block9.js

_updateViewableItems(data: any) {
    const {getItemCount} = this.props;

    this._viewabilityTuples.forEach(tuple => {
      tuple.viewabilityHelper.onUpdate(
        getItemCount(data),
        // contentOffset of the list 
        this._scrollMetrics.offset,
        // 🌟 viewportHeight 
        this._scrollMetrics.visibleLength, 
        this._getFrameMetrics,
        this._createViewToken,
        tuple.onViewableItemsChanged,
        this.state,
      );
    });
  }