Adopt Swift 6 + concurrency + remove background diffing (#158)

jessesquires · web-flow · commit 3926d69fbbaf · 2026-01-05T10:00:48.000-08:00
Closes #157. - Adopt Swift 6 - Support Concurrency - Remove background diffing Maintaining `CollectionViewDriverOptions.diffOnBackgroundQueue` is too large of an effort for very little benefit. It was always questionable, and against guidance from the UIKit team. **See this blog post for more:** https://www.jessesquires.com/blog/2024/12/19/diffable-data-source-main-actor-inconsistency **From the UIKit team:** > We have seen a number of issues stemming from usage of diffable data source on background queues/threads, and the performance benefits of doing this are generally minimal due to the fact that only the diffing of identifiers in the old & new snapshots happens on the background queue/thread; the work to set up and execute the Ul updates and animations for cells always happens on the main thread. Therefore, we made the decision to restrict diffable data source to the main actor when using Swift Concurrency, as this ensures correctness in all cases and is nearly always the best approach anyways. If you were previously applying snapshots from a background queue, we recommend you update your implementation to do so on the main queue instead.
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -7,6 +7,14 @@ NEXT
 
 - TBA
 
+0.2.0
+-----
+
+This release closes the [0.2.0 milestone](https://github.com/jessesquires/ReactiveCollectionsKit/milestone/3?closed=1).
+
+**Breaking Changes:**
+- Adopt Swift 6 and Swift Concurrency, remove `CollectionViewDriverOptions.diffOnBackgroundQueue`. ([@jessesquires](https://github.com/jessesquires), [#157](https://github.com/jessesquires/ReactiveCollectionsKit/issues/157), [#158](https://github.com/jessesquires/ReactiveCollectionsKit/pull/158)) **See linked issue and pull request for decision to remove `diffOnBackgroundQueue`.**
+
 0.1.9
 -----
 
diff --git a/Example/Sources/List/ListViewController.swift b/Example/Sources/List/ListViewController.swift
@@ -20,7 +20,6 @@ final class ListViewController: ExampleViewController, CellEventCoordinator {
     lazy var driver: CollectionViewDriver = {
         let driver = CollectionViewDriver(
             view: self.collectionView,
-            options: .init(diffOnBackgroundQueue: true),
             emptyViewProvider: sharedEmptyViewProvider,
             cellEventCoordinator: self
         )
diff --git a/Package.swift b/Package.swift
@@ -41,16 +41,5 @@ let package = Package(
             ]
         )
     ],
-    swiftLanguageModes: [.v5]
+    swiftLanguageModes: [.v6]
 )
-
-#warning("Remove after Swift 6 language mode")
-let swiftSettings = [
-    SwiftSetting.enableExperimentalFeature("StrictConcurrency")
-]
-
-for target in package.targets {
-    var settings = target.swiftSettings ?? []
-    settings.append(contentsOf: swiftSettings)
-    target.swiftSettings = settings
-}
diff --git a/README.md b/README.md
@@ -91,7 +91,7 @@ driver.update(viewModel: updated)
 ## Requirements
 
 - iOS 16.0+
-- Swift 5.10+
+- Swift 6.0+
 - Xcode 26.0+
 - [SwiftLint](https://github.com/realm/SwiftLint)
 
diff --git a/ReactiveCollectionsKit.xcodeproj/project.pbxproj b/ReactiveCollectionsKit.xcodeproj/project.pbxproj
@@ -288,7 +288,7 @@
 				SWIFT_ACTIVE_COMPILATION_CONDITIONS = DEBUG;
 				SWIFT_OPTIMIZATION_LEVEL = "-Onone";
 				SWIFT_STRICT_CONCURRENCY = complete;
-				SWIFT_VERSION = 5.0;
+				SWIFT_VERSION = 6.0;
 				TARGETED_DEVICE_FAMILY = "1,2";
 				VERSIONING_SYSTEM = "apple-generic";
 			};
@@ -351,7 +351,7 @@
 				SWIFT_COMPILATION_MODE = wholemodule;
 				SWIFT_OPTIMIZATION_LEVEL = "-O";
 				SWIFT_STRICT_CONCURRENCY = complete;
-				SWIFT_VERSION = 5.0;
+				SWIFT_VERSION = 6.0;
 				TARGETED_DEVICE_FAMILY = "1,2";
 				VALIDATE_PRODUCT = YES;
 				VERSIONING_SYSTEM = "apple-generic";
diff --git a/Sources/CollectionViewDriver.swift b/Sources/CollectionViewDriver.swift
@@ -92,7 +92,7 @@ public final class CollectionViewDriver: NSObject {
 
