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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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/**
   * 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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_viewabilityTuples: Array<ViewabilityHelperCallbackTuple> = [];

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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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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

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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;

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