const MyTab = TabNavigator({
    tab1:{screen:TabScreen1},
    tab2:{screen:TabScreen2}
}


class MainScreen extends React.Component{

    constructor(){
        super();
        this.state = {
            loading:true
        }
    }

    componentWillMount(){
        //fetch login
        //set loading:false when fetch is done
    }

    render(){
        !this.state.loading && <MyTab/>
    }
}