block16.js

_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;
    // Using map, so the time complexity would be o(n) 
    const nextItems = new Map(
      viewableIndicesToCheck.map(ii => {
        const viewable = createViewToken(ii, true);
        return [viewable.key, viewable];
      }),
    );

    const changed = [];
    for (const [key, viewable] of nextItems) {
      if (!prevItems.has(key)) {
        changed.push(viewable);
      }
    }
    for (const [key, viewable] of prevItems) {
      if (!nextItems.has(key)) {
        changed.push({...viewable, isViewable: false});
      }
    }
    if (changed.length > 0) {
      this._viewableItems = nextItems;
      onViewableItemsChanged({
        viewableItems: Array.from(nextItems.values()),
        changed,
        viewabilityConfig: this._config,
      });
    }
  }
}