ViewModel 函数：无法赋值：'error' 是 'let' 常量

Question

我正在努力学习 swift ViewModel，并且我创建了一个函数来处理错误。

但是当我在 ViewController 文件中调用该函数时，它会显示：

Cannot assign to value: 'error' is a 'let' constant

有没有办法解决这个问题？我仍然是一名学生，正在寻找最好的建议。

非常感谢。

下面是我的代码，你们可以帮帮我吗。

SignUpError.swift ：

class SingUpError {
    func errorMessage(name: String?, lastName: String?,email: String?, password: String?, error: String?){

        enum ErrorMessage: String {
            case noName = "Name cannot be empty"
            case noLastName = "Last name cannot be empty"
            case noEmail = "Email cannot be empty"
            case noPassword = "Password cannot be empty"
        }

        if name == "" {
            error = ErrorMessage.noName.rawValue
        }
    }
}

NewUserVC.swift ：

class NewUserVC: UIViewController {

    // Var
    var vm = SingUpError()

    // Outlet Text Fields

    @IBOutlet weak var nameText: UITextField!
    @IBOutlet weak var lastNameText: UITextField!
    @IBOutlet weak var emailText: UITextField!
    @IBOutlet weak var passwordText: UITextField!

    // Outlet Labels

    @IBOutlet weak var nameErrLbl: UILabel!
    @IBOutlet weak var lastNameErrLbl: UILabel!
    @IBOutlet weak var emailErrLbl: UILabel!
    @IBOutlet weak var passwordErrLbl: UILabel!


    @IBAction func submiButton(_ sender: UIButton) {
        submitData()
    }

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

    func submitData() {
        guard
            let name = nameText.text,
            let lastName = lastNameText.text,
            let email = emailText.text,
            let password = passwordText.text
            else {return}

        vm.errorMessage(name: name, lastName: lastName, email: email, password: password, error: nameErrLbl.text)
    }
}

Answer 1

我不完全确定你想用SingUpError 做什么（你的意思是SignUpError？）但这是我的尝试。基本上我将枚举移出 func 并让 func 返回一个字符串

class SingUpError  {
    enum ErrorMessage: String {
        case noName = "Name cannot be empty"
        case noLastName = "Last name cannot be empty"
        case noEmail = "Email cannot be empty"
        case noPassword = "Password cannot be empty"
    }

    func errorMessage(name: String?, lastName: String?,email: String?, password: String?) -> String? {
        var message: String?

        if name == nil || name.isEmpty {
            message = ErrorMessage.noName.rawValue
        } //else if lastName...

        return message
    }
}

然后在submitData

if let error = vm.errorMessage(name: name, lastName: lastName, email: email, password: password) {
     nameErrLbl.text = error
}

Answer 2

你试图给常量赋值。

你不能这样做，因为error 它是函数的参数。 如果您想从此函数中获取错误消息，您需要编写如下内容：

return ErrorMessage.noName.rawValue

改为

error = ErrorMessage.noName.rawValue

Answer 3

class SingUpError {
    func errorMessage(name: String?, lastName: String?,email: String?, password: String?) -> String? {

        guard let _ = name else { return ErrorMessage.noName.rawValue }

        // <this seems incomplete, but..>
        enum ErrorMessage: String { 
            case noName = "Name cannot be empty"
            case noLastName = "Last name cannot be empty"
            case noEmail = "Email cannot be empty"
            case noPassword = "Password cannot be empty"
        }

        return nil
    }
}

Answer 4

你需要做这样的事情

class SingUpError {
    func errorMessage(name: String?, lastName: String?,email: String?, password: String?, error: String?) -> String{

        var errorMessage : String = ""

        enum ErrorMessage: String {
            case noName = "Name cannot be empty"
            case noLastName = "Last name cannot be empty"
            case noEmail = "Email cannot be empty"
            case noPassword = "Password cannot be empty"
        }

        if name == "" {
            errorMessage = ErrorMessage.noName.rawValue
        }

        return errorMessage
    }
}  

Answer 5

当你在 swift 上声明一个函数时，它的参数默认是 'let' 常量。而且我们不能为常量赋值。

在您的情况下，您试图分配给“错误”变量，这是一个参数，因此是一个常量（let）。你不能这样做。

您可能会返回错误消息

class SingUpError {
    func errorMessage(name: String?, lastName: String?,email: String?, password: String?) -> String{

