Fix in flight retention

EssentialFeed/EssentialFeed/Feed Cache/LocalFeedLoader.swift

@@ -19,7 +19,7 @@ public final class LocalFeedLoader: FeedLoader {
 
 extension LocalFeedLoader {
     public typealias LoadResult = FeedLoader.Result
-    public func load(completion: @escaping (LoadResult) -> Void) {
+    public func load(completion: @escaping (LoadResult) -> Void) -> FeedLoaderTask? {
         store.retrieve { [weak self] result in
             guard let self else { return }
             switch result {
@@ -32,6 +32,7 @@ extension LocalFeedLoader {
                 completion(.success([]))
             }
         }
+        return nil
     }
 }
 

EssentialFeed/EssentialFeed/Feed Feature/FeedLoader.swift

@@ -4,8 +4,12 @@
 
 import Foundation
 
+public protocol FeedLoaderTask {
+    func cancel()
+}
 
 public protocol FeedLoader {
     typealias Result = Swift.Result<[FeedImage], Error>
-	func load(completion: @escaping (Result) -> Void)
+    @discardableResult
+	func load(completion: @escaping (Result) -> Void) -> FeedLoaderTask?
 }

EssentialFeed/EssentialFeed/FeedApi/RemoteFeedLoader.swift

@@ -23,7 +23,7 @@ public final class RemoteFeedLoader: FeedLoader {
         self.client = client
     }
 
-    public func load(completion:  @escaping (Result) -> Void) {
+    public func load(completion:  @escaping (Result) -> Void) -> FeedLoaderTask? {
         client.get(from: url) { [weak self] result in
             guard self != nil else { return }
             switch result {
@@ -33,6 +33,7 @@ public final class RemoteFeedLoader: FeedLoader {
                 completion(.failure(Error.connectivity))
             }
         }
+        return nil
     }
 
     private static func map(_ data: Data, from response: HTTPURLResponse) -> Result {

EssentialFeed/EssentialFeediOS/Composers/FeedLoaderPresentationAdapter.swift

@@ -13,13 +13,14 @@ final class FeedLoaderPresentationAdapter: FeedViewControllerDelegate {
     private let feedLoader: FeedLoader
     var presenter: FeedPresenter?
 
+    private var currentTask: FeedLoaderTask?
     init(feedLoader: FeedLoader) {
         self.feedLoader = feedLoader
     }
 
     func loadFeed() {
         presenter?.didStartLoadingFeed()
-        feedLoader.load { [weak self] result in
+        currentTask = feedLoader.load { [weak self] result in
             switch result {
             case let .success(feed):
                 self?.presenter?.didFinishLoadingFeed(with: feed)
@@ -32,4 +33,9 @@ final class FeedLoaderPresentationAdapter: FeedViewControllerDelegate {
     func didRequestFeedRefresh() {
         loadFeed()
     }
-}
+
+    func didRequestFeedLoadCancel() {
+        currentTask?.cancel()
+        currentTask = nil
+    }
+}

EssentialFeed/EssentialFeediOS/Composers/MainQueueDispatchDecorator.swift

@@ -21,12 +21,12 @@ final class MainQueueDispatchDecorator<T> {
 }
 
 extension MainQueueDispatchDecorator: FeedLoader where T == FeedLoader {
-    func load(completion: @escaping (FeedLoader.Result) -> Void) {
+    func load(completion: @escaping (FeedLoader.Result) -> Void) -> FeedLoaderTask? {
         decoratee.load { result in
             Self.dispatch {
                 completion(result)
             }
-        }
+        } 
     }
 }
 

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

@@ -9,6 +9,7 @@ import UIKit
 
 protocol FeedViewControllerDelegate {
     func didRequestFeedRefresh()
+    func didRequestFeedLoadCancel()
 }
 
 public final class FeedViewController: UITableViewControllerExtendedLifecycle, UITableViewDataSourcePrefetching {
@@ -28,6 +29,11 @@ public final class FeedViewController: UITableViewControllerExtendedLifecycle, U
         super.viewDidLoad()
     }
 
+    public override func viewWillDisappear(_ animated: Bool) {
+        super.viewWillDisappear(animated)
+        delegate?.didRequestFeedLoadCancel()
+    }
+
     public override func viewFirstAppearance() {
         super.viewFirstAppearance()
         refresh()

EssentialFeed/EssentialFeediOSTests/FeedUI/FeedUIIntegrationTests.swift

@@ -12,6 +12,13 @@ import EssentialFeediOS
 
 final class FeedUIIntegrationTests: XCTestCase {
 
+    func test_viewWillDisappear_requestsFeedLoadCancellation() {
+        let (sut, loader) = makeSUT()
+        sut.simulateAppearance()
+        sut.simulateDisappearance()
+        XCTAssertEqual(loader.feedCancelRequestsCount, 1)
+    }
+
     func test_feedView_hasTitle() {
         let (sut, _) = makeSUT()
 
@@ -397,10 +404,15 @@ final class FeedUIIntegrationTests: XCTestCase {
 
         private(set) var feedRequests = [(FeedLoader.Result) -> Void]()
         var loadFeedCallCount: Int { feedRequests.count }
-
+
+        var feedCancelRequestsCount = 0
+
 
-        func load(completion: @escaping (FeedLoader.Result) -> Void) {
+        func load(completion: @escaping (FeedLoader.Result) -> Void) -> FeedLoaderTask? {
             feedRequests.append(completion)
+            return TaskSpy { [weak self] in
+                self?.feedCancelRequestsCount += 1
+            }
         }
 
 
@@ -414,7 +426,7 @@ final class FeedUIIntegrationTests: XCTestCase {
 
         // MARK: - FeedImageDataLoader
 
-       private struct TaskSpy: FeedImageDataLoaderTask {
+       private struct TaskSpy: FeedImageDataLoaderTask, FeedLoaderTask {
             let cancelCallback: () -> Void
             func cancel() {
                cancelCallback()
@@ -517,6 +529,11 @@ private extension FeedViewController {
         endAppearanceTransition()
     }
 
+    func simulateDisappearance() {
+        beginAppearanceTransition(false, animated: false)
+        endAppearanceTransition()
+    }
+
     func replaceRefreshControlWithFake() {
         let fake = FakeRefreshControl()
         refreshControl?.allTargets.forEach{ target in