Need a progress bar during init a document

I have no idea how to do it: on MacOS, in Document.init(configuration: ReadConfiguration) I decode file, and restore objects from data, which in some cases could take a long time. Document isn't fully inited, so I have no access to it. But would like to have a progress bar on screen (easier to wait for done, for now). I know size, progress value, but no idea how to make view from object during init.

I know, this question may be very stupid.

init(configuration: ReadConfiguration) throws {
        guard let data = configuration.file.regularFileContents
        else {
            throw CocoaError(.fileReadCorruptFile)
        }
        let decoder = JSONDecoder()
        let flat = try decoder.decode(FlatDoc.self, from: data)
        
        print ("reverting \(flat.objects.count) objects...")
     ///This takes time, a lot of time, need progress bar
     ///Total is `flat.objects.count`, current is `objects.count`
     /// `Show me a way to the (next) progress bar!`
        revertObjects(from: flat.objects)
    
        print ("...done")
       
        
    }

update: I defined var flatObjects in Document, and I can convert in .onAppear.

struct TestApp: App {
     @State var isLoading = false
...
     ContentView(document: file.$document)
                .onAppear {
                    isLoading = true
                    Task {file.document.revertObjects()}
                    isLoading = false
                }
            if isLoading {
                ProgressView()
            }
...
}

But progress bar never shows, only rainbow ball