Crash on data reload with 0 items and sections

[natenash203]

First off, great work on the layout. I use the layout for a couple sections with three or four items each. Each section has a header and the items are populated by data retrieved from a network call. I have a method that removes all objects from the data source and sets everything back to zero. For example, 0 sections, with 0 items. Essentially an empty array. The method looks a bit like this:

- (IBAction)resetButtonTapped:(id)sender
{
    [self.dataSource removeAllObjects];
    [self.collectionView reloadData];
}

Whenever I reset everything back to zero, and call reloadData, the following exception is thrown:

*** Assertion failure in -[UICollectionViewData layoutAttributesForSupplementaryElementOfKind:atIndexPath:], /SourceCache/UIKit_Sim/UIKit-3318/UICollectionViewData.m:853
2014-10-19 19:18:22.554 Hord[31192:4215382] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'request for layout attributes for supplementary view CHTCollectionElementKindSectionHeader in section 0 when there are only 0 sections in the collection view'

A similar issue was reported on another custom UICollectionViewLayout here. However, the other layout appears to have some sort of cache invalidation logic built in.

It looks like the layout object is still trying to return attributes for a header that no longer exists. Any advice you have on resolving this issue is much appreciated. Thanks in advance!

[natenash203]

As a follow up, I thought I'd share a full example of what I describe in my initial filing. Below is the view controller code. It is based off a storyboard with a UICollectionViewController. Layout object for the UICollectionViewController is set toCHTCollectionViewWaterfallLayout. Two different UIBarButtonItems are connected to the deleteFirstSection and deleteAllSections methods.

#import "GTCollectionViewController.h"

@interface GTCollectionViewController ()

@property (nonatomic, strong) NSMutableArray *dataSource;

@end

@implementation GTCollectionViewController

static NSString * const reuseIdentifier = @"Cell";

- (void)viewDidLoad {
    [super viewDidLoad];

    self.dataSource = [NSMutableArray arrayWithObjects:@{@"sectionName" : @"one",
                                                         @"sectionData": @[@"itemOne", @"itemTwo", @"itemThree"]},
                                                        @{@"sectionName" : @"two",
                                                            @"sectionData": @[@"itemFour", @"itemFive", @"itemSix"]},
                                                        @{@"sectionName" : @"three",
                                                            @"sectionData": @[@"itemSeven", @"itemEight", @"itemNine", @"itemTen"]}, nil];

    CHTCollectionViewWaterfallLayout *waterfall = (CHTCollectionViewWaterfallLayout *)self.collectionViewLayout;

    waterfall.columnCount = 3;

    [self.collectionView registerClass:[UICollectionReusableView class] forSupplementaryViewOfKind:CHTCollectionElementKindSectionHeader withReuseIdentifier:@"header"];
}


#pragma mark <UICollectionViewDataSource>

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView {

    NSLog(@"Called Sections");
    return self.dataSource.count;
}

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section
{
    NSArray *array = self.dataSource[section][@"sectionData"];
    return array.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath {

    NSArray *array = self.dataSource[indexPath.section][@"sectionData"];

    UICollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"cell" forIndexPath:indexPath];

    UILabel *label = (UILabel *)[cell viewWithTag:99];
    label.text = array[indexPath.item];

    cell.contentView.backgroundColor = [UIColor greenColor];

    return cell;
}

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{

    UICollectionReusableView *view;
    if (kind == CHTCollectionElementKindSectionHeader) {
        view = [collectionView dequeueReusableSupplementaryViewOfKind:kind withReuseIdentifier:@"header" forIndexPath:indexPath];
        view.backgroundColor = [UIColor redColor];

    }

    return view;
}

#pragma mark - CHTCollectionViewDelegateWaterfallLayout
- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout sizeForItemAtIndexPath:(NSIndexPath *)indexPath
{
    return CGSizeMake(arc4random() % 50 + 50, arc4random() % 50 + 50);
}

- (CGFloat)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout heightForHeaderInSection:(NSInteger)section
{
    return 100;
}

- (IBAction)deleteFirstSection:(id)sender
{

    //Will throw exception on last section to be removed

    [self.dataSource removeObjectAtIndex:0];
    [self.collectionView deleteSections:[NSIndexSet indexSetWithIndex:0]];

}

- (IBAction)deleteAllSections:(id)sender
{

    //Will always throw exception

    [self.dataSource removeAllObjects];
    [self.collectionView reloadData];

}
@end

[jhariani]

I'm experiencing this as well.

[chiahsien]

Can anyone fix this issue?
My little boy just delivered so I will not be available for months.
Sorry 😭

[jefflen]

I'm experiencing this issue as well, but I solved this by never returns zero in the collection view data source,

- (NSInteger)numberOfSectionsInCollectionView:(UICollectionView *)collectionView
{
    NSInteger count = [self sectionTitles].count;
    return (count == 0) ? 1 : count;
}

and then check and returns an supplementary view with empty contents,

- (UICollectionReusableView *)collectionView:(UICollectionView *)collectionView
           viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    NSArray *sectionTitles = [self sectionTitles];

    MyCollectionHeaderView *headerView = [collectionView dequeueReusableSupplementaryViewOfKind:kind
                                                                            withReuseIdentifier:kind
                                                                                   forIndexPath:indexPath];

    if (sectionTitles.count == 0) {
        headerView.titleLabel.text = @"";
        return headerView;
    }
    else { 
        ...
    }
}

you may need to check for other data source methods as well for this modification.

[chiahsien]

Interesting. Can't numberOfSectionsInCollectionView return 0? I can't find any document addressing this, anyone?

[jefflen]

I tried to modify the CHTCollectionViewWaterfallLayout class without success so just use this patch method instead. Not sure why it won't works when return zero...

[dginsburg]

Fixed in PR #93

[chiahsien]

Thanks @dginsburg !

[hebinda]

self.floatingSectionIndex = -1;

Initialization of floatingSectionIndex is needed in prepareLayout. When search result filtered down sections, the current floating section doesn't exist anymore

[Isuru-Nanayakkara]

I'm getting this error in the Swift version.

[satashiabhavin]

deinit {
SDWebImageDownloader.shared.cancelAllDownloads()
}