Add async composition to load more feed pages

Author: caiozullo

EssentialApp/EssentialApp/SceneDelegate.swift

@@ -143,18 +143,35 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate {
 		}
 		.eraseToAnyPublisher()
 	}
+
+	private func loadMoreRemoteFeed(last: FeedImage?) async throws -> Paginated<FeedImage> {
+		async let cachedItems = try await loadLocalFeed()
+		async let newItems = try await loadRemoteFeed(after: last)
+		
+		let items = try await cachedItems + newItems
+		
+		await store.schedule { [store] in
+			let localFeedLoader = LocalFeedLoader(store: store, currentDate: Date.init)
+			try? localFeedLoader.save(items)
+		}
+		
+		return try await makePage(items: items, last: newItems.last)
+	}
 
 	private func makeRemoteLoadMoreLoader(last: FeedImage?) -> AnyPublisher<Paginated<FeedImage>, Error> {
-		localFeedLoader.loadPublisher()
-			.zip(makeRemoteFeedLoader(after: last))
-			.map { (cachedItems, newItems) in
-				(cachedItems + newItems, newItems.last)
+		Deferred {
+			Future { completion in
+				Task.immediate {
+					do {
+						let feed = try await self.loadMoreRemoteFeed(last: last)
+						completion(.success(feed))
+					} catch {
+						completion(.failure(error))
+					}
+				}
 			}
-			.map(makePage)
-			.receive(on: scheduler)
-			.caching(to: localFeedLoader)
-			.subscribe(on: scheduler)
-			.eraseToAnyPublisher()
+		}
+		.eraseToAnyPublisher()
 	}
 
 	private func makeRemoteFeedLoader(after: FeedImage? = nil) -> AnyPublisher<[FeedImage], Error> {

EssentialApp/EssentialAppTests/FeedAcceptanceTests.swift

@@ -11,22 +11,27 @@ import EssentialFeediOS
 class FeedAcceptanceTests: XCTestCase {
 
 	func test_onLaunch_displaysRemoteFeedWhenCustomerHasConnectivity() throws {
-		let feed = try launch(httpClient: .online(response), store: .empty)
+		let store = try CoreDataFeedStore.empty
+		let feed = try launch(httpClient: .online(response), store: store)
 
 		XCTAssertEqual(feed.numberOfRenderedFeedImageViews(), 2)
 		XCTAssertEqual(feed.renderedFeedImageData(at: 0), makeImageData0())
 		XCTAssertEqual(feed.renderedFeedImageData(at: 1), makeImageData1())
 		XCTAssertTrue(feed.canLoadMoreFeed)
 
-		feed.simulateLoadMoreFeedAction()
+		try store.withWaitingChanges {
+			feed.simulateLoadMoreFeedAction()
+		}
 
 		XCTAssertEqual(feed.numberOfRenderedFeedImageViews(), 3)
 		XCTAssertEqual(feed.renderedFeedImageData(at: 0), makeImageData0())
 		XCTAssertEqual(feed.renderedFeedImageData(at: 1), makeImageData1())
 		XCTAssertEqual(feed.renderedFeedImageData(at: 2), makeImageData2())
 		XCTAssertTrue(feed.canLoadMoreFeed)
 
-		feed.simulateLoadMoreFeedAction()
+		try store.withWaitingChanges {
+			feed.simulateLoadMoreFeedAction()
+		}
 
 		XCTAssertEqual(feed.numberOfRenderedFeedImageViews(), 3)
 		XCTAssertEqual(feed.renderedFeedImageData(at: 0), makeImageData0())
@@ -41,7 +46,9 @@ class FeedAcceptanceTests: XCTestCase {
 		let onlineFeed = try launch(httpClient: .online(response), store: sharedStore)
 		onlineFeed.simulateFeedImageViewVisible(at: 0)
 		onlineFeed.simulateFeedImageViewVisible(at: 1)
-		onlineFeed.simulateLoadMoreFeedAction()
+		try sharedStore.withWaitingChanges {
+			onlineFeed.simulateLoadMoreFeedAction()
+		}
 		onlineFeed.simulateFeedImageViewVisible(at: 2)
 
 		let offlineFeed = try launch(httpClient: .offline, store: sharedStore)
@@ -186,6 +193,25 @@ class FeedAcceptanceTests: XCTestCase {
 
 @MainActor
 extension CoreDataFeedStore {
+	private struct Timeout: Error {}
+	
+	func withWaitingChanges(_ action: () -> Void, timeout: TimeInterval = 1) throws {
+		let state = try retrieve()?.timestamp
+		action()
+		
+		let maxDate = Date() + timeout
+			
+		while Date() <= maxDate {
+			if try retrieve()?.timestamp != state {
+				return
+			}
+			
+			RunLoop.current.run(until: Date())
+		}
+
+		throw Timeout()
+	}
+	
 	static var empty: CoreDataFeedStore {
 		get throws {
 			try CoreDataFeedStore(storeURL: URL(fileURLWithPath: "/dev/null"), contextQueue: .main)