Use of @StateObject in Document-based SwiftUI application: getting instances instead of references

Question

I am trying to use a @Stateobject in my document as source of truth for the dataModel behind the document. However, it seems I am doing something wrong as I appear to get different instances for the document and the content view, which is in line with the warnings I am receiving: Accessing StateObject's object without being installed on a View. This will create a new instance each time.

Can anybody help? I am pretty sure this has a very simple fix...

Here is the code:

Observed Object:

class TestObject:ObservableObject{
    @Published var text: String
    init(){
        text = "initString"
    }
}

App:

@main
struct FileOpen5App: App {
    var body: some Scene {
        DocumentGroup(newDocument: FileOpen5Document()) { file in
            ContentView(document: file.$document, obj: file.document.docObject) /// instance warning
        }
    }
}

Document (partial):

struct FileOpen5Document: FileDocument {
    //var text: String
    @StateObject var docObject = TestObject()

    init() {
        docObject.text = "DocText" /// instance warning
    }
...

ContentView:

struct ContentView: View {
    @Binding var document: FileOpen5Document
    @ObservedObject var obj: TestObject
    
    var body: some View {
        Text(obj.text)
    }
}
'''

As the warning alludes to, @StateObject is for use inside a View -- you're trying to own it inside a FileDocument. What happens if you just remove the property wrapper? — jnpdx
– jnpdx, Commented Mar 9, 2022 at 19:29
I originally designed it to have the data object directly as a variable of the document, however, I get problems if I re-shape the data in response to user input as the View will hit it and cause out-of-bounds errors etc. I thought putting it into a observable object, I can prevent those errors (object will change notifications) — nonresidentalien
– nonresidentalien, Commented Mar 9, 2022 at 20:17

malhal · Accepted Answer · 2022-03-09 22:12:39Z

1

@StateObject is just for Views, try ReferenceFileDocument but before you resort to using classes I highly recommend figuring out if you can stick to structs because Swift and SwiftUI work best with value types. See https://developer.apple.com/documentation/swift/choosing_between_structures_and_classes

answered Mar 9, 2022 at 22:12

malhal

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