屏幕中间的反应导航选项卡

Question

我在react-native项目中使用react-navigation，我想在我的屏幕中间选项卡导航器默认它给出顶部或底部位置。

const Routes =   {
  RECENT: { screen : Upcomings},
  UPNEXT : { screen : Upcomings},
  UPCOMING :{ screen : Upcomings},
};

const TabConfig = {
  stateName: 'MainNavigation',
  tabBarOptions: {
    activeTintColor: '#e91e63',
    labelStyle: {
      fontSize: 12,
    },style: {
    backgroundColor: '#ffffff',
    borderTopWidth: 1,
    borderTopColor: 'gray',
  }
  },
};
// register all screens of the app (including internal ones)
export const screenRoute = (SignIn) => {
  return StackNavigator({
    LOG_IN:{screen:Login},
    SIGN_UP:{screen:SignUp},
    CHARITY:{screen:Charity},
    FETCH_FRIEND:{screen:FetchFriend},
    DASHBOARD:{screen: Dashboard},
    PAYPAL:{screen: Paypal},
    FPASSWORD:{screen: ForgetPassword},
    ADDFRIEND:{screen: AddFriend},
    UPCOMINGS : {screen :TabNavigator(Routes, TabConfig)},
    },{
    headerMode: 'none',
    mode:'modal',
    initialRouteName: SignIn ? 'UPCOMINGS':'LOG_IN'
  });
};

这是否可以通过任何方式提前完成。