When does the UIColletionView calls his datasource and delegate methods

Question

I created a UICollectionView with a custom cell. In the viewDidLoad() method I was registering my cellclass.

   collectionView?.register(CustomCell.self, forCellWithReuseIdentifier: cellId)
  collectionView?.reloadData()

While debugging my code to check the behavior I saw that in the line where I was registering my collectionView and called collectionView.reloadData() with the custom Cellclass none of the datasource and delegate methods got executed like cellForItem at etc.. ? Why is that so ?

Answer 1

viewDidLoad() is called when your view has finished loading but before the view is laid out i.e. before its frame is properly set.

Judging from my observations a collection view calls its data source methods (like cellForItem(at:)) lazily during the layout process. After all, it doesn't need to call the data source methods at all if its size is .zero or if its position is currently off-screen!

So to put it short:

There is no reason why your collection view's data source methods should be called in viewDidLoad().

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As far as I know calling reloadData() only discards all visible cells and decoration views so that they must be reloaded during the next layout pass. When the collection view is currently visible a new layout pass is triggered immediately but in viewDidLoad() the collection view isn't even on screen yet. (viewDidAppear() has not been called yet.)