How to get state from parent in react native? -

i have tabnavigator :

        <tabnavigator>              <tabnavigator.item                 selected={this.state.selectedtab === 'home'}                 title="home"                 rendericon={() => <image source={icon1}/>}                 renderselectedicon={() => <image source={require('./img/slider-left.png')}/>}                 onpress={() => this.setstate({ selectedtab: 'home' })}                 >                 <mainnav selectedtab={this.state.selectedtab}                     getselectedtab = {() => this.state.selectedtab}/>              </tabnavigator.item>             <tabnavigator.item                 selected={this.state.selectedtab === 'profile'}                 title="profile"                 rendericon={() => <image source={icon1}/>}                 renderselectedicon={() => <image source={icon1}/>}                  onpress={() => this.setstate({ selectedtab: 'profile' })}                 >                 <topicnav selectedtab={this.state.selectedtab} /*this not work*//>             </tabnavigator.item>          </tabnavigator> 

and in mainnav , topicnav （in seperated files） both have navigator stack, , backandroid process physical events, 2 backandroid can event @ same time, this:

backandroid.addeventlistener('hardwarebackpress', () => {     if (_navigator.getcurrentroutes().length === 1  ) {         return false;     }      _navigator.pop();     return true; }); 

i need know current shown tab, , respond shown one. how can parent state selectedtab when changes? thanks.