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

Not the most elegant but quite simple and working solution I stuck with for now. Works only with linear layout (not grid) but it's fine for me.

// retrieve data to be inserted
NSArray *fetchedObjects = [managedObjectContext executeFetchRequest:fetchRequest error:nil];
NSMutableArray *objects = [fetchedObjects mutableCopy];
[objects addObjectsFromArray:self.messages];

// self.messages is a DataSource array
self.messages = objects;

// calculate index paths to be updated (we are inserting 
// fetchedObjects.count of objects at the top of collection view)
NSMutableArray *indexPaths = [NSMutableArray new];
for (int i = 0; i < fetchedObjects.count; i ++) {
    [indexPaths addObject:[NSIndexPath indexPathForItem:i inSection:0]];
}

// calculate offset of the top of the displayed content from the bottom of contentSize
CGFloat bottomOffset = self.collectionView.contentSize.height - self.collectionView.contentOffset.y;

// performWithoutAnimation: cancels default collection view insertion animation
[UIView performWithoutAnimation:^{

    // capture collection view image representation into UIImage
    UIGraphicsBeginImageContextWithOptions(self.collectionView.bounds.size, NO, 0);
    [self.collectionView drawViewHierarchyInRect:self.collectionView.bounds afterScreenUpdates:YES];
    UIImage *snapshotImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    // place the captured image into image view laying atop of collection view
    self.snapshot.image = snapshotImage;
    self.snapshot.hidden = NO;

    [self.collectionView performBatchUpdates:^{
        // perform the actual insertion of new cells
        [self.collectionView insertItemsAtIndexPaths:indexPaths];
    } completion:^(BOOL finished) {
        // after insertion finishes, scroll the collection so that content position is not
        // changed compared to such prior to the update
        self.collectionView.contentOffset = CGPointMake(0, self.collectionView.contentSize.height - bottomOffset);
        [self.collectionView.collectionViewLayout invalidateLayout];

        // and hide the snapshot view
        self.snapshot.hidden = YES;
    }];
}];