Infrastructure optimizations

EssentialApp/EssentialApp/FeedViewAdapter.swift

@@ -11,18 +11,27 @@ final class FeedViewAdapter: ResourceView {
     private weak var controller: ListViewController?
     private let imageLoader: (URL) -> FeedImageDataLoader.Publisher
     private let selection: (FeedImage) -> Void
+    private let currentFeed: [FeedImage: CellController]
 
     private typealias ImageDataPresentationAdapter = LoadResourcePresentationAdapter<Data, WeakRefVirtualProxy<FeedImageCellController>>
     private typealias LoadMorePresentationAdapter = LoadResourcePresentationAdapter<Paginated<FeedImage>, FeedViewAdapter>
 
-    init(controller: ListViewController? = nil, imageLoader: @escaping (URL) -> FeedImageDataLoader.Publisher, selection: @escaping (FeedImage) -> Void) {
+    init(currentFeed: [FeedImage: CellController] = [:], controller: ListViewController? = nil, imageLoader: @escaping (URL) -> FeedImageDataLoader.Publisher, selection: @escaping (FeedImage) -> Void) {
+        self.currentFeed = currentFeed
         self.controller = controller
         self.imageLoader = imageLoader
         self.selection = selection
     }
 
     func display(_ viewModel: Paginated<FeedImage>) {
+        guard let controller = controller else { return }
+
+        var currentFeed = self.currentFeed
         let feed: [CellController] = viewModel.items.map { model in
+            if let controller = currentFeed[model] {
+                return controller
+            }
+
             let adapter = ImageDataPresentationAdapter(loader: { [imageLoader] in
                 imageLoader(model.url)
             })
@@ -40,25 +49,32 @@ final class FeedViewAdapter: ResourceView {
                 errorView: WeakRefVirtualProxy(view),
                 mapper: UIImage.tryMake)
 
-            return CellController(id: model, view)
+            let controller = CellController(id: model, view)
+            currentFeed[model] = controller
+            return controller
         }
 
         guard let loadMorePublisher = viewModel.loadMorePublisher else {
-            controller?.display(feed)
+            controller.display(feed)
             return
         }
 
         let loadMoreAdapter = LoadMorePresentationAdapter(loader: loadMorePublisher)
         let loadMore = LoadMoreCellController(callback: loadMoreAdapter.loadResource)
 
         loadMoreAdapter.presenter = LoadResourcePresenter(
-            resourceView: self,
+            resourceView: FeedViewAdapter(
+                currentFeed: currentFeed,
+                controller: controller,
+                imageLoader: imageLoader,
+                selection: selection
+            ),
             loadingView: WeakRefVirtualProxy(loadMore),
             errorView: WeakRefVirtualProxy(loadMore))
 
         let loadMoreSection = [CellController(id: UUID(), loadMore)]
 
-        controller?.display(feed, loadMoreSection)
+        controller.display(feed, loadMoreSection)
     }
 }
 

EssentialApp/EssentialApp/LoadResourcePresentationAdapter.swift

@@ -10,17 +10,24 @@ import EssentialFeediOS
 final class LoadResourcePresentationAdapter<Resource, View: ResourceView> {
     private let loader: () -> AnyPublisher<Resource, Error>
     private var cancellable: Cancellable?
+    private var isLoading = false
     var presenter: LoadResourcePresenter<Resource, View>?
 
     init(loader: @escaping () -> AnyPublisher<Resource, Error>) {
         self.loader = loader
     }
 
     func loadResource() {
+        guard !isLoading else { return }
+
         presenter?.didStartLoading()
+        isLoading = true
 
         cancellable = loader()
             .dispatchOnMainQueue()
+            .handleEvents(receiveCancel: { [weak self] in
+                self?.isLoading = false
+            })
             .sink(
                 receiveCompletion: { [weak self] completion in
                     switch completion {
@@ -29,6 +36,8 @@ final class LoadResourcePresentationAdapter<Resource, View: ResourceView> {
                     case let .failure(error):
                         self?.presenter?.didFinishLoading(with: error)
                     }
+
+                    self?.isLoading = false
                 }, receiveValue: { [weak self] resource in
                     self?.presenter?.didFinishLoading(with: resource)
                 })

