Switched to using a DispatchQueue in the MetaStore

BackgroundTransferRevised-Example.xcodeproj/project.pbxproj

@@ -14,8 +14,7 @@
 		437CD5112D94C62800A909A6 /* AppDelegate.swift in Sources */ = {isa = PBXBuildFile; fileRef = 437CD50E2D94C62800A909A6 /* AppDelegate.swift */; };
 		4395E65E2DB00E4100637803 /* BackgroundDownloadMetaStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4395E65C2DB00E4100637803 /* BackgroundDownloadMetaStore.swift */; };
 		4395E65F2DB00E4100637803 /* BackgroundDownloadService.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4395E65B2DB00E4100637803 /* BackgroundDownloadService.swift */; };
-		4395E6632DB00E8300637803 /* BackgroundDownloadTaskStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4395E6622DB00E8300637803 /* BackgroundDownloadTaskStore.swift */; };
-		4395E6652DB00ECD00637803 /* BackgroundDownloadDelegator.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4395E6642DB00ECD00637803 /* BackgroundDownloadDelegator.swift */; };
+		4395E6652DB00ECD00637803 /* BackgroundDownloadDelegate.swift in Sources */ = {isa = PBXBuildFile; fileRef = 4395E6642DB00ECD00637803 /* BackgroundDownloadDelegate.swift */; };
 		43D1CF5A2D80860E00AC1ED9 /* Preview Assets.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = 43D1CF592D80860E00AC1ED9 /* Preview Assets.xcassets */; };
 		43D1CF642D80860E00AC1ED9 /* BackgroundTransferRevised_ExampleTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 43D1CF632D80860E00AC1ED9 /* BackgroundTransferRevised_ExampleTests.swift */; };
 		43D1CF832D808A5000AC1ED9 /* Assets.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = 43D1CF7E2D808A5000AC1ED9 /* Assets.xcassets */; };
@@ -43,8 +42,7 @@
 		437CD5132D94C66C00A909A6 /* Info.plist */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		4395E65B2DB00E4100637803 /* BackgroundDownloadService.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadService.swift; sourceTree = "<group>"; };
 		4395E65C2DB00E4100637803 /* BackgroundDownloadMetaStore.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadMetaStore.swift; sourceTree = "<group>"; };
-		4395E6622DB00E8300637803 /* BackgroundDownloadTaskStore.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadTaskStore.swift; sourceTree = "<group>"; };
-		4395E6642DB00ECD00637803 /* BackgroundDownloadDelegator.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadDelegator.swift; sourceTree = "<group>"; };
+		4395E6642DB00ECD00637803 /* BackgroundDownloadDelegate.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadDelegate.swift; sourceTree = "<group>"; };
 		43D1CF4F2D80860C00AC1ED9 /* BackgroundTransferRevised-Example.app */ = {isa = PBXFileReference; explicitFileType = wrapper.application; includeInIndex = 0; path = "BackgroundTransferRevised-Example.app"; sourceTree = BUILT_PRODUCTS_DIR; };
 		43D1CF592D80860E00AC1ED9 /* Preview Assets.xcassets */ = {isa = PBXFileReference; lastKnownFileType = folder.assetcatalog; path = "Preview Assets.xcassets"; sourceTree = "<group>"; };
 		43D1CF5F2D80860E00AC1ED9 /* BackgroundTransferRevised-ExampleTests.xctest */ = {isa = PBXFileReference; explicitFileType = wrapper.cfbundle; includeInIndex = 0; path = "BackgroundTransferRevised-ExampleTests.xctest"; sourceTree = BUILT_PRODUCTS_DIR; };
@@ -110,10 +108,9 @@
 		4395E65D2DB00E4100637803 /* BackgroundDownload */ = {
 			isa = PBXGroup;
 			children = (
-				4395E6622DB00E8300637803 /* BackgroundDownloadTaskStore.swift */,
 				4395E65B2DB00E4100637803 /* BackgroundDownloadService.swift */,
 				4395E65C2DB00E4100637803 /* BackgroundDownloadMetaStore.swift */,
-				4395E6642DB00ECD00637803 /* BackgroundDownloadDelegator.swift */,
+				4395E6642DB00ECD00637803 /* BackgroundDownloadDelegate.swift */,
 			);
 			path = BackgroundDownload;
 			sourceTree = "<group>";
@@ -300,8 +297,7 @@
 				437CD5102D94C62800A909A6 /* BackgroundTransferRevised_ExampleApp.swift in Sources */,
 				4395E65E2DB00E4100637803 /* BackgroundDownloadMetaStore.swift in Sources */,
 				4395E65F2DB00E4100637803 /* BackgroundDownloadService.swift in Sources */,
-				4395E6632DB00E8300637803 /* BackgroundDownloadTaskStore.swift in Sources */,
-				4395E6652DB00ECD00637803 /* BackgroundDownloadDelegator.swift in Sources */,
+				4395E6652DB00ECD00637803 /* BackgroundDownloadDelegate.swift in Sources */,
 				43D1CF842D808A5000AC1ED9 /* ImageLoader.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;

BackgroundTransferRevised-Example/Application/AppDelegate.swift

@@ -9,24 +9,13 @@ import UIKit
 import OSLog
 
 class AppDelegate: NSObject, UIApplicationDelegate {
-    static var shared: AppDelegate?
-
-    private var backgroundCompletionHandler: (() -> Void)?
-    private let logger = Logger(subsystem: "com.williamboles",
-                                category: "appDelegate")
-
-    // MARK: - Background
+    // MARK: - UIApplicationDelegate
 
     func application(_ application: UIApplication,
                      handleEventsForBackgroundURLSession identifier: String,
                      completionHandler: @escaping () -> Void) {
-        self.backgroundCompletionHandler = completionHandler
-    }
-
-    func backgroundDownloadsComplete() {
-        logger.info("Triggering background session completion handler")
-
-        backgroundCompletionHandler?()
-        backgroundCompletionHandler = nil
+        Task {
+            await BackgroundDownloadService.shared.saveBackgroundCompletionHandler(completionHandler)
+        }
     }
 }

