Passing SwiftUI View size to AppKit?

I'm using NSTabViewController to switch between SwiftUI Views, then resize my window frame size according to current View's size.

The question is, how can I pass the View's size to an AppKit Controller efficiently?

My code:

// My observable class

class SizeObservable: ObservableObject {
    static let shared = SizeObservable()
    
    @Published var firstSize: NSSize?
}

// My first test View

struct FirstView: View {
    @ObservedObject private var contentSize: SizeObservable = SizeObservable.shared
    
    var body: some View {
        VStack {
            Text("First")
        }
        .frame(width: 400, height: 280)
        .background(GeometryReader { geo in
            Rectangle()
                .fill(Color.clear)
                .onAppear(perform: {
                    contentSize.firstSize = geo.size
                })
        })
    }
}

// In my window controller

func testSize() {
    var firstSize: NSSize!

    SizeObservable.shared.$firstSize.sink { size in
        print(size)  // Breakpoint here
        firstSize = size
    }
    .store(in: &subscribers)

    print(firstSize)  // Breakpoint here
}

And I got below outputs:

nil
nil
Optional((400.0, 280.0))

Corresponding to breakpoints order: size(nil) -> firstSize(nil) -> size(Optional((400.0, 280.0)))

This means that I assigned nil to firstSize the first time I got size, just before size becomes Optional((400.0, 280.0)).

So how can I determine whether size is safe to use?

Or how can I pass my View's size to AppKit?

Any ideas? :)