UICollectionView Invalidate Layout On Bounds Changes

Question

I currently have the following snippet for calculating UICollectionViewCells sizes:

- (CGSize)collectionView:(UICollectionView *)mainCollectionView
                  layout:(UICollectionViewLayout *)collectionViewLayout
  sizeForItemAtIndexPath:(NSIndexPath *)atIndexPath
{
    CGSize bounds = mainCollectionView.bounds.size;
    bounds.height /= 4;
    bounds.width /= 4;
    return bounds;
}

This works. However, I'm now adding a keyboard observer in viewDidLoad (which is triggering the the delegate and data source methods for the UICollectionView before it appears and resizes itself from the storyboard). The bounds are thus wrong. I also would like to support rotation. What's a good way of handling these two edge cases and re-calculating the sizes if the UICollectionView changes size?

Answer 1

The solution for invalidating your layout when the bounds of the collection view changes is to override shouldInvalidateLayoutForBoundsChange: and return YES. It's also stated in the documentation: https://developer.apple.com/documentation/uikit/uicollectionviewlayout/1617781-shouldinvalidatelayoutforboundsc

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds 
{
     return YES;
}

This should cover rotation support as well. If it doesn't, implement viewWillTransitionToSize:withTransitionCoordinator:

- (void)viewWillTransitionToSize:(CGSize)size withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator
{
    [super viewWillTransitionToSize:size
          withTransitionCoordinator:coordinator];

    [coordinator animateAlongsideTransition:^(id<UIViewControllerTransitionCoordinatorContext> context)
     {
         [self.collectionView.collectionViewLayout invalidateLayout];
     }
                                 completion:^(id<UIViewControllerTransitionCoordinatorContext> context)
     {
     }];
}

Answer 2

1. You should handle the case when the collection view size is changed. If you change the orientation or constraints, viewWillLayoutSubviews method will be triggered.

2. You should invalidate the current collection view layout. After the layout is invalidated by using invalidateLayout method, the UICollectionViewDelegateFlowLayout methods will be triggered.

Here's the example code:

- (void)viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    [mainCollectionView.collectionViewLayout invalidateLayout];
}