Merge pull request #93 from essentialdevelopercom/swift-concurrency

Migrate Pagination composition to async/await

Author: mapostolakis

EssentialApp/EssentialApp/CombineHelpers.swift

@@ -6,32 +6,6 @@ import Foundation
 import Combine
 import EssentialFeed
 
-public extension Paginated {
-	init(items: [Item], loadMorePublisher: (() -> AnyPublisher<Self, Error>)?) {
-		self.init(items: items, loadMore: loadMorePublisher.map { publisher in
-			return { completion in
-				publisher().subscribe(Subscribers.Sink(receiveCompletion: { result in
-					if case let .failure(error) = result {
-						completion(.failure(error))
-					}
-				}, receiveValue: { result in
-					completion(.success(result))
-				}))
-			}
-		})
-	}
-	
-	var loadMorePublisher: (() -> AnyPublisher<Self, Error>)? {
-		guard let loadMore = loadMore else { return nil }
-		
-		return {
-			Deferred {
-				Future(loadMore)
-			}.eraseToAnyPublisher()
-		}
-	}
-}
-
 @MainActor
 public extension HTTPClient {
 	typealias Publisher = AnyPublisher<(Data, HTTPURLResponse), Error>

EssentialApp/EssentialApp/FeedUIComposer.swift

@@ -3,18 +3,17 @@
 //
 
 import UIKit
-import Combine
 import EssentialFeed
 import EssentialFeediOS
 
 @MainActor
 public final class FeedUIComposer {
 	private init() {}
 
-	private typealias FeedPresentationAdapter = LoadResourcePresentationAdapter<Paginated<FeedImage>, FeedViewAdapter>
+	private typealias FeedPresentationAdapter = AsyncLoadResourcePresentationAdapter<Paginated<FeedImage>, FeedViewAdapter>
 
 	public static func feedComposedWith(
-		feedLoader: @MainActor @escaping () -> AnyPublisher<Paginated<FeedImage>, Error>,
+		feedLoader: @MainActor @escaping () async throws -> Paginated<FeedImage>,
 		imageLoader: @MainActor @escaping (URL) async throws -> Data,
 		selection: @MainActor @escaping (FeedImage) -> Void = { _ in }
 	) -> ListViewController {

EssentialApp/EssentialApp/FeedViewAdapter.swift

@@ -14,7 +14,7 @@ final class FeedViewAdapter: ResourceView {
 	private let currentFeed: [FeedImage: CellController]
 
 	private typealias ImageDataPresentationAdapter = AsyncLoadResourcePresentationAdapter<Data, WeakRefVirtualProxy<FeedImageCellController>>
-	private typealias LoadMorePresentationAdapter = LoadResourcePresentationAdapter<Paginated<FeedImage>, FeedViewAdapter>
+	private typealias LoadMorePresentationAdapter = AsyncLoadResourcePresentationAdapter<Paginated<FeedImage>, FeedViewAdapter>
 
 	init(currentFeed: [FeedImage: CellController] = [:], controller: ListViewController, imageLoader: @escaping (URL) async throws -> Data, selection: @escaping (FeedImage) -> Void) {
 		self.currentFeed = currentFeed
@@ -54,12 +54,12 @@ final class FeedViewAdapter: ResourceView {
 			return controller
 		}
 
-		guard let loadMorePublisher = viewModel.loadMorePublisher else {
+		guard let loadMoreAsync = viewModel.loadMore else {
 			controller.display(feed)
 			return
 		}
 
-		let loadMoreAdapter = LoadMorePresentationAdapter(loader: loadMorePublisher)
+		let loadMoreAdapter = LoadMorePresentationAdapter(loader: loadMoreAsync)
 		let loadMore = LoadMoreCellController(callback: loadMoreAdapter.loadResource)
 
 		loadMoreAdapter.presenter = LoadResourcePresenter(

EssentialApp/EssentialApp/SceneDelegate.swift

@@ -49,7 +49,7 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate {
 
 	private lazy var navigationController = UINavigationController(
 		rootViewController: FeedUIComposer.feedComposedWith(
-			feedLoader: makeRemoteFeedLoaderWithLocalFallback,
+			feedLoader: loadRemoteFeedWithLocalFallback,
 			imageLoader: loadLocalImageWithRemoteFallback,
 			selection: showComments))
 
@@ -96,44 +96,59 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate {
 		}
 	}
 
-	private func makeRemoteFeedLoaderWithLocalFallback() -> AnyPublisher<Paginated<FeedImage>, Error> {
-		makeRemoteFeedLoader()
-			.receive(on: scheduler)
-			.caching(to: localFeedLoader)
-			.fallback(to: localFeedLoader.loadPublisher)
-			.map(makeFirstPage)
-			.eraseToAnyPublisher()
+	private func loadRemoteFeedWithLocalFallback() async throws -> Paginated<FeedImage> {
+		do {
+			let feed = try await loadAndCacheRemoteFeed()
+			return makeFirstPage(items: feed)
+		} catch {
+			let feed = try await loadLocalFeed()
+			return makeFirstPage(items: feed)
+		}
 	}
 
-	private func makeRemoteLoadMoreLoader(last: FeedImage?) -> AnyPublisher<Paginated<FeedImage>, Error> {
-		localFeedLoader.loadPublisher()
-			.zip(makeRemoteFeedLoader(after: last))
-			.map { (cachedItems, newItems) in
-				(cachedItems + newItems, newItems.last)
-			}
-			.map(makePage)
-			.receive(on: scheduler)
-			.caching(to: localFeedLoader)
-			.subscribe(on: scheduler)
-			.eraseToAnyPublisher()
+	private func loadAndCacheRemoteFeed() async throws -> [FeedImage] {
+		let feed = try await loadRemoteFeed()
+		await store.schedule { [store] in
+			let localFeedLoader = LocalFeedLoader(store: store, currentDate: Date.init)
+			try? localFeedLoader.save(feed)
+		}
+		return feed
+	}
+
+	private func loadLocalFeed() async throws -> [FeedImage] {
+		try await store.schedule { [store] in
+			let localFeedLoader = LocalFeedLoader(store: store, currentDate: Date.init)
+			return try localFeedLoader.load()
+		}
 	}
 
-	private func makeRemoteFeedLoader(after: FeedImage? = nil) -> AnyPublisher<[FeedImage], Error> {
+	private func loadRemoteFeed(after: FeedImage? = nil) async throws -> [FeedImage] {
 		let url = FeedEndpoint.get(after: after).url(baseURL: baseURL)
+		let (data, response) = try await httpClient.get(from: url)
+		return try FeedItemsMapper.map(data, from: response)
+	}
+
+	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 httpClient
-			.getPublisher(url: url)
-			.tryMap(FeedItemsMapper.map)
-			.eraseToAnyPublisher()
+		return try await makePage(items: items, last: newItems.last)
 	}
 
 	private func makeFirstPage(items: [FeedImage]) -> Paginated<FeedImage> {
 		makePage(items: items, last: items.last)
 	}
 
 	private func makePage(items: [FeedImage], last: FeedImage?) -> Paginated<FeedImage> {
-		Paginated(items: items, loadMorePublisher: last.map { last in
-			{ self.makeRemoteLoadMoreLoader(last: last) }
+		Paginated(items: items, loadMore: last.map { last in
+			{ @MainActor @Sendable in try await self.loadMoreRemoteFeed(last: last) }
 		})
 	}
 

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)