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I have a feeling this will also crash with a regular remove(dataSource:) call for the last section, but this is how I came across it.

ComposedDataSource.removeAll calls updateDelegate?.dataSource(self, didDeleteSections: IndexSet(indexes)) on line 164, which triggers collectionView.collectionViewLayout.layoutAttributesForElements(in: collectionView.bounds) in CollectionViewWrapper.

FlowLayout is then queried, but the collection view and the data sources are out-of-sync. The collection view thinks it has more than 0 sections, but they've actually just been deleted, so when backgroundViewClass(in:forSectionAt:) is called it tries to get a mapping for a section that – to the data source – doesn't exist.

I've seen this on master and performance but my attempts to fix it have been fruitless :(

Attached is a screenshot of the stacktrace.

While trying to get ComposedDataSource to work I noticed that the methods on DataSourceUpdateDelegate seem to be somewhat duplicated.

At first I had assumed that I should use func dataSource(_ dataSource: DataSource, willPerform updates: [DataSourceUpdate]) and func dataSource(_ dataSource: DataSource, didPerform updates: [DataSourceUpdate]), but then I noticed the func dataSource(_ dataSource: DataSource, did*) functions.

Looking at DataSourceUpdate it has equivalents for each of the func dataSource(_ dataSource: DataSource, did*) functions. Do these have explicit different uses, or could they be consolidated? I saw that in CollectionViewWrapper both functions are handled, but in the func dataSource(_ dataSource: DataSource, did*) functions the messages are passed to the collection view, but in func dataSource(_ dataSource: DataSource, didPerform updates: [DataSourceUpdate]) only the placeholder is updated.

Using only the DataSourceUpdate methods would maybe make implementations smaller (e.g. the message passing in SegmentedDataSource would be much smaller), and would IMO be the better solution.

What do you think?