物理键返回指定页面：
componentWillMount(){
BackHandler.addEventListener('hardwareBackPress',()=>this._back());
}
_back(){
const {navigate} = this.props.navigation;
navigate('My');
return true;
}物理键退出App：
componentWillMount(){
BackHandler.addEventListener('hardwareBackPress',()=>this._back());
}
_back(){
let timer=(new Date()).getTime();
if(timer-this.lastBackPressed < 2000){
BackHandler.exitApp();
return false;
}
this.lastBackPressed=timer;
ToastAndroid.show('再按一次退出应用',ToastAndroid.SHORT);
return true;
}