Scroll

There are 2 scroll components ListView and ScrollView. ScrollView more commonly used for small amounts of content when performance is less important, and ListView when you're rendering large amounts of content and performance matters.

One thing that they both provide is an onScroll function, and additionally you need to provide a scrollEventThrottle which will call the onScroll at a throttled rate. Typically that value should be set to 16 to receive all scroll events, otherwise put it higher if you care less.

The event.nativeEvent will have a JSON structure like so.

{
    "contentOffset": {
        "x": 0,
        "y": 2395.333333333333
    },
    "layoutMeasurement": {
        "width": 414,
        "height": 736
    },
    "contentSize": {
        "width": 414,
        "height": 5000
    },
    "zoomScale": 1,
    "updatedChildFrames": [],
    "contentInset": {
        "right": 0,
        "top": 0,
        "left": 0,
        "bottom": 0
    }
}

The one thing to call out is the contentSize and the contentOffset. The contentSize will have the height of all rendered content inside of it, and the contentOffset will provide a current x and y of how far things have scrolled. You can have horizontal, ListViews and ScrollViews which is where the x distance comes into play.

As an example lets say you want to change the background color of a view on scroll.

var SampleApp = React.createClass({
  componentWillMount: function() {
    this._animatedValue = new Animated.Value(0);
  },
  render: function() {
    var interpolatedColor = this._animatedValue.interpolate({
      inputRange: [0, 5000],
      outputRange: ['rgba(255,255,255,1)', 'rgba(51,156,177, 1)'],
      extrapolate: 'clamp'
    });

    var event = Animated.event([
      {
        nativeEvent: { 
          contentOffset: {
            y: this._animatedValue
          }
        }
      }
    ])

    return (
      <View style={styles.container}>
        <ScrollView style={{flex: 1}} onScroll={event} scrollEventThrottle={16}>
          <Animated.View style={{height: 5000, backgroundColor: interpolatedColor}} />   
        </ScrollView>
      </View>
    );
  }
});

We setup our _animatedValue and create our interpolate that spans the range of our content. In our case we've defined it as 5000, so this is slightly impracticle. But that's what extrapolation is for! In our case we'll clamp it but it might make sense to leave it as the default extend here.

Now we use Animated.event to map our event from the JSON, to the contentOffset.y which is the distance the user has scrolled. Then we can simply pass that into the onScroll and as the user scrolls it'll automatically call setValue on our this._animatedValue which will cause the interpolate to control the background color.

The result would look something like this.

Simple Color Animation on Scroll

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