Use UIDiffableDataSource.apply to perform diff and only update what…

…'s changed (with animation)

Since the diffable data source diffing happens on its own dedicated queue (not the main queue) that may also run work on the main thread, we had to replace the `ImmediateWhenOnMainQueueScheduler` with the `ImmediateWhenOnMainThreadScheduler`.

Also, we had to rearrange the setup of some tests since the diffable data source eagerly preloads cells.

EssentialApp/EssentialApp/CombineHelpers.swift

@@ -117,8 +117,8 @@ private extension FeedCache {
 }
 
 extension Publisher {
-	func dispatchOnMainQueue() -> AnyPublisher<Output, Failure> {
-		receive(on: DispatchQueue.immediateWhenOnMainQueueScheduler).eraseToAnyPublisher()
+	func dispatchOnMainThread() -> AnyPublisher<Output, Failure> {
+		receive(on: DispatchQueue.immediateWhenOnMainThreadScheduler).eraseToAnyPublisher()
 	}
 }
 
@@ -168,6 +168,39 @@ extension DispatchQueue {
 			DispatchQueue.main.schedule(after: date, interval: interval, tolerance: tolerance, options: options, action)
 		}
 	}
+
+	static var immediateWhenOnMainThreadScheduler: ImmediateWhenOnMainThreadScheduler {
+		ImmediateWhenOnMainThreadScheduler()
+	}
+
+	struct ImmediateWhenOnMainThreadScheduler: Scheduler {
+		typealias SchedulerTimeType = DispatchQueue.SchedulerTimeType
+		typealias SchedulerOptions = DispatchQueue.SchedulerOptions
+
+		var now: SchedulerTimeType {
+			DispatchQueue.main.now
+		}
+
+		var minimumTolerance: SchedulerTimeType.Stride {
+			DispatchQueue.main.minimumTolerance
+		}
+
+		func schedule(options: SchedulerOptions?, _ action: @escaping () -> Void) {
+			guard Thread.isMainThread else {
+				return DispatchQueue.main.schedule(options: options, action)
+			}
+
+			action()
+		}
+
+		func schedule(after date: SchedulerTimeType, tolerance: SchedulerTimeType.Stride, options: SchedulerOptions?, _ action: @escaping () -> Void) {
+			DispatchQueue.main.schedule(after: date, tolerance: tolerance, options: options, action)
+		}
+
+		func schedule(after date: SchedulerTimeType, interval: SchedulerTimeType.Stride, tolerance: SchedulerTimeType.Stride, options: SchedulerOptions?, _ action: @escaping () -> Void) -> Cancellable {
+			DispatchQueue.main.schedule(after: date, interval: interval, tolerance: tolerance, options: options, action)
+		}
+	}
 }
 
 typealias AnyDispatchQueueScheduler = AnyScheduler<DispatchQueue.SchedulerTimeType, DispatchQueue.SchedulerOptions>
@@ -176,6 +209,10 @@ extension AnyDispatchQueueScheduler {
 	static var immediateOnMainQueue: Self {
 		DispatchQueue.immediateWhenOnMainQueueScheduler.eraseToAnyScheduler()
 	}
+
+	static var immediateOnMainThread: Self {
+		DispatchQueue.immediateWhenOnMainThreadScheduler.eraseToAnyScheduler()
+	}
 }
 
 extension Scheduler {

EssentialApp/EssentialApp/LoadResourcePresentationAdapter.swift

@@ -24,7 +24,7 @@ final class LoadResourcePresentationAdapter<Resource, View: ResourceView> {
 		isLoading = true
 
 		cancellable = loader()
-			.dispatchOnMainQueue()
+			.dispatchOnMainThread()
 			.handleEvents(receiveCancel: { [weak self] in
 				self?.isLoading = false
 			})

EssentialApp/EssentialAppTests/FeedAcceptanceTests.swift

@@ -86,7 +86,7 @@ class FeedAcceptanceTests: XCTestCase {
 		httpClient: HTTPClientStub = .offline,
 		store: InMemoryFeedStore = .empty
 	) -> ListViewController {
-		let sut = SceneDelegate(httpClient: httpClient, store: store, scheduler: .immediateOnMainQueue)
+		let sut = SceneDelegate(httpClient: httpClient, store: store, scheduler: .immediateOnMainThread)
 		sut.window = UIWindow(frame: CGRect(x: 0, y: 0, width: 390, height: 1))
 		sut.configureWindow()
 
