block2.js

/**
   * Called when the viewability of rows changes, as defined by the
   * `viewabilityConfig` prop.
   */
  onViewableItemsChanged?: ?(info: {
    viewableItems: Array<ViewToken>,
    changed: Array<ViewToken>,
    ...
  }) => void,
  
export type ViewToken = {
  item: any,
  // The key of this item
  key: string,
  index: ?number,
  // indicated whether this item is viewable or not
  isViewable: boolean,
  section?: any,
  ...
};