Each post in a feed has to send an impression event exactly once when it has been at least 50% visible for at least 1 second. How would you design this impression tracker on top of UITableView/UICollectionView?
A tracker on the controller side registers candidates in willDisplay/didEndDisplaying, computes the visible ratio in scrollViewDidScroll from the overlap between the cell frame and the viewport, and sends the event once 1 second has passed since the ratio crossed 50%.
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