Large UICollectionViewCell stopped being displayed when scrolling

Question

The same behavior of UICollectionView as described here has been led to this question. Even though I decided to post my own one, because I did further investigations, I didn't want to post in a comment or in edit of the question mentioned above.

What happens?:

When large cells being displayed in a UICollectionView with a UICollectionViewFlowLayout, after scrolling the collection view to a certain offset, the cells will disappear.

When scrolling further until another cell comes into visible area, the vanished/hidden cell becomes visible again.

I tested with a vertical scrolling collection view and full-width-cells, but I'm rather sure, that it would also happen with similar setups for horizontal scrolling.

What are large cells?:

The described behavior happens with cells higher than twice the display height (960.f + 1.f on 3,5 inch displays, 1136.f + 1.f on 4 inch).

What exactly happens?:

When the scrolling offset of the collection view exceeds cell.frame.origin.y + displayHeightOfHardware the cells hidden property is set to YES and -collectionView:didEndDisplayingCell:forItemAtIndexPath: gets called (e.g. the first cell changes to hidden when scrollingOffset.y reaches 481.f on 3,5-inch-iPhone).

As described above, when scrolling until next cell comes into view, the hidden cell gets displayed again (i.e. hidden property changes to NO) and furthermore, when scrolling far enough the cell will never vanish again, when it shouldn't, no matter where you scroll to.

This changes when working with cells larger than triple-display-height (1441.f/1705.f). Those show the same behavior, but it stays the same, no matter how far they're being scrolled up and down.

What else?:

The situation can not be fixed by overriding -(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds to return YES.

The cells cannot being forced to display with setting the hidden property to NO programmatically after they were hidden (in didEndDisplayingCell for example)

So, whats the question?:

I'm pretty sure, that this is a bug in UICollectionView/Controller/Cell/Layout and I'll submit a TSI at Apple. But for the meantime: Has anyone any ideas for a quick hack solution?

Answer 1

i have a VERY dirty and internal solution for this problem:

@interface UICollectionView ()
- (CGRect)_visibleBounds;
@end

@interface MyCollectionView : UICollectionView

@end

@implementation MyCollectionView

- (CGRect)_visibleBounds {
    CGRect rect = [super _visibleBounds];
    rect.size.height = [self heightOfLargestVisibleCell];
    return rect;
}

- (float)heightOfLargestVisibleCell {
    // do your calculations for current max cellHeight and return it 
    return 1234;
}

@end

Answer 2

I have a workaround that seems to be working for me and should not run amok of Apple's rules for iOS applications.

The key is the observation that the large cells bounds are the issue. I've worked around that by ensuring that one edge of the cell is within the viewable area of the scrollable content region. You'll obviously need to subclass the UICollectionViewFlowLayout class or UICollectionViewLayout depending on your needs and make use of the contentOffset value to track where you are in the UIScrollView.

I also had to ensure:

- (BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds 

returns YES or face a runtime exception indicating the layout was invalid. I keep the edge of the larger cell bound to the left edge in my case. This way you can avoid the erroneous bounds intersection detection for these larger cells.

This does create more work depending on how you would like the contents of the cell to be rendered as the width/height of the cell is being updated as you scroll. In my case, the subviews within the cell are relatively simple and do not require a lot of fiddling with.

As requested here is an example of my layoutAttributesInRect

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
    NSMutableArray* attributes = [NSMutableArray array];
    NSArray *vertical = myVerticalCellsStore.cells;
    NSInteger startRow = floor(rect.origin.y * (vertical.count)/ (vertical.count * verticalViewHeight + verticalViewSpacing * 2));
    startRow = (startRow < 0) ? 0 : startRow;

    for (NSInteger i = startRow; i < vertical.count && (rect.origin.y + rect.size.height >= i * verticalViewHeight); i++) {
        NSArray *horizontals = myHorizontalStore.horizontalCells;
        UICollectionViewLayoutAttributes *verticalAttr = [self layoutAttributesForSupplementaryViewOfKind:@"vertical" atIndexPath:[NSIndexPath indexPathForItem:0 inSection:i]];
        if (CGRectIntersectsRect(verticalAttr.frame, rect)) {
            [attributes addObject:verticalAttr];
        }

        BOOL foundAnElement = NO;
        for (NSInteger j = 0 ; j < horizontals.count; j++) {
            MYViewLayoutAttributes *attr = (MyViewLayoutAttributes *)[self layoutAttributesForItemAtIndexPath:[NSIndexPath indexPathForItem:j inSection:i]];
            if (CGRectIntersectsRect(rect, attr.frame)) {
                [attributes addObject: attr];
                foundAnElement = YES;
            }
            else if (foundAnElement) {
                break;
            }
        }
    }
    return attributes;
}

This is my sanitized code. Basically I calculate about were the first cell should be based on the cell height. In my case that is fixed, so the calculation is pretty easy. But my horizontal elements have various widths. So the inner loop is really about figuring out the right number of horizontal cells to include in the attributes array. There I'm using the CGRectIntersectsRect to determine if the cell intersects. Then the loop keeps going until the intersection fails. And if at least one horizontal cell has been found the loop will break. Hope that helps.