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// 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,
    }
}

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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

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

sueLan

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

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

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_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

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

sueLan

viewabilityConfig = {
    waitForInteraction: true,
    viewAreaCoveragePercentThreshold: 95,
    itemVisiblePercentThreshold: 75
  }

  onViewableItemsChanged = ({viewableItems, changed}) => {
    console.log("Visible items are", viewableItems);
    console.log("Changed in this iteration", changed);
  };

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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

sueLan

const timestamp = e.timeStamp;
let visibleLength = this._selectLength(e.nativeEvent.layoutMeasurement);
let contentLength = this._selectLength(e.nativeEvent.contentSize);
let offset = this._selectOffset(e.nativeEvent.contentOffset);
let dOffset = offset - this._scrollMetrics.offset;

// ...
 this._scrollMetrics = {
      contentLength,
      dt,