         // workaround for swift initialization rules.
         // the "real" init is below.
-        self._dataSource = DiffableDataSource(view: view, diffOnBackgroundQueue: false)
+        self._dataSource = DiffableDataSource(view: view)
 
         super.init()
 
@@ -106,7 +106,6 @@ public final class CollectionViewDriver: NSObject {
         // `self` owns the `_dataSource`, so we know that `self` will always exist.
         self._dataSource = DiffableDataSource(
             view: view,
-            diffOnBackgroundQueue: options.diffOnBackgroundQueue,
             cellProvider: { [unowned self] view, indexPath, itemIdentifier in
                 self._cellProvider(
                     collectionView: view,
diff --git a/Sources/CollectionViewDriverOptions.swift b/Sources/CollectionViewDriverOptions.swift
@@ -15,11 +15,6 @@ import Foundation
 
 /// Defines various options to customize behavior of a ``CollectionViewDriver``.
 public struct CollectionViewDriverOptions: Hashable {
-    /// Specifies whether or not to perform diffing on a background queue.
-    /// Pass `true` to perform diffing in the background,
-    /// pass `false` to perform diffing on the main thread.
-    public let diffOnBackgroundQueue: Bool
-
     /// Specifies whether or not the ``CollectionViewDriver`` should
     /// perform a hard `reloadData()` when replacing the ``CollectionViewModel`` with
     /// a new one, or if it should always perform a diff.
@@ -31,13 +26,10 @@ public struct CollectionViewDriverOptions: Hashable {
     /// Initializes a `CollectionViewDriverOptions` object.
     ///
     /// - Parameters:
-    ///   - diffOnBackgroundQueue: Whether or not to perform diffing on a background queue. Default is `false`.
     ///   - reloadDataOnReplacingViewModel: Whether or not to reload or diff during replacement. Default is `false`.
     public init(
-        diffOnBackgroundQueue: Bool = false,
         reloadDataOnReplacingViewModel: Bool = false
     ) {
-        self.diffOnBackgroundQueue = diffOnBackgroundQueue
         self.reloadDataOnReplacingViewModel = reloadDataOnReplacingViewModel
     }
 }
diff --git a/Sources/DebugDescriptions.swift b/Sources/DebugDescriptions.swift
@@ -133,7 +133,6 @@ private func debugDescriptionBuilder<Target: TextOutputStream>(
             debugDescriptionBuilder(
                 elements: [
                     (.type(CollectionViewDriverOptions.self), indent + 2),
-                    (.field(label: "diffOnBackgroundQueue", value: options.diffOnBackgroundQueue), indent + 4),
                     (.field(label: "reloadDataOnReplacingViewModel", value: options.reloadDataOnReplacingViewModel), indent + 4),
                     (.end, indent + 2)
                 ],
@@ -203,7 +202,6 @@ func driverOptionsDebugDescription(_ options: CollectionViewDriverOptions) -> St
     debugDescriptionBuilder(
         elements: [
             (.type(CollectionViewDriverOptions.self), 0),
-            (.field(label: "diffOnBackgroundQueue", value: options.diffOnBackgroundQueue), 2),
             (.field(label: "reloadDataOnReplacingViewModel", value: options.reloadDataOnReplacingViewModel), 2),
             (.end, 0)
         ],
diff --git a/Sources/DiffableDataSource.swift b/Sources/DiffableDataSource.swift
@@ -26,34 +26,23 @@ final class DiffableDataSource: UICollectionViewDiffableDataSource<AnyHashable,
     // Thus, unowned is safe here.
     private unowned let _collectionView: UICollectionView
 
-    private let _diffOnBackgroundQueue: Bool
-
-    private lazy var _diffingQueue = DispatchQueue(
-        label: "com.jessesquires.ReactiveCollectionsKit",
-        qos: .userInteractive,
-        autoreleaseFrequency: .workItem
-    )
-
     var logger: Logging?
 
