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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,

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_viewabilityTuples: Array<ViewabilityHelperCallbackTuple> = [];

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/**
   * Called when the viewability of rows changes, as defined by the
   * `viewabilityConfig` prop.
   */
  onViewableItemsChanged?: ?(info: {
    viewableItems: Array<ViewToken>,
    changed: Array<ViewToken>,
    ...
  }) => void,
  

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if (this.props.viewabilityConfigCallbackPairs) {
      this._viewabilityTuples = this.props.viewabilityConfigCallbackPairs.map(
        pair => ({
          viewabilityHelper: new ViewabilityHelper(pair.viewabilityConfig),
          onViewableItemsChanged: pair.onViewableItemsChanged,
        }),
      );
    } else if (this.props.onViewableItemsChanged) {
      this._viewabilityTuples.push({
        viewabilityHelper: new ViewabilityHelper(this.props.viewabilityConfig),

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

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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();
}

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_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, 

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

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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) {