SwiftUI @State 变量未更新

Question

我的代码基本上和this question 一样。我遇到的问题是，当tapGesture 事件发生时，工作表显示（调用sheet 代码）但调试显示showUserEditor 为假（在这种情况下，工作表如何显示......？）并且 selectedUserId 仍然是 nil （因此在打开它时会崩溃......）

观点：

struct UsersView: View {
    @Environment(\.managedObjectContext)
    private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \User.nickname, ascending: true)],
        animation: .default)
    private var users: FetchedResults<User>
    
    @State private var selectedUserId : NSManagedObjectID? = nil
    @State private var showUserEditor = false
    
    var body: some View {
        NavigationView {
            List {
                ForEach(users) { user in
                    UserRowView(user: user)
                        .onTapGesture {
                            self.selectedUserId = user.objectID
                            self.showUserEditor = true
                        }
                    }
                }
            }.sheet(isPresented: $showUserEditor) {
                UserEditorView(userId: self.selectedUserId!)
            }
        }
}

如果您愿意，我可以发布编辑器和行，但它们似乎与问题无关，因为魔术应该发生在视图中。

Answer 1

所以，我仍然没有弄清楚为什么问题中发布的代码不起作用，使用来自 @loremipsum 的指针我通过使用另一个 .sheet() 方法得到了一个工作代码，该方法采用可选的 Item 和不是布尔标志。代码现在看起来像这样并且可以工作，但如果有人能解释为什么发布的代码不起作用，我会很感激。

struct UsersView: View {
    @Environment(\.managedObjectContext)
    private var viewContext

    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \User.nickname, ascending: true)],
        animation: .default)
    private var users: FetchedResults<User>
    
    @State private var selectedUser : User? = nil
    
    var body: some View {
        NavigationView {
            List {
                ForEach(users) { user in
                    UserRowView(user: user)
                        .onTapGesture {
                            self.selectedUser = user
                        }
                    }.onDelete(perform: deleteItems)
                }
        }.sheet(item: $selectedUser, onDismiss: nil) { user in
            UserEditorView(user: user)
        }
    }
}

Answer 2

struct == immutable 和 SwiftUI 决定 struct 何时初始化并重新加载

不建议使用依赖于 SwiftUI 在特定时间更新非包装变量的代码。您无法控制此过程。

要使您的第一个设置工作，您需要对变量使用 SwiftUI 包装器

.sheet(isPresented: $showUserEditor) {
            //struct == immutable SwiftUI wrappers load the entire struct when there are changes
            //With your original setup this variable gets created/set when the body is loaded so the orginal value of nil is what is seen in the next View
            UserEditorView1(userId: $selectedUserId)
        }

struct UserEditorView1: View {
    //This is what you orginal View likely looks like it won't work because of the struct being immutable and SwiftUI controlling when the struct is reloaded

    //let userId: NSManagedObjectID? <---- Depends on specific reload steps

    //To make it work you would use a SwiftUI wrapper so the variable gets updated when SwiftUI descides to update it which is invisible to the user
    @Binding var userId: NSManagedObjectID?
    //This setup though now requres you to go fetch the object somehow and put it into the View so you can edit it.
    //It is unnecessary though because SwiftUI provides the .sheet init with item where the item that is set gets passed directly vs waiting for the SwiftUi update no optionals
    var body: some View {
        Text(userId?.description ?? "nil userId")
    }
}

您的答案代码不起作用，因为您的参数是可选的并且Binding 不喜欢可选参数

struct UserEditorView2: View {
    //This is the setup that you posted in the Answer code and it doesn't work becaue of the ? Bindings do not like nil. You have to create wrappers to compensate for this
    //But unecessary because all CoreData objects are ObservableObjects so you dont need Binding here the Binding is built-in the object for editing the variables
    @Binding var user: User?
    
    var body: some View {
        TextField("nickname", text: $user.nickname)
    }
}

现在可以使用易于编辑的 CoreData 对象来处理代码

struct UsersView: View {
    @Environment(\.managedObjectContext)
    private var viewContext
    
    @FetchRequest(
        sortDescriptors: [NSSortDescriptor(keyPath: \User.nickname, ascending: true)],
        animation: .default)
    private var users: FetchedResults<User>
    //Your list view would use the CoreData object to trigger a sheet when the new value is available. When nil there will not be a sheet available for showing
    @State private var selectedUser : User? = nil
    
    var body: some View {
        NavigationView {
            List {
                ForEach(users) { user in
                    UserRowView(user: user)
                        .onTapGesture {
                            self.selectedUser = user
                        }
                }
            }
        }.sheet(item: $selectedUser, onDismiss: nil) { user in //This gives you a non-optional user so you don't have to compensate for nil in the next View
            UserEditorView3(user: user)
        }
    }
}

那么工作表中的视图将如下所示

struct UserEditorView3: View {
    //I mentioned the ObservedObject in my comment 
    @ObservedObject var user: User
    var body: some View {
        //If your nickname is a String? you have to compensate for that optional but it is much simpler to do it from here
        TextField("nickname", text: $user.nickname.bound)
    }
}


//This comes from another very popular SO question (couldn't find it to quote it) that I could not find and is necessary when CoreData does not let you define a variable as non-optional and you want to use Binding for editing
extension Optional where Wrapped == String {
    var _bound: String? {
        get {
            return self
        }
        set {
            self = newValue
        }
    }
    public var bound: String {
        get {
            //This just give you an empty String when the variable is nil
            return _bound ?? ""
        }
        set {
            _bound = newValue.isEmpty ? nil : newValue
        }
    }
}