = "!$&'()*+,;="

        var allowed = CharacterSet.urlQueryAllowed
        allowed.remove(charactersIn: generalDelimitersToEncode + subDelimitersToEncode)
        return allowed
    ()