Question

For example, there are 100 rows in tableView, heightForRowAtIndexPath: was executed already before the 100th row was show.

Why? In order to calculate contentSize of tableView?

Thanks!

Answer 1

From Apple’s documentation :

Every time a table view is displayed, it calls tableView:heightForRowAtIndexPath: on the delegate for each of its rows

So yes, it’s called prior to displaying the cell. If you think about it, if the table did it the other way around, it would create a cell with a standard height, put the content in it, get the real height, and resize/shift the cells. That would not be really nice.

This may cause performance issues, so you should only use it if you have cells of different heights :

The method allows the delegate to specify rows with varying heights. If this method is implemented, the value it returns overrides the value specified for the rowHeight property of UITableView for the given row.

There are performance implications to using tableView:heightForRowAtIndexPath: instead of the rowHeight property. Every time a table view is displayed, it calls tableView:heightForRowAtIndexPath: on the delegate for each of its rows, which can result in a significant performance problem with table views having a large number of rows (approximately 1000 or more).

Otherwise, if all your cells are the same height, use the rowHeight property, and do not implement tableView:heightForRowAtIndexPath:.