@@ -97,7 +97,7 @@ class FeedAcceptanceTests: XCTestCase {
 	}
 
 	private func enterBackground(with store: InMemoryFeedStore) {
-		let sut = SceneDelegate(httpClient: HTTPClientStub.offline, store: store, scheduler: .immediateOnMainQueue)
+		let sut = SceneDelegate(httpClient: HTTPClientStub.offline, store: store, scheduler: .immediateOnMainThread)
 		sut.sceneWillResignActive(UIApplication.shared.connectedScenes.first!)
 	}
 

EssentialApp/EssentialAppTests/FeedUIIntegrationTests.swift

@@ -180,7 +180,7 @@ class FeedUIIntegrationTests: XCTestCase {
 	func test_loadMoreActions_requestMoreFromLoader() {
 		let (sut, loader) = makeSUT()
 		sut.simulateAppearance()
-		loader.completeFeedLoading()
+		loader.completeFeedLoading(with: [makeImage()])
 
 		XCTAssertEqual(loader.loadMoreCallCount, 0, "Expected no requests before until load more action")
 
@@ -209,13 +209,13 @@ class FeedUIIntegrationTests: XCTestCase {
 		sut.simulateAppearance()
 		XCTAssertFalse(sut.isShowingLoadMoreFeedIndicator, "Expected no loading indicator once view appears")
 
-		loader.completeFeedLoading(at: 0)
+		loader.completeFeedLoading(with: [makeImage()], at: 0)
 		XCTAssertFalse(sut.isShowingLoadMoreFeedIndicator, "Expected no loading indicator once loading completes successfully")
 
 		sut.simulateLoadMoreFeedAction()
 		XCTAssertTrue(sut.isShowingLoadMoreFeedIndicator, "Expected loading indicator on load more action")
 
-		loader.completeLoadMore(at: 0)
+		loader.completeLoadMore(with: [makeImage()], at: 0)
 		XCTAssertFalse(sut.isShowingLoadMoreFeedIndicator, "Expected no loading indicator once user initiated loading completes successfully")
 
 		sut.simulateLoadMoreFeedAction()
@@ -278,10 +278,9 @@ class FeedUIIntegrationTests: XCTestCase {
 		let (sut, loader) = makeSUT()
 
 		sut.simulateAppearance()
-		loader.completeFeedLoading(with: [image0, image1])
-
 		XCTAssertEqual(loader.loadedImageURLs, [], "Expected no image URL requests until views become visible")
-
+
+		loader.completeFeedLoading(with: [image0, image1])
 		sut.simulateFeedImageViewVisible(at: 0)
 		XCTAssertEqual(loader.loadedImageURLs, [image0.url], "Expected first image URL request once first view becomes visible")
 
@@ -436,9 +435,9 @@ class FeedUIIntegrationTests: XCTestCase {
 		let (sut, loader) = makeSUT()
 
 		sut.simulateAppearance()
-		loader.completeFeedLoading(with: [image0, image1])
 		XCTAssertEqual(loader.loadedImageURLs, [], "Expected no image URL requests until image is near visible")
-
+
+		loader.completeFeedLoading(with: [image0, image1])
 		sut.simulateFeedImageViewNearVisible(at: 0)
 		XCTAssertEqual(loader.loadedImageURLs, [image0.url], "Expected first image URL request once first image is near visible")
 

EssentialFeed/EssentialFeediOS/Shared UI/Controllers/ListViewController.swift

@@ -73,7 +73,7 @@ public final class ListViewController: UITableViewController, UITableViewDataSou
 			snapshot.appendItems(cellControllers, toSection: section)
 		}
 
-		dataSource.applySnapshotUsingReloadData(snapshot)
+		dataSource.apply(snapshot)
 	}
 
 	public func display(_ viewModel: ResourceLoadingViewModel) {