1.29 SwiftUI学习笔记及报错问题记录

原创已于 2025-07-17 10:14:45 修改 · 2k 阅读

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于 2022-01-29 18:12:09 首次发布

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本文介绍SwiftUI中Form的使用方法及其在数据输入控件中的应用，并探讨Future模式如何实现异步操作及结果处理。

1.Form

A container for grouping controls used for data entry, such as in settings or inspectors.

对用于数据输入的控件(如设置或检查器)进行分组的容器

用法：

var body: some View {
        Form {
            Section(header: Text("Notifications")) {
                Picker("Notify Me About", selection: $notifyMeAbout) {
                    Text("Direct Messages").tag(NotifyMeAboutType.directMessages)
                    Text("Mentions").tag(NotifyMeAboutType.mentions)
                    Text("Anything").tag(NotifyMeAboutType.anything)
                }
                Toggle("Play notification sounds", isOn: $playNotificationSounds)
                Toggle("Send read receipts", isOn: $sendReadReceipts)
            }
            Section(header: Text("User Profiles")) {
                Picker("Profile Image Size", selection: $profileImageSize) {
                    Text("Large").tag(ProfileImageSize.large)
                    Text("Medium").tag(ProfileImageSize.medium)
                    Text("Small").tag(ProfileImageSize.small)
                }
                Button("Clear Image Cache") {}
            }
        }
    
}

2.Future

A publisher that eventually produces a single value and then finishes or fails.

最终生成单个值，之后成功或失败的发布者

Future.Promise

A type that represents a closure to invoke in the future, when an element or error is available.、当元素或错误可用时，表示将来要调用的闭包的类型

typealias Promise = (Result<Output, Failure>) -> Void

3.eraseToAnyPublisher()

用来将类型擦除，最后得到一个AnyPublisher类型的发布者


func eraseToAnyPublisher() -> AnyPublisher<Self.Output, Self.Failure>

二、报错问题记录

1.Cannot infer key path type from context; consider explicitly specifying a root type

Insert '<#Root#>'

 ContentView().environment(\.managedObjectContext, persistenceController.container.viewContext)

Cannot find 'NSEntityDescription' and 'NSManagedObject' in scope

init(context: NSManagedObjectContext = PersistenceController.shared.container.viewContext) {
        _viewModel = StateObject(wrappedValue: TodoViewModel(context: context))
    }

解决办法 import CoreData