import React from "react";
import SwiperFlatList from "react-native-swiper-flatlist";

export default class MyComponent extends React.Component {
// INSIDE A COMPONENT
  constructor(props) {
    super(props);
    this.currentIndex = 0;
    this._updateIndex = this._updateIndex.bind(this);
    this.viewabilityConfig = {
      itemVisiblePercentThreshold: 50
    };
  }

  _updateIndex({ viewableItems }) {
  // getting the first element visible index
    this.currentIndex = viewableItems[0].index;
  }

  render() {
    return  // ...
      <SwiperFlatList
              loop={false}
              index={0}
              onViewableItemsChanged={this._updateIndex}
              viewabilityConfig={this.viewabilityConfig}
            >
             {data}
            </SwiperFlatList>;
            }
}