Customising position of header in UICollectionView layout causes NSInternalInconsistencyException error

Question

I am trying to customise the positions of the headers in a UICollectionView using a subclassed UICollectionViewFlowLayout class (based loosely on the code for stacked headers which is shown enter link description here).

As a minimal test, let's say I just want to add a fixed offset to the position of all headers:

• I add all headers to the array returned by layoutAttributesForElementsInRect so that all are always processed (this may be the cause of the problem, I'm not sure)
• I then update each header by adding a fixed offset in layoutAttributesForSupplementaryViewOfKind

The full implementation is included at the end of this post.

(By the way, I know that adding all headers, including those outside the rect, is not strictly speaking necessary in the first step, but this is a simplified example of a more complex customisation in position I want to make which would cause all headers to be displayed in the draw rect.)

However, when I run the code I get the following NSInternalInconsistencyException:

2014-01-15 00:41:50.130 CollectionStackedHeaders[60777:70b] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException',
reason: 'layout attributes for supplementary item at index path (<NSIndexPath: 0x8a7db90> {length = 2, path = 0 - 0})
changed from <UICollectionViewLayoutAttributes: 0x8a7f8b0> index path: (<NSIndexPath: 0x8a7d9c0> {length = 2, path = 0 - 0}); element kind: (UICollectionElementKindSectionHeader); frame = (0 0; 320 50);
        to   <UICollectionViewLayoutAttributes: 0x8a7fb80> index path: (<NSIndexPath: 0x8a7db90> {length = 2, path = 0 - 0}); element kind: (UICollectionElementKindSectionHeader); frame = (0 50; 320 50); zIndex = 1024;
without invalidating the layout'

It seems that this is caused by the update of the attributes, as if I comment out the following two lines it works fine:

attributes.zIndex = 1024;
attributes.frame = frame;

What is causing this error, and what can I do to get my simple example up and running?

Here is the full class implementation for this simple example:

@implementation myStackedHeaderFlowLayout

- (NSArray*)layoutAttributesForElementsInRect:(CGRect)rect {
    // Call super to get elements
    NSMutableArray* answer = [[super layoutAttributesForElementsInRect:rect] mutableCopy];

    // As a test, always add first header to the answer array
    NSArray* indexes = [NSArray arrayWithObjects: [NSNumber numberWithInt:0], nil];
    for (NSNumber* sectionNumber in indexes) {
        NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:[sectionNumber integerValue]];
        UICollectionViewLayoutAttributes* layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];
        if (layoutAttributes) {
            [answer removeObject:layoutAttributes]; // remove if already present
            [answer addObject:layoutAttributes];
        }
    }

    return answer;
}

- (UICollectionViewLayoutAttributes*)layoutAttributesForSupplementaryViewOfKind:(NSString*)kind atIndexPath:(NSIndexPath*)indexPath {
    // Call super to get base attributes
    UICollectionViewLayoutAttributes* attributes = [super layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];

    if ([kind isEqualToString:UICollectionElementKindSectionHeader]) {
        CGRect frame = attributes.frame;
        frame.origin.y += 50;
        // Update attributes position here - causes the problem
        attributes.zIndex = 1024;
        attributes.frame = frame;
    }

    return attributes;
}

- (UICollectionViewLayoutAttributes*)initialLayoutAttributesForAppearingSupplementaryElementOfKind:(NSString*)kind atIndexPath:(NSIndexPath*)indexPath {
    UICollectionViewLayoutAttributes* attributes = [self layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    return attributes;
}

- (UICollectionViewLayoutAttributes*)finalLayoutAttributesForDisappearingSupplementaryElementOfKind:(NSString*)kind atIndexPath:(NSIndexPath*)indexPath {
    UICollectionViewLayoutAttributes* attributes = [self layoutAttributesForSupplementaryViewOfKind:kind atIndexPath:indexPath];
    return attributes;
}

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

@end

Answer 1

layout attributes for supplementary item at index path (<NSIndexPath>) 
changed from <UICollectionViewLayoutAttributes> 
to <UICollectionViewLayoutAttributes>
without invalidating the layout

In my experience, the NSInternalInconsistencyException with the description above is thrown when the array returned from layoutAttributesForElementsInRect: contains two UICollectionViewLayoutAttributes objects with the same index path and (supplementary) element category.

Answer 2

You're receiving this error because you're adjusting the frame from (0 0; 320 50) to (0 50; 320 50) without re-validating the layout (likely you're doing this inadvertently).

Typically, it is because you're referencing the same IndexPath for two different layout elements but providing a different frame value for each.

Consider the following:

NSIndexPath *indexPath = [NSIndexPath indexPathForItem:0 inSection:0];

UICollectionViewLayoutAttributes *newAttribute1 = [UICollectionViewLayoutAttributes layoutAttributesForDecorationViewOfKind:UICollectionElementKindSectionHeader withIndexPath:indexPath];
newAttribute1.frame = CGRectMake(0, 50, 320, 50);
[attributes addObject:newAttribute1];

UICollectionViewLayoutAttributes *newAttribute2 = [UICollectionViewLayoutAttributes layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionFooter withIndexPath:indexPath];
newAttribute2.frame = CGRectMake(0, 0, 320, 50);
[attributes addObject:newAttribute2];

Each is using the same IndexPath and thus it causes an NSInternalInconsistencyException

Answer 3

OK, I'm not 100% sure why, but replacing the layoutAttributesForElementsInRect with the following seemed to do the trick:

- (NSArray*)layoutAttributesForElementsInRect:(CGRect)rect {
    // Call super to get elements
    NSMutableArray* answer = [[super layoutAttributesForElementsInRect:rect] mutableCopy];

    NSUInteger maxSectionIndex = 0;
    for (NSUInteger idx=0; idx < [answer count]; ++idx) {
        UICollectionViewLayoutAttributes *layoutAttributes = answer[idx];

        if (layoutAttributes.representedElementCategory == UICollectionElementCategoryCell || layoutAttributes.representedElementCategory == UICollectionElementCategorySupplementaryView) {
            // Keep track of the largest section index found in the rect (maxSectionIndex)
            NSUInteger sectionIndex = (NSUInteger)layoutAttributes.indexPath.section;
            if (sectionIndex > maxSectionIndex) {
                maxSectionIndex = sectionIndex;
            }
        }
        if ([layoutAttributes.representedElementKind isEqualToString:UICollectionElementKindSectionHeader]) {
            // Remove layout of header done by our super, as we will do it right later
            [answer removeObjectAtIndex:idx];
            idx--;
        }
    }

    // Re-add all section headers for sections >= maxSectionIndex
    for (NSUInteger idx=0; idx <= maxSectionIndex; ++idx) {
        NSIndexPath* indexPath = [NSIndexPath indexPathForItem:0 inSection:idx];
        UICollectionViewLayoutAttributes *layoutAttributes = [self layoutAttributesForSupplementaryViewOfKind:UICollectionElementKindSectionHeader atIndexPath:indexPath];
        if (layoutAttributes) {
            [answer addObject:layoutAttributes];
        }
    }

    return answer;
}

I can only imagine that before layoutAttributesForElementsInRect was being called early before the header I had added to the control for the first section was properly initialised, and so programatically determining what headers were present avoided this? Any thoughts would be welcome, but with the above the issue is resolved.