使用所需的便利初始化解码时如何修复“调用中的额外参数”？（编码器:)？

Question

我对应用程序开发相对较新，我正在尝试将持久性添加到使用 SwiftUI 编写的家庭作业管理应用程序中。我在我的“UserData”类中实现 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后，我在 self.init( ） 线。我的代码到底有什么问题？

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 是一个类，与结构不同，类没有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。如果没有它，Swift 可以找到的最接近的初始化程序是 init()，它继承自 NSObject - 除了参数名称不匹配，因此会出错。

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) 传递的参数需要被解包，因为你有可选项，而初始化程序不需要可选项。

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

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 - 你不需要它们，因为这些值将来自初始化程序