EssentialApp/EssentialApp/NullStore.swift

@@ -0,0 +1,29 @@
+//
+// Created by Rodrigo Porto.
+// Copyright © 2025 PortoCode. All Rights Reserved.
+//
+
+import Foundation
+import EssentialFeed
+
+class NullStore: FeedStore & FeedImageDataStore {
+    func deleteCachedFeed(completion: @escaping DeletionCompletion) {
+        completion(.success(()))
+    }
+
+    func insert(_ feed: [LocalFeedImage], timestamp: Date, completion: @escaping InsertionCompletion) {
+        completion(.success(()))
+    }
+
+    func retrieve(completion: @escaping RetrievalCompletion) {
+        completion(.success(.none))
+    }
+
+    func insert(_ data: Data, for url: URL, completion: @escaping (InsertionResult) -> Void) {
+        completion(.success(()))
+    }
+
+    func retrieve(dataForURL url: URL, completion: @escaping (FeedImageDataStore.RetrievalResult) -> Void) {
+        completion(.success(.none))
+    }
+}

EssentialApp/EssentialApp/SceneDelegate.swift

@@ -3,6 +3,7 @@
 // Copyright © 2025 PortoCode. All Rights Reserved.
 //
 
+import os
 import UIKit
 import CoreData
 import Combine
@@ -15,11 +16,19 @@ class SceneDelegate: UIResponder, UIWindowSceneDelegate {
         URLSessionHTTPClient(session: URLSession(configuration: .ephemeral))
     }()
 
+    private lazy var logger = Logger(subsystem: "portocode.EssentialApp", category: "main")
+
     private lazy var store: FeedStore & FeedImageDataStore = {
-        try! CoreDataFeedStore(
-            storeURL: NSPersistentContainer
-                .defaultDirectoryURL()
-                .appendingPathComponent("feed-store.sqlite"))
+        do {
+            return try CoreDataFeedStore(
+                storeURL: NSPersistentContainer
+                    .defaultDirectoryURL()
+                    .appendingPathComponent("feed-store.sqlite"))
+        } catch {
+            assertionFailure("Failed to instantiate CoreData store with error: \(error.localizedDescription)")
+            logger.fault("Failed to instantiate CoreData store with error: \(error.localizedDescription)")
+            return NullStore()
+        }
     }()
 
     private lazy var localFeedLoader: LocalFeedLoader = {

EssentialApp/EssentialAppTests/CommentsUIIntegrationTests.swift

@@ -27,9 +27,14 @@ class CommentsUIIntegrationTests: XCTestCase {
         sut.simulateAppearance()
         XCTAssertEqual(loader.loadCommentsCallCount, 1, "Expected a loading request once view appears")
 
+        sut.simulateUserInitiatedReload()
+        XCTAssertEqual(loader.loadCommentsCallCount, 1, "Expected no request until previous completes")
+
+        loader.completeCommentsLoading(at: 0)
         sut.simulateUserInitiatedReload()
         XCTAssertEqual(loader.loadCommentsCallCount, 2, "Expected another loading request once user initiates a reload")
 
+        loader.completeCommentsLoading(at: 1)
         sut.simulateUserInitiatedReload()
         XCTAssertEqual(loader.loadCommentsCallCount, 3, "Expected yet another loading request once user initiates another reload")
     }
@@ -205,6 +210,7 @@ class CommentsUIIntegrationTests: XCTestCase {
 
         func completeCommentsLoading(with comments: [ImageComment] = [], at index: Int = 0) {
             requests[index].send(comments)
+            requests[index].send(completion: .finished)
         }
 
         func completeCommentsLoadingWithError(at index: Int = 0) {

EssentialApp/EssentialAppTests/FeedUIIntegrationTests.swift

@@ -37,14 +37,19 @@ class FeedUIIntegrationTests: XCTestCase {
 
     func test_loadFeedActions_requestFeedFromLoader() {
         let (sut, loader) = makeSUT()
-        XCTAssertEqual(loader.loadFeedCallCount, 0, "Expected no loading requests before view appears")
+        XCTAssertEqual(loader.loadFeedCallCount, 0, "Expected no loading requests before view is loaded")
 
         sut.simulateAppearance()
-        XCTAssertEqual(loader.loadFeedCallCount, 1, "Expected a loading request once view appears")
+        XCTAssertEqual(loader.loadFeedCallCount, 1, "Expected a loading request once view is loaded")
 
