SwiftUI：如何将核心数据对象传递给视图？

Question

What I want to do: pass a reference to a NSManagedObject of a FetchRequest (here from: TestView) to another Child-View (here: LogRectangle).

What I tried: These are basically the important lines :

List(testObjects, id: \.self) { habit in
    LogRectangle(testObject: testObject)
}

And this is the whole code:

struct TestView : View {
    @Environment(\.managedObjectContext) var managedObjectContext
    @FetchRequest(
        entity: TestObject.entity(),
        sortDescriptors: [NSSortDescriptor(keyPath: \TestObject.name, ascending: true)]
    ) var testObjects: FetchedResults<TestObject>


    var body: some View {
        NavigationView {
            HStack {
                Button(action: {
                    let testObject = TestObject(context: self.managedObjectContext)
                    habit.name = "TestObject String"
                    do {
                        try self.managedObjectContext.save()
                    } catch {
                        // handle the Core Data error
                    }
                }) {
                    Text("Insert example TestObject")
                }
                List(testObjects, id: \.self) { habit in
                    LogRectangle(testObject: testObject)
                }
            }
                .navigationBarTitle("Test")
        }
    }
}

struct LogRectangle : View {
    var habit : TestObject

    var body: some View {
        Text(habit.name)
            .font(.title)
            .foregroundColor(.white)        
    }
}

Here I get this error on the line with the text in the class LogRectangle.

Cannot convert value of type 'String?' to expected argument type 'LocalizedStringKey'

Answer 1

You are encountering this error perhaps because the attribute, name , is defined as an optional string in the data model.

The Text struct does not take an optional value. It has to be an explicit value that is:

• A value conforming to the StringProtocol
• A literal string that can be used for loading a localized string

Therefore, you will need to make some changes to the model that you are passing to the LogRectangle . These change can either be made at the data model level (and then updating the associated NSManagedObject class, or you can have a computed property on the model to provide a non-optional value of the attribute.

Answer 2

You may have better luck using the testObject indices for the list rather than iterating on the actual objects. Try this:

List(testObjects.indices, id: \.self) { index in
     LogRectangle(testObject: testObjects[index])
}