使用所需的便利初始化解码时如何修复 "Extra argument in call"？（编码器:)？

Question

我是应用程序开发的新手，正在尝试为使用 SwiftUI 编写的家庭作业管理应用程序添加持久性。我在我的 "UserData" class 中实现了 NSCoding 并且能够毫无问题地创建编码函数，但是在我设置解码函数之后我在 self.init() 行。我的代码究竟有什么问题？

class UserData: NSObject, NSCoding, ObservableObject {

    // MARK:- Properties

    @Published var assignments: [Assignment]
    @Published var courses: [Course] = []

    @Published var username: String = ""
    @Published var appState: AppState = AppState(willPresentSplashScreen: false)

...

 // MARK:- NSCoding

    func encode(with coder: NSCoder) {
        coder.encode(assignments, forKey: PropertyKey.assignments)
        coder.encode(courses, forKey: PropertyKey.courses)
        coder.encode(username, forKey: PropertyKey.username)
        coder.encode(appState, forKey: PropertyKey.appState)
    }

    required convenience init?(coder decoder: NSCoder) {

        let assignments = decoder.decodeObject(forKey: PropertyKey.assignments) as? [Assignment]
        let courses = decoder.decodeObject(forKey: PropertyKey.courses) as? [Course]
        let username = decoder.decodeObject(forKey: PropertyKey.username) as? String
        let appState = decoder.decodeObject(forKey: PropertyKey.appState) as? AppState

        self.init(assignments: assignments, courses: courses, username: username, appState: appState)
    }
}

Answer 1

UserData 是一个 class，并且 classes 与结构不同，不会获得默认的成员初始化器。您需要创建一个接受这些参数的初始化程序。没有它，Swift 可以找到的最接近的初始化器是从 NSObject 继承的 init() - 除了参数名称不匹配所以它给出了一个错误。

您需要创建成员初始化器：

init(assignments: [Assignment], courses: [Course], username: String, appState: AppState) {
    self.assignments = assignments
    self.courses = courses
    self.username = username
    self.appState = appState
}

这会给你一个错误，你从 init?(coder decoder: NSCoder) 传递的参数需要解包，因为你有可选值而初始化程序不期望可选值。

init?(coder decoder: NSCoder) 是一个失败的初始化器，所以你可以 return nil 如果一切都不对：

required convenience init?(coder decoder: NSCoder) {

        guard let assignments = decoder.decodeObject(forKey: PropertyKey.assignments) as? [Assignment],
            let courses = decoder.decodeObject(forKey: PropertyKey.courses) as? [Course],
            let username = decoder.decodeObject(forKey: PropertyKey.username) as? String,
            let appState = decoder.decodeObject(forKey: PropertyKey.appState) as? AppState else {
                return nil
        }

        self.init(assignments: assignments, courses: courses, username: username, appState: appState)
}

我可能会去掉分配给 courses、username 和 appState 的默认值——您不需要它们，因为这些值将来自初始化程序

使用所需的便利初始化解码时如何修复 "Extra argument in call"？（编码器:)？

How do I fix "Extra argument in call" when decoding with required convenience init?(coder:)?

ios

swift

nscoding