UICollectionReusableView method not being called

I want my sections in the UICollectionView to have a header with an image.

I have followed these steps:

• at the storyboard, assigned a header as an accessory for my UICollectionView
• gave it an identifier
• created a subclass of UICollectionReusableView for it
• assigned the custom class to the class at the storyboard.
• put an ImageView at the header accessory
• made an outlet for the ImageView at the custom class in the .h file
• Implemented the following at viewDidLoad:

[self.collectionView registerClass:[ScheduleHeaderView class] forSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:headerIdentifier];

-(UICollectionReusableView *)collectionView:(UICollectionView *)collectionView viewForSupplementaryElementOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath
{
    UICollectionReusableView *reusableview = nil;

    if (kind == UICollectionElementKindSectionHeader)
    {
        ScheduleHeaderView *headerView = [collectionView dequeueReusableSupplementaryViewOfKind:UICollectionElementKindSectionHeader withReuseIdentifier:headerIdentifier forIndexPath:indexPath];

        headerView.headerImageView.image = [UIImage imageNamed:@"blah.png"];

        reusableview = headerView;
    }

    return reusableview;
}

I know that the datasource and delegate methods are working because I can see all the cells and its sections. However, I don't have my headers. I put in a breakpoint at the method above and it's never being called.

What am i doing wrong?

It seems that you have to give your header a non-zero size or collectionView:viewForSupplementaryElementOfKind:atIndexPath isn't called. So either set the headerReferenceSize property of the flow layout like so:

flowLayout.headerReferenceSize = CGSizeMake(self.collectionView.frame.size.width, 100.f);

Swift 5+

flowLayout.headerReferenceSize = CGSize(CGSize(width: self.collectionView.frame.size.width, height: 100))

or, implement collectionView:layout:referenceSizeForHeaderInSection if you want to vary the size by section.

You answered the question, but in words I understand better:

To make the method being called, add these methods:

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForHeaderInSection:(NSInteger)section {
    return CGSizeMake(60.0f, 30.0f);
}

- (CGSize)collectionView:(UICollectionView *)collectionView layout:(UICollectionViewLayout *)collectionViewLayout referenceSizeForFooterInSection:(NSInteger)section {
    return CGSizeMake(60.0f, 30.0f);
}

...and go from there.

Swift 4 Xcode 9.1 Beta 2

Add @objc

@objc func collectionView(_ collectionView: UICollectionView, layout  collectionViewLayout: UICollectionViewLayout, referenceSizeForFooterInSection section: Int) -> CGSize{
    let size = CGSize(width: 400, height: 50)
    return size
}

Credits: https://medium.com/@zonble/your-delegation-methods-might-not-be-called-in-swift-3-c6065ed7b4cd