sueLan

for (let idx = first; idx <= last; idx++) {
      const metrics = getFrameMetrics(idx);
      if (!metrics) {
        continue;
      }
      // The top of current item cell, relative to the screen 
      const top = metrics.offset - scrollOffset;
      // The bottom of current item cell 
      const bottom = top + metrics.length;
      if (top < viewportHeight && bottom > 0) {

sueLan

type State = {
  // The range of the rendered items, 
  // used for the optimization to reduce the scan size
  first: number,
  last: number,
  ...
};

sueLan

// The map storing the cell layout info
{ [cellKey]: {
      // offset of the item cell
      offset: number, 
      // length of the item cell. width or height determined by the direction of the VirtualizedList
      length: number, 
      index: number,
      inLayout: boolean,
    }
}

sueLan

export type ViewabilityConfig = {
  /**
   * Minimum amount of time (in milliseconds) that an item must be physically viewable before the
   * viewability callback will be fired. A high number means that scrolling through content without
   * stopping will not mark the content as viewable.
   */
  minimumViewTime?: number,

  /**
   * Percent of viewport that must be covered for a partially occluded item to count as