        enum ErrorMessage: String {
            case noName = "Name cannot be empty"
            case noLastName = "Last name cannot be empty"
            case noEmail = "Email cannot be empty"
            case noPassword = "Password cannot be empty"
        }

        if name == "" {
            return ErrorMessage.noName.rawValue
        }

        return ""
    }
}

Answer 6

首先，将此枚举从类 SignUpError 中取出

enum ErrorMessage: String {
        case noName = "Name cannot be empty"
        case noLastName = "Last name cannot be empty"
        case noEmail = "Email cannot be empty"
        case noPassword = "Password cannot be empty"
    }

其次，你可以像这样修改你的函数：

func errorMessage(name: String?, lastName: String?,email: String?, password: String?) -> String? {
    if name == "" {
        let error = ErrorMessage.noName.rawValue
        return error
    }
    return nil
}

现在在你的 ViewController 中，你可以修改你的函数来得到这样的 errorMessage

func submitData() {
    //YOUR EXISTING CODE HERE    
    if let errorMessage = vm.errorMessage(name: name, lastName: lastName, email: email, password: password) {
        print("errorMessage:\(errorMessage)")
        return
    }
}

或者您可以尝试使用相同的方法，但将错误作为 inout 参数传递。 similar question here

Answer 7

正如其他答案所说，您不能分配给 'error' 参数，因为它是一个常量（如果您愿意，可以使用 let 变量）。现在您可以按照建议的方式修改函数以返回字符串，但最好采用更结构化的方法并实际抛出错误。

这样的事情会做：

enum sumbissionErrors: Error {
    case noName
    case noLastName
    case noEmail
    case noPassword

    var description: String {
        switch self {
        case .noName:
            return "Name cannot be empty"
        case .noLastName:
            return "Last name cannot be empty"
        case .noEmail:
            return "Email cannot be empty"
        case .noPassword:
            return "Password cannot be empty"
        }
    }
}

func submitData() throws {
    if nameText.text == nil || nameText.text!.isEmpty {
        throw sumbissionErrors.noName
    }

    if lastNameText.text == nil || lastNameText.text!.isEmpty {
        throw sumbissionErrors.noLastName
    }

    if emailText.text == nil || emailText.text!.isEmpty {
        throw sumbissionErrors.noEmail
    }

    if passwordText.text == nil || passwordText.text!.isEmpty {
        throw sumbissionErrors.noPassword
    }

    // Do whatever you want now that the data is valid.
}

@IBAction func submitButton(_ sender: UIButton) {
    do {
        try submitData()
    }
    catch {
        if let sumbmissionError = error as? sumbissionErrors {
            switch sumbmissionError {
            case .noName:
                nameErrLbl.text = sumbmissionError.description
            case .noLastName:
                lastNameErrLbl.text = sumbmissionError.description
            case .noEmail:
                emailErrLbl.text = sumbmissionError.description
            case .noPassword:
                passwordErrLbl.text = sumbmissionError.description
            }
        } else {
            // This will probably never happen but it indicates that some other more general error occured and needs to be handled here.
        }
    }
}

这只是一个示例，就像您的原件一样，只会抛出它遇到的第一个错误。您可能希望重新构建所有内容以允许标记多个错误。 IE。有可能不止一个是空的。

Answer 8

如果您想采用与以前相同的方法，那么您需要更改函数中的一些小事情。

首先让函数参数为常量意味着你不能分配值，但如果你仍然需要为你的参数分配一些值，那么在函数中将有一种使用 inout 关键字的方式

class SingUpError {
    func errorMessage(name: String?, lastName: String?,email: String?, password: String?, error: inout String?){

        enum ErrorMessage: String {
            case noName = "Name cannot be empty"
            case noLastName = "Last name cannot be empty"
            case noEmail = "Email cannot be empty"
            case noPassword = "Password cannot be empty"
        }

        if name == "" {
            error = ErrorMessage.noName.rawValue
        }
    }
}

打电话

vm.errorMessage(name: name, lastName: lastName, email: email, password: password, error: &nameErrLbl.text)

这里也有一些参考链接

When to use inout parameters?