UICollectionView flowLayout not wrapping cells cor

Question

I have a UICollectionView with a FLowLayout. It will work as I expect most of the time, but every now and then one of the cells does not wrap properly. For example, the the cell that should be on in the first "column" of the third row if actually trailing in the second row and there is just an empty space where it should be (see diagram below). All you can see of this rouge cell is the left hand side (the rest is cut off) and the place it should be is empty.

This does not happen consistently; it is not always the same row. Once it has happened, I can scroll up and then back and the cell will have fixed itself. Or, when I press the cell (which takes me to the next view via a push) and then pop back, I will see the cell in the incorrect position and then it will jump to the correct position.

The scroll speed seems to make it easier to reproduce the problem. When I scroll slowly, I can still see the cell in the wrong position every now and then, but then it will jump to the correct position straight away.

The problem started when I added the sections insets. Previously, I had the cells almost flush against the collection bounds (little, or no insets) and I did not notice the problem. But this meant the right and left of the collection view was empty. Ie, could not scroll. Also, the scroll bar was not flush to the right.

I can make the problem happen on both Simulator and on an iPad 3.

I guess the problem is happening because of the left and right section insets... But if the value is wrong, then I would expect the behavior to be consistent. I wonder if this might be a bug with Apple? Or perhaps this is due to a build up of the insets or something similar.

---

Follow up: I have been using this answer below by Nick for over 2 years now without a problem (in case people are wondering if there are any holes in that answer - I have not found any yet). Well done Nick.

Answer 1

I have added a bug report to Apple. What works for me is to set bottom sectionInset to a value less than top inset.

Answer 2

Put this into the viewController that owns the collection view

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

Answer 3

A Swift version of Nick Snyder's answer:

class NDCollectionViewFlowLayout : UICollectionViewFlowLayout {
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        let attributes = super.layoutAttributesForElements(in: rect)
        let contentSize = collectionViewContentSize
        return attributes?.filter { $0.frame.maxX <= contentSize.width && $0.frame.maxY < contentSize.height }
    }
}

Answer 4

i discovered similar problems in my iPhone application. Searching the Apple dev forum brought me this suitable solution which worked in my case and will probably in your case too:

Subclass UICollectionViewFlowLayout and override shouldInvalidateLayoutForBoundsChange to return YES.

//.h
@interface MainLayout : UICollectionViewFlowLayout
@end

and

//.m
#import "MainLayout.h"
@implementation MainLayout
-(BOOL)shouldInvalidateLayoutForBoundsChange:(CGRect)newBounds{
    return YES;
}
@end

Answer 5

I just experienced a similar issue but found a very different solution.

I am using a custom implementation of UICollectionViewFlowLayout with a horizontal scroll. I am also creating custom frame locations for each cell.

The problem that I was having was that [super layoutAttributesForElementsInRect:rect] wasn't actually returning all of the UICollectionViewLayoutAttributes that should be displayed on screen. On calls to [self.collectionView reloadData] some of the cells would suddenly be set to hidden.

What I ended up doing was to create a NSMutableDictionary that cached all of the UICollectionViewLayoutAttributes that I have seen so far and then include any items that I know should be displayed.

- (NSArray *)layoutAttributesForElementsInRect:(CGRect)rect {

    NSArray * originAttrs = [super layoutAttributesForElementsInRect:rect];
    NSMutableArray * attrs = [NSMutableArray array];
    CGSize calculatedSize = [self calculatedItemSize];

    [originAttrs enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes * attr, NSUInteger idx, BOOL *stop) {
        NSIndexPath * idxPath = attr.indexPath;
        CGRect itemFrame = [self frameForItemAtIndexPath:idxPath];
        if (CGRectIntersectsRect(itemFrame, rect))
        {
            attr = [self layoutAttributesForItemAtIndexPath:idxPath];
            [self.savedAttributesDict addAttribute:attr];
        }
    }];

    // We have to do this because there is a bug in the collection view where it won't correctly return all of the on screen cells.
    [self.savedAttributesDict enumerateKeysAndObjectsUsingBlock:^(NSString *key, NSArray * cachedAttributes, BOOL *stop) {

        CGFloat columnX = [key floatValue];
        CGFloat leftExtreme = columnX; // This is the left edge of the element (I'm using horizontal scrolling)
        CGFloat rightExtreme = columnX + calculatedSize.width; // This is the right edge of the element (I'm using horizontal scrolling)

        if (leftExtreme <= (rect.origin.x + rect.size.width) || rightExtreme >= rect.origin.x) {
            for (UICollectionViewLayoutAttributes * attr in cachedAttributes) {
                [attrs addObject:attr];
            }
        }
    }];

    return attrs;
}

Here is the category for NSMutableDictionary that the UICollectionViewLayoutAttributes are being saved correctly.

#import "NSMutableDictionary+CDBCollectionViewAttributesCache.h"

@implementation NSMutableDictionary (CDBCollectionViewAttributesCache)

- (void)addAttribute:(UICollectionViewLayoutAttributes*)attribute {

    NSString *key = [self keyForAttribute:attribute];

    if (key) {

        if (![self objectForKey:key]) {
            NSMutableArray *array = [NSMutableArray new];
            [array addObject:attribute];
            [self setObject:array forKey:key];
        } else {
            __block BOOL alreadyExists = NO;
            NSMutableArray *array = [self objectForKey:key];

            [array enumerateObjectsUsingBlock:^(UICollectionViewLayoutAttributes *existingAttr, NSUInteger idx, BOOL *stop) {
                if ([existingAttr.indexPath compare:attribute.indexPath] == NSOrderedSame) {
                    alreadyExists = YES;
                    *stop = YES;
                }
            }];

            if (!alreadyExists) {
                [array addObject:attribute];
            }
        }
    } else {
        DDLogError(@"%@", [CDKError errorWithMessage:[NSString stringWithFormat:@"Invalid UICollectionVeiwLayoutAttributes passed to category extension"] code:CDKErrorInvalidParams]);
    }
}

- (NSArray*)attributesForColumn:(NSUInteger)column {
    return [self objectForKey:[NSString stringWithFormat:@"%ld", column]];
}

- (void)removeAttributesForColumn:(NSUInteger)column {
    [self removeObjectForKey:[NSString stringWithFormat:@"%ld", column]];
}

- (NSString*)keyForAttribute:(UICollectionViewLayoutAttributes*)attribute {
    if (attribute) {
        NSInteger column = (NSInteger)attribute.frame.origin.x;
        return [NSString stringWithFormat:@"%ld", column];
    }

    return nil;
}

@end

Answer 6

This might be a little late but make sure you are setting your attributes in prepare() if possible.

My issue was that the cells were laying out, then getting update in layoutAttributesForElements. This resulted in a flicker effect when new cells came into view.

By moving all the attribute logic into prepare, then setting them in UICollectionViewCell.apply() it eliminated the flicker and created butter smooth cell displaying

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