+        sut.simulateUserInitiatedReload()
+        XCTAssertEqual(loader.loadFeedCallCount, 1, "Expected no request until previous completes")
+
+        loader.completeFeedLoading(at: 0)
         sut.simulateUserInitiatedReload()
         XCTAssertEqual(loader.loadFeedCallCount, 2, "Expected another loading request once user initiates a reload")
 
+        loader.completeFeedLoading(at: 1)
         sut.simulateUserInitiatedReload()
         XCTAssertEqual(loader.loadFeedCallCount, 3, "Expected yet another loading request once user initiates another reload")
     }
@@ -504,6 +509,27 @@ class FeedUIIntegrationTests: XCTestCase {
         XCTAssertEqual(view0?.isShowingImageLoadingIndicator, false, "Expected no loading indicator when image loads successfully after view becomes visible again")
     }
 
+    func test_feedImageView_configuresViewCorrectlyWhenTransitioningFromNearVisibleToVisibleWhileStillPreloadingImage() {
+        let (sut, loader) = makeSUT()
+
+        sut.simulateAppearance()
+        loader.completeFeedLoading(with: [makeImage()])
+
+        sut.simulateFeedImageViewNearVisible(at: 0)
+        let view0 = sut.simulateFeedImageViewVisible(at: 0)
+
+        XCTAssertEqual(view0?.renderedImage, nil, "Expected no rendered image when view becomes visible while still preloading image")
+        XCTAssertEqual(view0?.isShowingRetryAction, false, "Expected no retry action when view becomes visible while still preloading image")
+        XCTAssertEqual(view0?.isShowingImageLoadingIndicator, true, "Expected loading indicator when view becomes visible while still preloading image")
+
+        let imageData = UIImage.make(withColor: .red).pngData()!
+        loader.completeImageLoading(with: imageData, at: 0)
+
+        XCTAssertEqual(view0?.renderedImage, imageData, "Expected rendered image after image preloads successfully")
+        XCTAssertEqual(view0?.isShowingRetryAction, false, "Expected no retry action after image preloads successfully")
+        XCTAssertEqual(view0?.isShowingImageLoadingIndicator, false, "Expected no loading indicator after image preloads successfully")
+    }
+
     func test_feedImageView_doesNotRenderLoadedImageWhenNotVisibleAnymore() {
         let (sut, loader) = makeSUT()
 
@@ -531,6 +557,32 @@ class FeedUIIntegrationTests: XCTestCase {
         wait(for: [exp], timeout: 1.0)
     }
 
+    func test_feedImageView_doesNotLoadImageAgainUntilPreviousRequestCompletes() {
+        let image = makeImage(url: URL(string: "http://url-0.com")!)
+        let (sut, loader) = makeSUT()
+        sut.simulateAppearance()
+        loader.completeFeedLoading(with: [image])
+
+        sut.simulateFeedImageViewNearVisible(at: 0)
+        XCTAssertEqual(loader.loadedImageURLs, [image.url], "Expected first request when near visible")
+
+        sut.simulateFeedImageViewVisible(at: 0)
+        XCTAssertEqual(loader.loadedImageURLs, [image.url], "Expected no request until previous completes")
+
+        loader.completeImageLoading(at: 0)
+        sut.simulateFeedImageViewVisible(at: 0)
+        XCTAssertEqual(loader.loadedImageURLs, [image.url, image.url], "Expected second request when visible after previous complete")
+
+        sut.simulateFeedImageViewNotVisible(at: 0)
+        sut.simulateFeedImageViewVisible(at: 0)
+        XCTAssertEqual(loader.loadedImageURLs, [image.url, image.url, image.url], "Expected third request when visible after canceling previous complete")
+
+        sut.simulateLoadMoreFeedAction()
+        loader.completeLoadMore(with: [image, makeImage()])
+        sut.simulateFeedImageViewVisible(at: 0)
+        XCTAssertEqual(loader.loadedImageURLs, [image.url, image.url, image.url], "Expected no request until previous completes")
+    }
+
     // MARK: - Helpers
 
     private func makeSUT(

EssentialApp/EssentialAppTests/Helpers/FeedUIIntegrationTests+LoaderSpy.swift

@@ -30,6 +30,7 @@ extension FeedUIIntegrationTests {
             feedRequests[index].send(Paginated(items: feed, loadMorePublisher: { [weak self] in
                 self?.loadMorePublisher() ?? Empty().eraseToAnyPublisher()
             }))
+            feedRequests[index].send(completion: .finished)
         }
 