     // MARK: Init
 
     init(
         view: UICollectionView,
-        diffOnBackgroundQueue: Bool,
         cellProvider: @escaping DiffableDataSource.CellProvider,
         supplementaryViewProvider: @escaping DiffableDataSource.SupplementaryViewProvider
     ) {
         self._collectionView = view
-        self._diffOnBackgroundQueue = diffOnBackgroundQueue
         super.init(collectionView: view, cellProvider: cellProvider)
         self.supplementaryViewProvider = supplementaryViewProvider
     }
 
-    convenience init(view: UICollectionView, diffOnBackgroundQueue: Bool) {
+    convenience init(view: UICollectionView) {
         self.init(
             view: view,
-            diffOnBackgroundQueue: diffOnBackgroundQueue,
             cellProvider: { _, _, _ in nil },
             supplementaryViewProvider: { _, _, _ in nil }
         )
@@ -66,12 +55,8 @@ final class DiffableDataSource: UICollectionViewDiffableDataSource<AnyHashable,
 
         let snapshot = DiffableSnapshot(viewModel: viewModel)
         self.applySnapshotUsingReloadData(snapshot) {
-            // UIKit guarantees `completion` is called on the main queue.
-            dispatchPrecondition(condition: .onQueue(.main))
-            MainActor.assumeIsolated {
-                completion?()
-                self.logger?.log("DataSource reload snapshot completed")
-            }
+            completion?()
+            self.logger?.log("DataSource reload snapshot completed")
         }
     }
 
@@ -89,33 +74,18 @@ final class DiffableDataSource: UICollectionViewDiffableDataSource<AnyHashable,
         // We need to inspect the current collection view state first, then pass this info downstream.
         let visibleItemIdentifiers = self._visibleItemIdentifiers()
 
-        if self._diffOnBackgroundQueue {
-            self.logger?.log("DataSource using background queue")
-            self._diffingQueue.async {
-                self._applySnapshot(
-                    from: source,
-                    to: destination,
-                    withVisibleItems: visibleItemIdentifiers,
-                    animated: animated,
-                    completion: completion
-                )
-            }
-        } else {
-            self.logger?.log("DataSource using main queue")
-            dispatchPrecondition(condition: .onQueue(.main))
-            self._applySnapshot(
-                from: source,
-                to: destination,
-                withVisibleItems: visibleItemIdentifiers,
-                animated: animated,
-                completion: completion
-            )
-        }
+        self._applySnapshot(
+            from: source,
+            to: destination,
+            withVisibleItems: visibleItemIdentifiers,
+            animated: animated,
+            completion: completion
+        )
     }
 
     // MARK: Private
 
-    nonisolated private func _applySnapshot(
+    private func _applySnapshot(
         from source: CollectionViewModel,
         to destination: CollectionViewModel,
         withVisibleItems visibleItemIdentifiers: Set<UniqueIdentifier>,
@@ -153,59 +123,49 @@ final class DiffableDataSource: UICollectionViewDiffableDataSource<AnyHashable,
         destinationSnapshot.reconfigureItems(itemsToReconfigure)
 
         // Apply the snapshot with item reconfigure updates.
-        //
-        // Swift 6 complains about 'call to main actor-isolated instance method' here.
-        // However, call this method from a background thread is valid according to the docs.
         self.apply(destinationSnapshot, animatingDifferences: animated) { [weak self] in
-            // UIKit guarantees `completion` is called on the main queue.
-            dispatchPrecondition(condition: .onQueue(.main))
-
             guard let self else {
-                MainActor.assumeIsolated {
-                    completion?()
-                }
+                completion?()
                 return
             }
 
