SwiftUI@State变量不是'；t更新

我的代码基本上与中的相同。我遇到的问题是，当

tapperstate

事件发生时，工作表显示（调用

sheet

代码），但调试显示

showUserEditor

为false（在这种情况下，工作表如何显示…？），

selectedUserId

仍为

nil

（因此在展开时崩溃…）

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!)
            }
        }
}

struct UsersView:View{
@环境（\.managedObjectContext）
私有变量viewContext
@获取请求(
SortDescriptor:[NSSortDescriptor（密钥路径：\User.昵称，升序：true）]，
动画：。默认设置）
私有var用户：FetchedResults
@状态私有变量selectedUserId:NSManagedObjectID？=nil
@状态私有变量showUserEditor=false
var body：一些观点{
导航视图{
名单{
ForEach（用户）{user in
UserRowView（用户：用户）
.ontapsigne{
self.selectedUserId=user.objectID
self.showUserEditor=true
}
}
}
}.sheet（显示：$showUserEditor）{
UserEditorView（用户ID: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。

因此，我仍然没有弄清楚问题中发布的代码为什么不起作用，使用@loremipsum的指针，我通过使用另一个

.sheet（）

方法获得了一个工作代码，接受可选项而不是布尔标志的项。代码现在看起来是这样的并且可以工作，但是如果有人能解释为什么发布的代码不能工作，我将不胜感激

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)
        }
    }
}

struct UsersView:View{
@环境（\.managedObjectContext）
私有变量viewContext
@获取请求(
SortDescriptor:[NSSortDescriptor（密钥路径：\User.昵称，升序：true）]，
动画：。默认设置）
私有var用户：FetchedResults
@状态私有变量selectedUser:用户？=nil
var body：一些观点{
导航视图{
名单{
ForEach（用户）{user in
UserRowView（用户：用户）
.ontapsigne{
self.selectedUser=用户
}
}.onDelete（执行：删除项）
}
}.sheet（项目：$selectedUser，onDismiss:nil）{中的用户
UserEditorView（用户：用户）
}
}
}

struct==immutable

和SwiftUI决定何时初始化并重新加载

struct

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

要使您的第一次安装工作正常，您需要为变量使用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")
    }
}

现在开始使用易于编辑的CoreData对象编写代码

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)
        }
    }
}

---

在将

showUserEditor

设置为true之前，至少将

selectedUserId

设置为true，交换行。或者将

工作表

init与中的

项

一起使用it@vadian我做到了。。。这是显而易见的方式，我改变了它，没有回头看。我将编辑问题以反映这一点。在任何情况下，据我所知，这对SwiftUI重画周期都不重要。另外，请注意，当我调试

工作表

代码时，即使是showUserEditor在打印变量时也是错误的，因此我在这里感到困惑。。。也许我不清楚调试@State变量时应该查找什么。@loremipsum-找到了您的意思：我试试看。有一个

.sheet

init，您可以在其中作为参数传递该项。当项目的

@状态

不是

nil

时，它会显示一张我可以键入答案的工作表。更好地解释一下，当工作表是在已经设置的变量中创建的，并且主体不会因为显示工作表而被重新加载时，结构是不可变的。

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)
        }
    }
}

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
        }
    }
}