老师您好！我想请问一下，我现在想把Toast加到主View上，老师的方法是这样的：

export default class First extends Component {

    render() {

        return(

            <View style={styles.container}>

           <TabNavigator>

    {this._renderTab(PopularPage,'tb_popular','最热',require('../../res/images/ic_polular.png'))}

    {this._renderTab(TrendingPage,'tb_trending','趋势',require('../../res/images/ic_trending.png'))}

    {this._renderTab(FavoritePage,'tb_favorite','收藏',require('../../res/images/ic_favorite.png'))}

    {this._renderTab(MyPage,'tb_my','我的',require('../../res/images/ic_my.png'))}

</TabNavigator>

<Toast ref={(toast)=>this.toast=toast}/>    //添加Toast 

            </View>

        );

    }

}

但用react-navigatiaon 里的方法是这样的：

const FirstApp = TabNavigator({

    Popular: {screen: homePage},

    Trending: {screen: TrendPage},

    Favorite:{ screen: FavoritePage},

    Mine:{screen:MinePage}

});

问题是<Toast ref={(toast)=>this.toast=toast}/>应该放到哪里？求指导一下