When UICollectionView will fill cells and get height in swift - ios

Question

I have a uiCollectionView with some elements. I need to know the collectionView's height before viewDidAppear to do some stuff magic. I use the below snippet code:
collectionView.collectionViewLayout.collectionViewContentSize.height
// or
collectionView.contentSize.height
However, this is only return true value in viewDidAppear and in viewWillAppear and viewDidLoad others return 0 (before viewDidAppear I need that). I need to know about collectionView's lifecycle and where the elements fill in the collectionView to determine it's height.

Answer 1

If you want to know about height of UICollectionView then use the code
collectionView.frame.size.height
If you want to know how much scrollable height is then use
collectionView.contentSize.height
The datasource methods of UICollectionView are called just before the viewDidLayoutSubviews function.

Answer 2

UICollectionView's loading is an asynchronous process so you can't be sure the loading is completed before viewDidAppear is called.
I suggest you to use KVO to observe value changes of your collectionView's contentSize, and perform your magic when the value changes.
Another option "dirty" option from my pov is to do something like
collectionView.reloadData()
DispatchQueue.main.async{
// This will be called when after reloadData() completes
}

Answer 3

Implement the following UICollectionViewDelegateFlowLayout
method which returns the size of specified item's cell before rendering it.
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
let height = collectionView.frame.size.height
// Your logic
}

Answer 4

How can you get contentSize of collectionView before the containing view(ViewController's view) is loaded?. I think you forgot the Life cycle of view.
If your controller contains subViews(like collectionView, tableView). Then first of all main parent view is loaded(viewDidLoad, viewWillApper or viewDidApper) and after that all subview is directly loaded in according to view hierarchy.
So you can only get full correctly contextSize after collectionView completely loaded. If you trying to access contentSize before view loaded then it return default value which is 0. So if you want to do some magic animation on collectionView you can hide when initialise, And after complete collectionView is loaded then Unhide it and perform animation.