elements left in stack

I want to check number of elements in my react navigation stack. if there was only one screen left and user press back button, application show them a message to exit the app. I had to use BackHandler in all of my screens because I hadn't had found a property which could show elements in stack.
this is my routes

const route1= {
	screen1,
	screen2
};

const route2= {
        screen3,
	screen4
};

export const stack1= createStackNavigator(route1);
export const stack2= createStackNavigator(route2);

export default {stack1, stack2};

this is my navigator

createNavigator = (routeName, params) => {
		this.navigator = createAppContainer(createSwitchNavigator(routes, { initialRouteName: routeName, initialRouteParams: params }));
	}

Commencez par examiner les points d’entrée createStackNavigator et createSwitchNavigator présentés dans l’issue, ainsi que le contexte de BackHandler de React Native. Déterminez si la documentation doit expliquer comment détecter un seul écran restant et gérer l’Exit-Prompt ; c’est terminé lorsque l’approche prise en charge est documentée avec un exemple correspondant à la configuration stack1/stack2.