react-navigation How to set tabBar margin corresponding to Animated Value

Question

I am using react-navigation Tab Navigation. I have a header above my tab navigation and it can collapse and expand corresponding to the scrollView.

This is my problem: When I scroll all the way up, the header will collapse and thats what I want but the tabBar will stay static(Please see the photo). Is there a way I could set the tabBar margin corresponding to the scrollview? So that there is no marginTop when the header is collapsed.

const Header_Maximum_Height = 40;
const Header_Minimum_Height = 0;

export default class App extends Component {

  render(){
    return (
      <View style={{flex:1, marginTop:30}}>
        <AppContainer/>
      </View>
    )
  }
}

class HomeScreen extends Component{
      constructor()
    {
         super();
         this.AnimatedHeaderValue = new Animated.Value(0);
     }
render() {

const AnimateHeaderBackgroundColor = this.AnimatedHeaderValue.interpolate(
   {
    inputRange: [ 0, ( Header_Maximum_Height - Header_Minimum_Height )  ],
    outputRange: [ '#009688', '#00BCD4' ],
    extrapolate: 'clamp'
   });

const AnimateHeaderHeight = this.AnimatedHeaderValue.interpolate(
      {
       inputRange: [ 0, ( Header_Maximum_Height - Header_Minimum_Height ) ],
       outputRange: [ Header_Maximum_Height, Header_Minimum_Height ],
       extrapolate: 'clamp'
                  });
          return (
                  <SafeAreaView style={{flex:1}}>

                  <Animated.View style={{height:AnimateHeaderHeight,width:'100%', backgroundColor:'gray'}}>
                    <Text style={{color:'white'}}> Collapsible Expandable Header </Text>
                  </Animated.View>

                    <Animated.ScrollView
                                      scrollEventThrottle = { 16 }
                                      onScroll = { Animated.event(
                                        [{ nativeEvent: { contentOffset: { y: this.AnimatedHeaderValue }}}]
                                  )}>

                        <ImageBackground
                        style={{width:375, height:400}}
                        source={require('./assets/pizza.jpg')}>
                        </ImageBackground>

                 </Animated.ScrollView>
                 </SafeAreaView>
                          );
                        }
                      }
    const tabBarHeight = 100

    const AppTabNavigator = createMaterialTopTabNavigator({

      Home:{
      screen:HomeScreen,
      navigationOptions: {
          header: null,
          tabBarVisible:true,
          activeTintColor: '#e91e63',
        }
      }, {
        tabBarOptions: {
         showLabel: true,
          style: {
              backgroundColor: 'rgba(22, 22, 22, 0)',
              position: 'absolute',
              Top:  Dimensions.get('window').height-tabBarHeight,
              left:0,
              right:0,
//I initially set the margin to 45 but as I scroll up How can I set the marginTop to 0 when I reach the top screen.
              marginTop:45
          },
          labelStyle:{
            fontSize:15,
            color:"white"
          }
        }
       }
      )

I also tried marginTop to this.AnimatedHeaderValue but doesn't seem to work. Any advise or comments would be really helpful.

Answer 1

You can add a listener on the Animated.event that would allow you to get the value of your y:

<Animated.ScrollView
  scrollEventThrottle={16}
  onScroll={Animated.event(
    [{ nativeEvent: { contentOffset: { y: this.AnimatedHeaderValue }}}],
    {
      useNativeDriver: true,
      listener: event => {
        const offsetY = event.nativeEvent.contentOffset.y
        this.onScrollChange(offsetY)
      }
    },
  )}
>
  <ImageBackground
    style={{ width: 375, height: 400 }}
    source={require('./assets/pizza.jpg')}
  />
</Animated.ScrollView>

Here, you just need to define the onScrollChange in your component. Because your marginTop is in your top-level <App />, it will be a little tricky to change, so you could potentially use Redux or a Context to update and use this value.

You could also put the margin at the same level and use a state, but that would force you to do the same for every page depending on your architecture.

---

Because you need to change on runtime the tabBarOptions, you're going to need to create and provide a custom tabBarComponent to your navigator:

const AppTabNavigator = createMaterialTopTabNavigator(
  {
    Home: {
      screen: HomeScreen,
      navigationOptions: {
        header: null,
        tabBarVisible: true,
        activeTintColor: '#e91e63',
      },
    },
  },
  {
    tabBarComponent: CustomTabBar
  },
)

This component will extend the default component used by react-navigation, and hook into a state.

import React from 'react'
import { MaterialTopTabBar } from 'react-navigation'

export default ({ hasMargin, ...props }) => (
  <MaterialTopTabBar
    {...props}
    showLabel
    labelStyle={{
      fontSize: 15,
      color: 'white',
    }}
    style={{
      backgroundColor: 'rgba(22, 22, 22, 0)',
      position: 'absolute',
      top: Dimensions.get('window').height-tabBarHeight,
      left: 0,
      right: 0,
      marginTop: hashMargin ? 45 : 0,
    }}
  />
)

The problem here is that you won't have the variable hasMargin. You will need to either choose to either connect your component using Redux or use a React context. Both require some understanding and configuration, so you might need to read up a bit on them. Once it's done, define onScrollChange to change your context or dispatch a redux action.