This is an example of using PreferenceKey with GeometryReader. Whenever this scaled red square changes geometry because of the space given by the parent view, the preference will update, updating the State variable. This will not causes State changes during view update issues.

GeometryReaderPreferenceKeyExample.swift

struct ContentView: View {
    @State private var size: CGFloat = 0

    var body: some View {
        Color.red
            .scaledToFit()
            .background(
                GeometryReader { geo in
                    Color.clear.preference(
                        key: SizePreferenceKey.self,
                        value: geo.size.width
                    )
                }
                .onPreferenceChange(SizePreferenceKey.self) { newSize in
                    print(newSize)
                    size = newSize
                }
            )
    }
}


struct SizePreferenceKey: PreferenceKey {
    static let defaultValue: CGFloat = 0

    static func reduce(value: inout CGFloat, nextValue: () -> CGFloat) {
        value = nextValue()
    }
}