BackgroundTransferRevised-Example/Application/BackgroundTransferRevised_ExampleApp.swift

@@ -16,13 +16,6 @@ struct BackgroundTransferRevised_ExampleApp: App {
     private let logger = Logger(subsystem: "com.williamboles",
                                 category: "app")
 
-
-    // MARK: - Init
-
-    init() {
-        AppDelegate.shared = appDelegate
-    }
-
     // MARK: - Scene
 
     var body: some Scene {
@@ -37,12 +30,12 @@ struct BackgroundTransferRevised_ExampleApp: App {
 
             logger.info("Files will be downloaded to: \(FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0].absoluteString)")
 
-//            //Exit app to test restoring app from a terminated state. Comment out to test restoring app from a suspended state.
-//            Task {
-//                logger.info("Simulating app termination by exit(0)")
-//
-//                exit(0)
-//            }
+            //Exit app to test restoring app from a terminated state.
+            Task {
+                logger.info("Simulating app termination by exit(0)")
+
+                exit(0)
+            }
         }
     }
 }

BackgroundTransferRevised-Example/Network/BackgroundDownload/BackgroundDownloadDelegate.swift

@@ -0,0 +1,117 @@
+//
+//  BackgroundDownloadDelegator.swift
+//  BackgroundTransferRevised-Example
+//
+//  Created by William Boles on 16/04/2025.
+//
+
+import Foundation
+import OSLog
+
+enum BackgroundDownloadError: Error {
+    case fileSystemError(_ underlyingError: Error)
+    case clientError(_ underlyingError: Error)
+    case serverError(_ underlyingResponse: URLResponse?)
+}
+
+final class BackgroundDownloadDelegate: NSObject, URLSessionDownloadDelegate {
+    private let metaStore: BackgroundDownloadMetaStore
+    private let logger: Logger
+    private let processingGroup: DispatchGroup
+    private let urlSessionDidFinishEventsCompletionHandler: (@Sendable () -> Void)
+
+    // MARK: - Init
+
+    init(metaStore: BackgroundDownloadMetaStore,
+         urlSessionDidFinishEventsCompletionHandler: @escaping (@Sendable () -> Void)) {
+        self.metaStore = metaStore
+        self.logger = Logger(subsystem: "com.williamboles",
+                             category: "background.download.delegate")
+        self.processingGroup = DispatchGroup()
+        self.urlSessionDidFinishEventsCompletionHandler = urlSessionDidFinishEventsCompletionHandler
+    }
+
+    // MARK: - URLSessionDownloadDelegate
+
+    func urlSession(_ session: URLSession,
+                    downloadTask: URLSessionDownloadTask,
+                    didFinishDownloadingTo location: URL) {
+        guard let fromURL = downloadTask.originalRequest?.url else {
+            logger.error("Unexpected nil URL for download task.")
+            return
+        }
+
+        logger.info("Download request completed for: \(fromURL.absoluteString)")
+
+        let tempLocation = FileManager.default.temporaryDirectory.appendingPathComponent(location.lastPathComponent)
+        try? FileManager.default.moveItem(at: location,
+                                          to: tempLocation)
+
+        processingGroup.enter()
+        metaStore.retrieveMetadata(key: fromURL.absoluteString) { [weak self] metadata in
+            defer {
+                self?.metaStore.removeMetadata(key: fromURL.absoluteString)
+                self?.processingGroup.leave()
+            }
+
+            guard let metadata else {
+                self?.logger.error("Unable to find existing download item for: \(fromURL.absoluteString)")
+                return
+            }
+
+            guard let response = downloadTask.response as? HTTPURLResponse,
+                  response.statusCode == 200 else {
+                self?.logger.error("Unexpected response for: \(fromURL.absoluteString)")
+                metadata.continuation?.resume(throwing: BackgroundDownloadError.serverError(downloadTask.response))
+                return
+            }
+
+            self?.logger.info("Download successful for: \(fromURL.absoluteString)")
+
+            do {
+                try FileManager.default.moveItem(at: tempLocation,
+                                                 to: metadata.toURL)
+                metadata.continuation?.resume(returning: metadata.toURL)
+            } catch {
+                self?.logger.error("File system error while moving file: \(error.localizedDescription)")
+                metadata.continuation?.resume(throwing: BackgroundDownloadError.fileSystemError(error))
+            }
+        }
+    }
+
+    func urlSession(_ session: URLSession,
+                    task: URLSessionTask,
+                    didCompleteWithError error: Error?) {
+        guard let error = error else {
+            return
+        }
+
+        guard let fromURL = task.originalRequest?.url else {
+            logger.error("Unexpected nil URL for task.")
+            return
+        }
+
+        logger.info("Download failed for: \(fromURL.absoluteString), error: \(error.localizedDescription)")
+
+        processingGroup.enter()
+        metaStore.retrieveMetadata(key: fromURL.absoluteString) { [weak self] metadata in
+            defer {
+                self?.metaStore.removeMetadata(key: fromURL.absoluteString)
+                self?.processingGroup.leave()
+            }
+
+            metadata?.continuation?.resume(throwing: BackgroundDownloadError.clientError(error))
+        }
+    }
+
+    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
+        logger.info("Did finish events for background session")
+
+        processingGroup.notify(queue: .global()) { [weak self] in
+            self?.logger.info("Processing group has finished")
+
+            self?.urlSessionDidFinishEventsCompletionHandler()
+        }
+    }
+}
+