Isolate the UIKit-facing API to the main actor

[radimvaculik]

Motivation

CellKit is a UITableView/UICollectionView data source layer — every one of its protocols is only ever exercised on the main thread. But the protocols are declared nonisolated, so under Swift 6 every consumer conformance reports:

warning: conformance of 'FooCellModel' to protocol 'CellConvertible' crosses into
main actor-isolated code and can cause data races; this is an error in the Swift 6 language mode

In an app I migrated to Swift 6 this accounted for 61 of 498 warnings — the second largest group. The only consumer-side workaround is to annotate every single conformance:

extension FooCellModel: CellModel, @MainActor ReusableCellConvertible {

which had to be repeated 45 times, plus a @preconcurrency import CellKit where a cell model is built off the main thread.

Change

@MainActor on the API that touches UIKit:

• CellKit — ReusableView, CellConfigurable, CellModel, SupplementaryViewModel, CellConvertible, CellModelSelectable, CellModelDataSourceDelegate, DataSource, CollectionSupplementaryViewModel, AbstractDataSource, CellModelDataSource, LazyCellProvider, LazySupplementaryViewProvider
• DiffableCellKit — DifferentiableCellModel, DifferentiableCellModelDataSource

DifferentiableCellModelWrapper needed a small rework: DifferenceKit's Differentiable/Equatable requirements are nonisolated, so differenceIdentifier is now computed once in a @MainActor init and stored, and == uses MainActor.assumeIsolated. Wrappers are only ever created and diffed by DifferentiableCellModelDataSource, which is now main actor-isolated, so the assumption holds.

MainActor.assumeIsolated requires iOS 13, so Package.swift platforms go from .v9 to .v13. DiffableCellKit already required iOS 13 throughout, and current SPM toolchains no longer accept iOS 9 anyway.

This is source-breaking

Any conformance that is not main actor-isolated will stop compiling. In practice these types are all cells, cell models and data sources, so this should be a no-op for real consumers — but it is a breaking change and probably warrants a minor version bump.

Verification

xcodebuild build passes for both the CellKit and DiffableCellKit schemes on generic/platform=iOS. SwiftLint reports the same 9 pre-existing violations as main — none added.

Building CellKit with SWIFT_STRICT_CONCURRENCY=complete succeeds; two warnings remain in SupplementaryElementKind, which reads UICollectionView.elementKindSectionHeader/Footer from a nonisolated RawRepresentable conformance. I left it alone because marking the enum @MainActor would break RawRepresentable, and the values are immutable strings. Happy to address it separately if you'd prefer.

I could not run the app-side end-to-end check (the consumer pins the released tag), so I have not empirically confirmed that all 45 @MainActor annotations become removable — though that is what the change is for.