Self-sizing cells: how do UITableView and UICollectionView compute height?
The gist
In self-sizing mode the cell's height is set not by the delegate but by Auto Layout. The collection fixes the width (the table's width or the item's width from the layout), asks the cell to compute a fitting size via systemLayoutSizeFitting, and gets the height from the contentView constraints. Until a cell is shown, an estimate is used: it is needed to compute contentSize and the scroll indicator without laying out every cell.
Hence the main requirement: inside contentView you need an unbroken chain of constraints from top to bottom, and multiline labels need numberOfLines = 0 and full horizontal constraints.
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