sueLan

_onUpdateSync(
    viewableIndicesToCheck,
    onViewableItemsChanged,
    createViewToken,
  ) {
    // Filter out indices that have gone out of view since this call was scheduled.
    viewableIndicesToCheck = viewableIndicesToCheck.filter(ii =>
      this._viewableIndices.includes(ii),
    );
    const prevItems = this._viewableItems;

sueLan

// Filter out indices that have gone out of view since this call was scheduled.
    viewableIndicesToCheck = viewableIndicesToCheck.filter(ii =>
      this._viewableIndices.includes(ii),
    );

sueLan

this._viewableIndices = viewableIndices;
 if (this._config.minimumViewTime) {
      const handle = setTimeout(() => {
        this._timers.delete(handle);
        this._onUpdateSync(
          viewableIndices,
          onViewableItemsChanged,
          createViewToken,
        );
      }, this._config.minimumViewTime);

sueLan

function _isViewable(
  viewAreaMode: boolean,
  viewablePercentThreshold: number,
  top: number,
  bottom: number,
  viewportHeight: number,
  itemLength: number,
): boolean {
  if (_isEntirelyVisible(top, bottom, viewportHeight)) {
    // Entirely visible 

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

_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, 

sueLan

class ViewabilityHelper {
  _config: ViewabilityConfig;
  _hasInteracted: boolean = false;
  /* A set of `timeoutID`, used for memory management */
  _timers: Set<number> = new Set();
  // Indexs of the viewable items
  _viewableIndices: Array<number> = [];
  // A map for viewable items
  _viewableItems: Map<string, ViewToken> = new Map();
}

sueLan

type ViewabilityHelperCallbackTuple = {
  viewabilityHelper: ViewabilityHelper,
  onViewableItemsChanged: (info: {
    viewableItems: Array<ViewToken>,
    changed: Array<ViewToken>,
    ...
  }) => void,
  ...
};