what is the difference heightForRowAt vs estimatedHeightForRowAt?

Question

Normally when i want set my row height then set

 func tableView(_ tableView: UITableView, heightForRowAt indexPath: IndexPath) -> CGFloat {
        return 300
    }

what is the benefited with estimatedHeightForRowAt when should i use it.

update! if i want to set estimatedHeightForRowAt:

 func tableView(_ tableView: UITableView, estimatedHeightForRowAt indexPath: IndexPath) -> CGFloat {

        return // what should i put here
    }

UPDATE:
Is there any effect tableview scrolling if i provide as my wish estimatedHeightForRowAt? . for example i give in estimatedHeightForRowAt 500

Answer 1

As specified in the iOS documentation:

If the table contains variable height rows, it might be expensive to calculate all their heights and so lead to a longer load time. Using estimation allows you to defer some of the cost of geometry calculation from load time to scrolling time.

Answer 2

This difference becomes more meaningful when you have some custom cells which their height is different from most of your other cells. In that case the OS use estimatedHeightForRowAt as a default (This optimizes your loading time), but then if specified a different height for a row then it would use that.

As for what to put there:

If there is no variation then you put the same 300. Basically you put your default/most expected height there. For the heightForRowAt depending on your needs you may want to do switch based on section/row and change height. If don't have any variations then still just put 300 there

---

I'm not sure if this is why you asked or not, but if you were asking this because you had dynamic tableViewCells with different heights, then the solution is:

1. Use Auto Layout when creating your table view cells.
2. Set the tableview rowHeight to UITableViewAutomaticDimension.
3. Set the estimatedRowHeight or implement the height estimation delegate method.

Basically do:

override func viewDidLoad() {  
    super.viewDidLoad()

    self.tableView.estimatedRowHeight = 80 // or any other number that makes sense for your cells
    self.tableView.rowHeight = UITableViewAutomaticDimension
}

For more see this RW Tutorial and this high voted question

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NOTE

If you don’t give the estimatedRowHeight then tableview will dequeue a lot more cells, because it’s trying to be smart and needs to prepare itself for smooth scrolling. It will trigger both your cellForRow & willDisplay callbacks

If you give estimatedRowHeight a value, then it won’t dequeue cells offscreen anymore