Reload UICollectionView header or footer?

Question

I have some data that is fetched in another thread that updates a UICollectionView's header. However, I've not found an efficient way of reloading a supplementary view such as a header or footer.

I can call collectionView reloadSections:, but this reloads the entire section which is unnecessary. collectionView reloadItemsAtIndexPaths: only seems to target cells (not supplementary views). And calling setNeedsDisplay on the header itself doesn't appear to work either. Am I missing something?

Answer 1

You can also use (the lazy way)

collectionView.collectionViewLayout.invalidateLayout() // swift

[[_collectionView collectionViewLayout] invalidateLayout] // objc

More complex would be to provide a context

collectionView.collectionViewLayout.invalidateLayout(with: context) // swift

[[_collectionView collectionViewLayout] invalidateLayoutWithContext:context] // objc

You can then make a or configure the context yourself to inform about what should be updated see: UICollectionViewLayoutInvalidationContext

It has a function in there that you can override:

invalidateSupplementaryElements(ofKind:at:) // swift

Another option is (if you have already loaded the correct header/footer/supplementary view) and you only want to update the view with the new data than you can use one of the following functions to retrieve it:

supplementaryView(forElementKind:at:) // get specific one visibleSupplementaryViews(ofKind:) // all visible ones

Same goes for visible cells with visibleCells. The advantage of just getting the view and not reloading a view entirely is that the cells retains it state. This is espically nice with table view cells when they use swipe to delete/edit/etc since that state is lost after reloading the cell.

If you feel fanatic you can of course also write some extensions to retrieve only cells/supplementary views of a given kind using generics

if let view = supplementaryView(forType: MySupplementaryView.self, at: indexPath) {
    configure(view, at indexPath)
}

this assumes that you have a function that registers/dequeues views in example with their class name. I made a post about this here