Dynamically setting layout on UICollectionView causes inexplicable contentOffset change

Solution 1:

UICollectionViewLayout contains the overridable method targetContentOffsetForProposedContentOffset: which allows you to provide the proper content offset during a change of layout, and this will animate correctly. This is available in iOS 7.0 and above

Solution 2:

I have been pulling my hair out over this for days and have found a solution for my situation that may help. In my case I have a collapsing photo layout like in the photos app on the ipad. It shows albums with the photos on top of each other and when you tap an album it expands the photos. So what I have is two separate UICollectionViewLayouts and am toggling between them with [self.collectionView setCollectionViewLayout:myLayout animated:YES] I was having your exact problem with the cells jumping before animation and realized it was the contentOffset. I tried everything with the contentOffset but it still jumped during animation. tyler's solution above worked but it was still messing with the animation.

Then I noticed that it happens only when there were a few albums on the screen, not enough to fill the screen. My layout overrides -(CGSize)collectionViewContentSize as recommended. When there are only a few albums the collection view content size is less than the views content size. That's causing the jump when I toggle between the collection layouts.

So I set a property on my layouts called minHeight and set it to the collection views parent's height. Then I check the height before I return in -(CGSize)collectionViewContentSize I ensure the height is >= the minimum height.

Not a true solution but it's working fine now. I would try setting the contentSize of your collection view to be at least the length of it's containing view.

edit: Manicaesar added an easy workaround if you inherit from UICollectionViewFlowLayout:

-(CGSize)collectionViewContentSize { //Workaround
    CGSize superSize = [super collectionViewContentSize];
    CGRect frame = self.collectionView.frame;
    return CGSizeMake(fmaxf(superSize.width, CGRectGetWidth(frame)), fmaxf(superSize.height, CGRectGetHeight(frame)));
}

Solution 3:

This issue bit me as well and it seems to be a bug in the transition code. From what I can tell it tries to focus on the cell that was closest to the center of the pre-transition view layout. However, if there doesn't happen to be a cell at the center of the view pre-transition then it still tries to center where the cell would be post-transition. This is very clear if you set alwaysBounceVertical/Horizontal to YES, load the view with a single cell and then perform a layout transition.

I was able to get around this by explicitly telling the collection to focus on a specific cell (the first cell visible cell, in this example) after triggering the layout update.

[self.collectionView setCollectionViewLayout:[self generateNextLayout] animated:YES];

// scroll to the first visible cell
if ( 0 < self.collectionView.indexPathsForVisibleItems.count ) {
    NSIndexPath *firstVisibleIdx = [[self.collectionView indexPathsForVisibleItems] objectAtIndex:0];
    [self.collectionView scrollToItemAtIndexPath:firstVisibleIdx atScrollPosition:UICollectionViewScrollPositionCenteredVertically animated:YES];
}

Solution 4:

Jumping in with a late answer to my own question.

The TLLayoutTransitioning library provides a great solution to this problem by re-tasking iOS7s interactive transitioning APIs to do non-interactive, layout to layout transitions. It effectively provides an alternative to setCollectionViewLayout, solving the content offset issue and adding several features:

1. Animation duration
2. 30+ easing curves (courtesy of Warren Moore's AHEasing library)
3. Multiple content offset modes

Custom easing curves can be defined as AHEasingFunction functions. The final content offset can be specified in terms of one or more index paths with Minimal, Center, Top, Left, Bottom or Right placement options.

To see what I mean, try running the Resize demo in the Examples workspace and playing around with the options.

The usage is like this. First, configure your view controller to return an instance of TLTransitionLayout:

- (UICollectionViewTransitionLayout *)collectionView:(UICollectionView *)collectionView transitionLayoutForOldLayout:(UICollectionViewLayout *)fromLayout newLayout:(UICollectionViewLayout *)toLayout
{
    return [[TLTransitionLayout alloc] initWithCurrentLayout:fromLayout nextLayout:toLayout];
}

Then, instead of calling setCollectionViewLayout, call transitionToCollectionViewLayout:toLayout defined in the UICollectionView-TLLayoutTransitioning category:

UICollectionViewLayout *toLayout = ...; // the layout to transition to
CGFloat duration = 2.0;
AHEasingFunction easing = QuarticEaseInOut;
TLTransitionLayout *layout = (TLTransitionLayout *)[collectionView transitionToCollectionViewLayout:toLayout duration:duration easing:easing completion:nil];

This call initiates an interactive transition and, internally, a CADisplayLink callback that drives the transition progress with the specified duration and easing function.

The next step is to specify a final content offset. You can specify any arbitrary value, but the toContentOffsetForLayout method defined in UICollectionView-TLLayoutTransitioning provides an elegant way to calculate content offsets relative to one or more index paths. For example, in order to have a specific cell to end up as close to the center of the collection view as possible, make the following call immediately after transitionToCollectionViewLayout:

NSIndexPath *indexPath = ...; // the index path of the cell to center
TLTransitionLayoutIndexPathPlacement placement = TLTransitionLayoutIndexPathPlacementCenter;
CGPoint toOffset = [collectionView toContentOffsetForLayout:layout indexPaths:@[indexPath] placement:placement];
layout.toContentOffset = toOffset;