         func completeFeedLoadingWithError(at index: Int = 0) {

EssentialFeed/EssentialFeed.xcodeproj/project.pbxproj

@@ -129,7 +129,7 @@
 		5BC4F6CB2CDAF0B20002D4CF /* CoreDataHelpers.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BC4F6CA2CDAF0B20002D4CF /* CoreDataHelpers.swift */; };
 		5BC4F6CD2CDAF1B30002D4CF /* ManagedCache.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BC4F6CC2CDAF1B30002D4CF /* ManagedCache.swift */; };
 		5BC4F6CF2CDAF1C60002D4CF /* ManagedFeedImage.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BC4F6CE2CDAF1C60002D4CF /* ManagedFeedImage.swift */; };
-		5BDE3C652D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataLoader.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BDE3C642D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataLoader.swift */; };
+		5BDE3C652D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BDE3C642D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataStore.swift */; };
 		5BDE3C672D6C225A005D520D /* CoreDataFeedStore+FeedStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BDE3C662D6C225A005D520D /* CoreDataFeedStore+FeedStore.swift */; };
 		5BE36BA62CD5845700ACC57C /* FeedCachePolicy.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BE36BA52CD5845700ACC57C /* FeedCachePolicy.swift */; };
 		5BF9F2F92CD9961400C8DB96 /* FeedStoreSpecs.swift in Sources */ = {isa = PBXBuildFile; fileRef = 5BF9F2F82CD9961400C8DB96 /* FeedStoreSpecs.swift */; };
@@ -313,7 +313,7 @@
 		5BC4F6CC2CDAF1B30002D4CF /* ManagedCache.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ManagedCache.swift; sourceTree = "<group>"; };
 		5BC4F6CE2CDAF1C60002D4CF /* ManagedFeedImage.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ManagedFeedImage.swift; sourceTree = "<group>"; };
 		5BDE3C632D6C1672005D520D /* FeedStore2.xcdatamodel */ = {isa = PBXFileReference; lastKnownFileType = wrapper.xcdatamodel; path = FeedStore2.xcdatamodel; sourceTree = "<group>"; };
-		5BDE3C642D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataLoader.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "CoreDataFeedStore+FeedImageDataLoader.swift"; sourceTree = "<group>"; };
+		5BDE3C642D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataStore.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "CoreDataFeedStore+FeedImageDataStore.swift"; sourceTree = "<group>"; };
 		5BDE3C662D6C225A005D520D /* CoreDataFeedStore+FeedStore.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "CoreDataFeedStore+FeedStore.swift"; sourceTree = "<group>"; };
 		5BE36BA52CD5845700ACC57C /* FeedCachePolicy.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = FeedCachePolicy.swift; sourceTree = "<group>"; };
 		5BF9F2F82CD9961400C8DB96 /* FeedStoreSpecs.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = FeedStoreSpecs.swift; sourceTree = "<group>"; };
@@ -890,7 +890,7 @@
 			children = (
 				5BF9F3092CDAD24D00C8DB96 /* CoreDataFeedStore.swift */,
 				5BDE3C662D6C225A005D520D /* CoreDataFeedStore+FeedStore.swift */,
-				5BDE3C642D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataLoader.swift */,
+				5BDE3C642D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataStore.swift */,
 				5BC4F6CA2CDAF0B20002D4CF /* CoreDataHelpers.swift */,
 				5BC4F6CC2CDAF1B30002D4CF /* ManagedCache.swift */,
 				5BC4F6CE2CDAF1C60002D4CF /* ManagedFeedImage.swift */,
@@ -1213,7 +1213,7 @@
 				5B88290A2D6A7B76006E0BD7 /* ResourceErrorViewModel.swift in Sources */,
 				5B8829142D6BAE59006E0BD7 /* FeedImageDataLoader.swift in Sources */,
 				5B8829082D6A7B12006E0BD7 /* ResourceLoadingViewModel.swift in Sources */,
-				5BDE3C652D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataLoader.swift in Sources */,
+				5BDE3C652D6C19D8005D520D /* CoreDataFeedStore+FeedImageDataStore.swift in Sources */,
 				5B88290F2D6A94C3006E0BD7 /* FeedImagePresenter.swift in Sources */,
 				5BBDA00E2D6FCCF000D68DF0 /* FeedCache.swift in Sources */,
 				5BC4F6CF2CDAF1C60002D4CF /* ManagedFeedImage.swift in Sources */,