-            MainActor.assumeIsolated {
-                // Once the snapshot with item reconfigures is applied,
-                // we need to find and apply supplementary view reconfigures, if needed.
-                //
-                // This is necessary to update all headers, footers, and supplementary views.
-                // Per notes above, supplementary views do not get reloaded / reconfigured
-                // automatically by `DiffableDataSource` when they change.
-                //
-                // To trigger updates on supplementary views with the existing APIs,
-                // the entire section must be reloaded. Yes, that sucks. We don't want to do that.
-                // That causes all items in the section to be hard-reloaded, too.
-                // Aside from the performance impact, doing that results in an ugly UI "flash"
-                // for all item cells in the collection. Gross.
-                //
-                // However, we can actually do much better than a hard reload!
-                // Instead of reloading the entire section, we can find and compare
-                // the supplementary views and manually reconfigure them if they changed.
-                //
-                // NOTE: this only matters if supplementary views are not static.
-                // That is, if they reflect data in the data source.
-                //
-                // For example, a header with a fixed title (e.g. "My Items") will NOT need to be reloaded.
-                // However, a header that displays changing data WILL need to be reloaded.
-                // (e.g. "My 10 Items")
-
-                // Check all the supplementary views and reconfigure them, if needed.
-                self._reconfigureSupplementaryViewsIfNeeded(from: source, to: destination)
-
-                // Finally, we're done and can call completion.
-                completion?()
-
-                self.logger?.log("DataSource diffing snapshot complete")
-            }
+            // Once the snapshot with item reconfigures is applied,
+            // we need to find and apply supplementary view reconfigures, if needed.
+            //
+            // This is necessary to update all headers, footers, and supplementary views.
+            // Per notes above, supplementary views do not get reloaded / reconfigured
+            // automatically by `DiffableDataSource` when they change.
+            //
+            // To trigger updates on supplementary views with the existing APIs,
+            // the entire section must be reloaded. Yes, that sucks. We don't want to do that.
+            // That causes all items in the section to be hard-reloaded, too.
+            // Aside from the performance impact, doing that results in an ugly UI "flash"
+            // for all item cells in the collection. Gross.
+            //
+            // However, we can actually do much better than a hard reload!
+            // Instead of reloading the entire section, we can find and compare
+            // the supplementary views and manually reconfigure them if they changed.
+            //
+            // NOTE: this only matters if supplementary views are not static.
+            // That is, if they reflect data in the data source.
+            //
+            // For example, a header with a fixed title (e.g. "My Items") will NOT need to be reloaded.
+            // However, a header that displays changing data WILL need to be reloaded.
+            // (e.g. "My 10 Items")
+
+            // Check all the supplementary views and reconfigure them, if needed.
+            self._reconfigureSupplementaryViewsIfNeeded(from: source, to: destination)
+
+            // Finally, we're done and can call completion.
+            completion?()
+
+            self.logger?.log("DataSource diffing snapshot complete")
         }
     }
 
     // MARK: Reconfiguring Cells
 
-    nonisolated private func _findItemsToReconfigure(
+    private func _findItemsToReconfigure(
         from source: CollectionViewModel,
         to destination: CollectionViewModel,
         withVisibleItems visibleItemIdentifiers: Set<UniqueIdentifier>
diff --git a/Tests/TestCollectionViewDriver.swift b/Tests/TestCollectionViewDriver.swift
@@ -430,23 +430,6 @@ final class TestCollectionViewDriver: UnitTestCase, @unchecked Sendable {
         self.waitForExpectations()
     }
 
-    @MainActor
-    func test_update_callsCompletion_withBackgroundDiffing() {
-        let driver = CollectionViewDriver(
-            view: self.collectionView,
-            options: .init(diffOnBackgroundQueue: true)
-        )
-
-        let expectation = self.expectation()
-
-        let newModel = self.fakeCollectionViewModel()
-        driver.update(viewModel: newModel, animated: true) { _ in
-            expectation.fulfillAndLog()
-        }
-
-        self.waitForExpectations()
-    }
-
     @MainActor
     func test_update_callsCompletion_withReloadOnReplace() {
         let driver = CollectionViewDriver(
diff --git a/Tests/TestCollectionViewDriverOptions.swift b/Tests/TestCollectionViewDriverOptions.swift
@@ -19,7 +19,6 @@ final class TestCollectionViewDriverOptions: XCTestCase {
 
     func test_defaultValues() {
         let options = CollectionViewDriverOptions()
-        XCTAssertFalse(options.diffOnBackgroundQueue)
         XCTAssertFalse(options.reloadDataOnReplacingViewModel)
     }
 
