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TableViewAutolayout.md

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Auto-layout in TableView's

iOS6 and prior

In iOS6 and prior, having a table view with alot of cells, where cells having various/different height, loading the table view would be slow. This because heightForRowAt:indexPath, which normally is an "intensive" method, needed to be called to calculate the table view height/content-size.

func tableView(_ tableView: UITableView, 
               heightForRowAt indexPath: IndexPath) -> CGFloat {
   //return the real height of the cell
}

iOS7

In iOS7 a new UITableViewDelegate method was introduced to solve this problem, estimatedHeightForRowAt:indexPath. Now the calculation of table view heigth/content-size could be "estimated" much faster thus the loading of the tableview would be faster.

However, the calcuation of the real height for cells would be deferred to later, at so called scroll-time. So better performance for loading a table and potentially a little less scroll-performance?

func tableView(_ tableView: UITableView, 
               heightForRowAt indexPath: IndexPath) -> CGFloat {                      
   //return the real height of the cell
}
         
func tableView(_ tableView: UITableView, 
               estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {
                        
   //return an estimated best guess of the height that most of the cells will have
   //should be greater than 0 and as close to the real size of the "median" cell
}

iOS8 and above

In iOS8 the 2 callbacks methods mentioned above can be skipped/ignored/removed and instead you can set the estimated height and row heigth on the table view.

But this requires obviously that all the cells in the table view are auto-layout'ed. So you cannot mix-and-match with cells that are auto-layout'ed and cells that aren't.

Also, this solution "assumes" that all cells are almost equal height. If you have a cell with 3x higher than another cell, you might observe some strange scroll behaviours. In those cases you can also implement the UITableViewDelegate method estimatedHeightForRowAt:indexPath to give a specific estimate for each cell.

tableView.estimatedRowHeight = 170.   //should be greater than 0 and as close to the real size of the "median" cell
tableView.rowHeight = UITableViewAutomaticDimension

Note that this also applies to section height.

tableView.estimatedSectionHeaderHeight = 25
tableView.sectionHeaderHeight = UITableViewAutomaticDimension

Note that if you set one of the properties (rowHeight or sectionHeaderHeight) you must set the other as well (estimatedRowHeight or estimatedSectionHeaderHeight), otherwise UIView-Encapsulated-Layout-Height constraint errors will most likely occur.