EssentialFeed/EssentialFeed/Feed Cache/Infrastructure/CoreData/CoreDataFeedStore+FeedImageDataLoader.swift → EssentialFeed/EssentialFeed/Feed Cache/Infrastructure/CoreData/CoreDataFeedStore+FeedImageDataStore.swift

@@ -20,7 +20,7 @@ extension CoreDataFeedStore: FeedImageDataStore {
     public func retrieve(dataForURL url: URL, completion: @escaping (FeedImageDataStore.RetrievalResult) -> Void) {
         perform { context in
             completion(Result {
-                try ManagedFeedImage.first(with: url, in: context)?.data
+                try ManagedFeedImage.data(with: url, in: context)
             })
         }
     }

EssentialFeed/EssentialFeed/Feed Cache/Infrastructure/CoreData/CoreDataFeedStore+FeedStore.swift

@@ -31,7 +31,7 @@ extension CoreDataFeedStore: FeedStore {
     public func deleteCachedFeed(completion: @escaping DeletionCompletion) {
         perform { context in
             completion(Result {
-                try ManagedCache.find(in: context).map(context.delete).map(context.save)
+                try ManagedCache.deleteCache(in: context)
             })
         }
     }

EssentialFeed/EssentialFeed/Feed Cache/Infrastructure/CoreData/ManagedCache.swift

@@ -18,8 +18,12 @@ extension ManagedCache {
         return try context.fetch(request).first
     }
 
+    static func deleteCache(in context: NSManagedObjectContext) throws {
+        try find(in: context).map(context.delete).map(context.save)
+    }
+
     static func newUniqueInstance(in context: NSManagedObjectContext) throws -> ManagedCache {
-        try find(in: context).map(context.delete)
+        try deleteCache(in: context)
         return ManagedCache(context: context)
     }
 

EssentialFeed/EssentialFeed/Feed Cache/Infrastructure/CoreData/ManagedFeedImage.swift

@@ -16,6 +16,12 @@ class ManagedFeedImage: NSManagedObject {
 }
 
 extension ManagedFeedImage {
+    static func data(with url: URL, in context: NSManagedObjectContext) throws -> Data? {
+        if let data = context.userInfo[url] as? Data { return data }
+
+        return try first(with: url, in: context)?.data
+    }
+
     static func first(with url: URL, in context: NSManagedObjectContext) throws -> ManagedFeedImage? {
         let request = NSFetchRequest<ManagedFeedImage>(entityName: entity().name!)
         request.predicate = NSPredicate(format: "%K = %@", argumentArray: [#keyPath(ManagedFeedImage.url), url])
@@ -25,17 +31,26 @@ extension ManagedFeedImage {
     }
 
     static func images(from localFeed: [LocalFeedImage], in context: NSManagedObjectContext) -> NSOrderedSet {
-        return NSOrderedSet(array: localFeed.map { local in
+        let images = NSOrderedSet(array: localFeed.map { local in
             let managed = ManagedFeedImage(context: context)
             managed.id = local.id
             managed.imageDescription = local.description
             managed.location = local.location
             managed.url = local.url
+            managed.data = context.userInfo[local.url] as? Data
             return managed
         })
+        context.userInfo.removeAllObjects()
+        return images
     }
 
     var local: LocalFeedImage {
         return LocalFeedImage(id: id, description: imageDescription, location: location, url: url)
     }
+
+    override func prepareForDeletion() {
+        super.prepareForDeletion()
+
+        managedObjectContext?.userInfo[url] = data
+    }
 }

EssentialFeed/EssentialFeediOS/Feed UI/Controllers/FeedImageCellController.swift

@@ -35,6 +35,8 @@ extension FeedImageCellController: UITableViewDataSource, UITableViewDelegate, U
         cell?.locationLabel.text = viewModel.location
         cell?.descriptionLabel.text = viewModel.description
         cell?.feedImageView.image = nil
+        cell?.feedImageContainer.isShimmering = true
+        cell?.feedImageRetryButton.isHidden = true
         cell?.onRetry = { [weak self] in
             self?.delegate.didRequestImage()
         }