@@ -29,21 +28,18 @@ final class TestCollectionViewDriverOptions: XCTestCase {
             options.debugDescription,
             """
             CollectionViewDriverOptions {
-              diffOnBackgroundQueue: false
               reloadDataOnReplacingViewModel: false
             }
             """
         )
 
         let options2 = CollectionViewDriverOptions(
-            diffOnBackgroundQueue: true,
             reloadDataOnReplacingViewModel: true
         )
         XCTAssertEqual(
             options2.debugDescription,
             """
             CollectionViewDriverOptions {
-              diffOnBackgroundQueue: true
               reloadDataOnReplacingViewModel: true
             }
             """
diff --git a/Tests/TestDebugDescriptionDriver.swift b/Tests/TestDebugDescriptionDriver.swift
@@ -52,7 +52,6 @@ final class TestDebugDescriptionDriver: XCTestCase {
             CollectionViewDriver \\{
               options:
                 CollectionViewDriverOptions \\{
-                  diffOnBackgroundQueue: false
                   reloadDataOnReplacingViewModel: false
                 \\}
               viewModel:
@@ -94,7 +93,6 @@ final class TestDebugDescriptionDriver: XCTestCase {
             CollectionViewDriver \\{
               options:
                 CollectionViewDriverOptions \\{
-                  diffOnBackgroundQueue: false
                   reloadDataOnReplacingViewModel: false
                 \\}
               viewModel:
@@ -155,7 +153,6 @@ final class TestDebugDescriptionDriver: XCTestCase {
             CollectionViewDriver \\{
               options:
                 CollectionViewDriverOptions \\{
-                  diffOnBackgroundQueue: false
                   reloadDataOnReplacingViewModel: false
                 \\}
               viewModel:
diff --git a/Tests/Utils/CollectionViewDriverOptions+Extensions.swift b/Tests/Utils/CollectionViewDriverOptions+Extensions.swift
@@ -16,6 +16,6 @@ import ReactiveCollectionsKit
 
 extension CollectionViewDriverOptions {
     static func test() -> Self {
-        .init(diffOnBackgroundQueue: false, reloadDataOnReplacingViewModel: true)
+        .init(reloadDataOnReplacingViewModel: true)
     }
 }

Original file line number	Diff line number	Diff line change
`@@ -20,7 +20,6 @@ final class ListViewController: ExampleViewController, CellEventCoordinator {`
`20`	`20`	`lazy var driver: CollectionViewDriver = {`
`21`	`21`	`let driver = CollectionViewDriver(`
`22`	`22`	`view: self.collectionView,`
`23`		`- options: .init(diffOnBackgroundQueue: true),`
`24`	`23`	`emptyViewProvider: sharedEmptyViewProvider,`
`25`	`24`	`cellEventCoordinator: self`
`26`	`25`	`)`
Original file line number	Diff line number	Diff line change
`@@ -41,16 +41,5 @@ let package = Package(`
`41`	`41`	`]`
`42`	`42`	`)`
`43`	`43`	`],`
`44`		`- swiftLanguageModes: [.v5]`
	`44`	`+ swiftLanguageModes: [.v6]`
`45`	`45`	`)`
`46`		`-`
`47`		`-#warning("Remove after Swift 6 language mode")`
`48`		`-let swiftSettings = [`
`49`		`- SwiftSetting.enableExperimentalFeature("StrictConcurrency")`
`50`		`-]`
`51`		`-`
`52`		`-for target in package.targets {`
`53`		`- var settings = target.swiftSettings ?? []`
`54`		`- settings.append(contentsOf: swiftSettings)`
`55`		`- target.swiftSettings = settings`
`56`		`-}`