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.