UICollectionView insert cells above maintaining position (like Messages.app)

Question

By default Collection View maintains content offset while inserting cells. On the other hand I\'d like to insert cells above the currently displaying ones so that they appea

Answer 1

Inspired by Bryan Pratte's solution I developed subclass of UICollectionViewFlowLayout to get chat behavior without turning collection view upside-down. This layout is written in Swift 3 and absolutely usable with RxSwift and RxDataSources because UI is completely separated from any logic or binding.

Three things were important for me:

1. If there is a new message, scroll down to it. It doesn't matter where you are in the list in this moment. Scrolling is realized with setContentOffset instead of scrollToItemAtIndexPath.
2. If you do "Lazy Loading" with older messages, then the scroll view shouldn't change and stays exactly where it is.
3. Add exceptions for the beginning. The collection view should behave "normal" till there are more messages than space on the screen.

My solution: https://gist.github.com/jochenschoellig/04ffb26d38ae305fa81aeb711d043068

Answer 2

if ([newMessages count] > 0)
{
    [self.collectionView reloadData];

    if (hadMessages)
        [self.collectionView scrollToItemAtIndexPath:[NSIndexPath indexPathForItem:[newMessages count] inSection:0] atScrollPosition:UICollectionViewScrollPositionTop animated:NO];
}

This seems to be working so far. Reload the collection, scroll the previously first message to the top without animation.