UICollectionView: custom FlowLayout crash if empty sections - ios

Question

I have a problem using custom flow layout, if my sections are empty i got a "EXC_ARTHMETIC(code=EXC_I386_DIV, subcode=0x0)".
I'm using custom FlowLayout to add custom header and custom section background(the crash is related to the background).
some code (please look at layoutAttributesForBackgroundAtSection: to see where the crash happens):
// UIViewController
-(void)loadView
{
[super loadView];
collectionViewFlowLayout *flowLayout = [[collectionViewFlowLayout alloc]init];
flowLayout.sectionInset = UIEdgeInsetsMake(35, 20, 0, 20);
[flowLayout setItemSize:cellSize];
[flowLayout setScrollDirection:UICollectionViewScrollDirectionVertical];
flowLayout.minimumInteritemSpacing = 0.0f;
self.collectionView = [[UICollectionView alloc]initWithFrame:(CGRectZero) collectionViewLayout:flowLayout];
[self.collectionView setTranslatesAutoresizingMaskIntoConstraints:NO];
[self.collectionView setBackgroundColor:[UIColor colorWithRed:246./256. green:246./256. blue:246./256. alpha:1]];
[self.view addSubview:self.collectionView];
[[self.collectionView.topAnchor constraintEqualToAnchor:self.view.topAnchor] setActive:YES];
[[self.collectionView.bottomAnchor constraintEqualToAnchor:self.view.bottomAnchor] setActive:YES];
[[self.collectionView.leftAnchor constraintEqualToAnchor:self.view.leftAnchor] setActive:YES];
[[self.collectionView.rightAnchor constraintEqualToAnchor:self.view.rightAnchor] setActive:YES];
[self.collectionView registerClass:[collectionViewCell class] forCellWithReuseIdentifier:reuseCellIdentifier];
[self.collectionView registerClass:[sectionHeaderReusableView class] forSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:reuseHeaderIdentifier];
[self.collectionView registerClass:[sectionBackgroundReusableView class]
forSupplementaryViewOfKind:KindSectionBackground
withReuseIdentifier:NSStringFromClass([sectionBackgroundReusableView class])];
[self.collectionView setShowsHorizontalScrollIndicator:NO];
[self.collectionView setShowsVerticalScrollIndicator:YES];
[self.collectionView setDataSource:self];
[self.collectionView setDelegate:self];
}
-(NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
return 6;
}
-(NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
return 0;
}
-(UICollectionReusableView *)collectionView:(UICollectionView *)collectionV
viewForSupplementaryElementOfKind:(NSString *)kind
atIndexPath:(NSIndexPath *)indexPath
{
UICollectionReusableView *reusableview = nil;
if (kind == UICollectionElementKindSectionHeader)
{
reusableview = [collectionV dequeueReusableSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:reuseHeaderIdentifier forIndexPath:indexPath];
NSString *sectionName = self.shelves[indexPath.section];
[(sectionHeaderReusableView*)reusableview configureWithTitle:sectionName];
}
if (kind == UICollectionElementKindSectionFooter)
{
}
if (kind == KindSectionBackground)
{
reusableview = [collectionView dequeueReusableSupplementaryViewOfKind:sectionBackground
withReuseIdentifier:NSStringFromClass([sectionBackgroundReusableView class])
forIndexPath:indexPath];
}
return reusableview;
}
-(CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout*)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section
{
return = KHeaderSize;
}
// FloawLayout subclass
-(NSArray *)layoutAttributesForElementsInRect:(CGRect)rect
{
NSMutableArray *attributes = [NSMutableArray arrayWithArray:[super layoutAttributesForElementsInRect:rect]];
// 1. get visible sections
NSInteger lastIndex = -1;
for(UICollectionViewLayoutAttributes * attr in attributes) {
lastIndex = attr.indexPath.section;
UICollectionViewLayoutAttributes * attr = [self layoutAttributesForBackgroundAtSection:lastIndex];
[attributes addObject:attr];
}
return attributes;
}
-(UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind
atIndexPath:(NSIndexPath *)indexPath
{
if([kind isEqualToString:KindSectionBackground]) {
return [self layoutAttributesForBackgroundAtSection:indexPath.section];
} else {
return [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
}
}
-(UICollectionViewLayoutAttributes *)layoutAttributesForBackgroundAtSection:(NSUInteger)section
{
NSIndexPath * indexPath =[NSIndexPath indexPathForItem:0
inSection:section];
UICollectionViewLayoutAttributes * attr = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:KindSectionBackground
withIndexPath:indexPath];
attr.hidden = NO;
attr.zIndex = -1;
// crash happen in the line below.
UICollectionViewLayoutAttributes * firstAttr = [selflayoutAttributesForItemAtIndexPath:indexPath];
CGRect frame;
frame.origin.x = firstAttr.frame.origin.x - self.sectionInset.left;
frame.origin.y = firstAttr.frame.origin.y - self.sectionInset.top;
frame.size.width = self.collectionView.bounds.size.width;
NSUInteger numItems = [self.collectionView numberOfItemsInSection:section];
CGFloat cellsPerLine = floorf(self.collectionView.bounds.size.width / self.itemSize.width);
NSUInteger numLines = ceilf(numItems / (float)cellsPerLine);
frame.size.height = numLines * firstAttr.size.height + (numLines-1)*self.minimumLineSpacing +
self.sectionInset.top + self.sectionInset.bottom;
attr.frame = frame;
return attr;
}
Could you please help to fix this, thank you.

Answer 1

Looks like layoutAttributesForItemAtIndexPath crashes when the section provided in the IndexPath is empty. This is probably a bug since it's supposed to return an optional (at least in swift). I had the same kind of issue and fixed it by checking the number of items in the section (swift example for section 1):
if dataSource.collectionView(collectionView, numberOfItemsInSection: 1) > 0 {
if let layoutAttributes = self.layoutAttributesForItem(at: IndexPath(row: 0, section: 1)) {
// update layoutAttributes or create your background attributes
}
}

Answer 2

One workaround would be to call collectionView.reloadData() in advance before we call any layout methods, this work for me.