pan-responder-handlers.js

...

handleGestureTerminationRequest = (e: GestureEvent, s: GestureState) =>
  this.props.shouldRelease(s);

handleGestureCapture = (e: GestureEvent, s: GestureState) =>
  this.props.shouldCapture(s);

handleGestureMove = (e: GestureEvent, { dx }: GestureState) => {
  const currentOffset: number =
    this.state.currentIndex * this.props.itemWidth;
  const resolvedOffset: number = currentOffset - dx;

  this.list.scrollToOffset({
    offset: resolvedOffset,
    animated: false,
  });
};

handleGestureRelease = (e: GestureEvent, { dx, vx }: GestureState) => {
  const currentOffset: number =
    this.state.currentIndex * this.props.itemWidth;
  const resolvedOffset: number = currentOffset - dx;
  const resolvedIndex: number = Math.round(
    (resolvedOffset +
      (dx > 0 ? -this.props.threshold : this.props.threshold)) /
      this.props.itemWidth
  );

  const absoluteVelocity: number = Math.round(Math.abs(vx));
  const velocityDifference: number =
    absoluteVelocity < 1 ? 0 : absoluteVelocity - 1;

  const newIndex: number =
    dx > 0
      ? Math.max(resolvedIndex - velocityDifference, 0)
      : Math.min(
          resolvedIndex + velocityDifference,
          this.props.data.length - 1
        );

  this.list.scrollToIndex({
    index: newIndex,
    animated: true,
    viewOffset: this.props.contentOffset,
  });

  this.setState(
    () => ({ currentIndex: newIndex }),
    () => this.props.onIndexChange(newIndex)
  );
};

...

<View
  style={[{ width: screenWidth }, style]}
  {...this.panResponder.panHandlers}
>
  <FlatList
    horizontal
    contentContainerStyle={[
      { paddingHorizontal: contentOffset },
      contentContainerStyle,
    ]}
    data={data}
    getItemLayout={this.getItemLayout}
    keyExtractor={extractKey}
    ref={this.getRef}
    scrollEnabled={false}
    showsHorizontalScrollIndicator={false}
    style={[styles.flatList, flatListStyle]}
    renderItem={({ item, index }) =>
      renderItem({
        item,
        currentIndex,
        itemIndex: index,
        itemCount: dataLength,
      })
    }
  />
</View>

...