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 }));
	}

首先查看 issue 中展示的 createStackNavigator 和 createSwitchNavigator 入口點，以及 React Native 的 BackHandler 情境。確定文件是否應說明如何偵測只剩下一個畫面並處理 Exit-Prompt；完成表示已透過一個與 stack1/stack2 設定相符的範例記錄支援的方法。