Implemented store

BackgroundTransfer-Example.xcodeproj/project.pbxproj

@@ -19,6 +19,7 @@
 		438F85472D6E241D00AF956D /* ImageLoader.swift in Sources */ = {isa = PBXBuildFile; fileRef = 438F85442D6E241D00AF956D /* ImageLoader.swift */; };
 		438F85482D6E241D00AF956D /* NetworkService.swift in Sources */ = {isa = PBXBuildFile; fileRef = 438F85452D6E241D00AF956D /* NetworkService.swift */; };
 		438F85492D6E241D00AF956D /* BackgroundDownloadService.swift in Sources */ = {isa = PBXBuildFile; fileRef = 438F85462D6E241D00AF956D /* BackgroundDownloadService.swift */; };
+		43DAF0CE2D8C75D3005900E7 /* BackgroundDownloadStore.swift in Sources */ = {isa = PBXBuildFile; fileRef = 43DAF0CD2D8C75D3005900E7 /* BackgroundDownloadStore.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -48,6 +49,7 @@
 		438F85442D6E241D00AF956D /* ImageLoader.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = ImageLoader.swift; sourceTree = "<group>"; };
 		438F85452D6E241D00AF956D /* NetworkService.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = NetworkService.swift; sourceTree = "<group>"; };
 		438F85462D6E241D00AF956D /* BackgroundDownloadService.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadService.swift; sourceTree = "<group>"; };
+		43DAF0CD2D8C75D3005900E7 /* BackgroundDownloadStore.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BackgroundDownloadStore.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -190,6 +192,7 @@
 				438F85442D6E241D00AF956D /* ImageLoader.swift */,
 				438F85452D6E241D00AF956D /* NetworkService.swift */,
 				438F85462D6E241D00AF956D /* BackgroundDownloadService.swift */,
+				43DAF0CD2D8C75D3005900E7 /* BackgroundDownloadStore.swift */,
 			);
 			path = Network;
 			sourceTree = "<group>";
@@ -307,6 +310,7 @@
 				43896B7A2D6906CE00FF34F8 /* CatCollectionViewCell.swift in Sources */,
 				3DEAF27720947086004FE44E /* AppDelegate.swift in Sources */,
 				438F85472D6E241D00AF956D /* ImageLoader.swift in Sources */,
+				43DAF0CE2D8C75D3005900E7 /* BackgroundDownloadStore.swift in Sources */,
 				438F85492D6E241D00AF956D /* BackgroundDownloadService.swift in Sources */,
 				3DEAF2AA20948C8A004FE44E /* NSObject+Name.swift in Sources */,
 				43896B7B2D6906CE00FF34F8 /* CatsViewController.swift in Sources */,

BackgroundTransfer-Example/Application/AppDelegate.swift

@@ -7,6 +7,7 @@
 //
 
 import UIKit
+import Foundation
 import os
 
 @UIApplicationMain
@@ -26,6 +27,8 @@ class AppDelegate: UIResponder, UIApplicationDelegate {
     }
 
     func applicationDidEnterBackground(_ application: UIApplication) {
+        os_log(.info, "Downloaded content will be saved to: %{public}@", 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.
         DispatchQueue.main.asyncAfter(deadline: .now()) {
             os_log(.info, "Simulating app termination by exit(0)")
@@ -48,7 +51,9 @@ class AppDelegate: UIResponder, UIApplicationDelegate {
 
     // MARK: - Background
 
-    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
-        BackgroundDownloadService.shared.backgroundCompletionHandler = completionHandler
+    func application(_ application: UIApplication, 
+                     handleEventsForBackgroundURLSession identifier: String,
+                     completionHandler: @escaping () -> Void) {
+        BackgroundDownloadService().backgroundCompletionHandler = completionHandler
     }
 }

BackgroundTransfer-Example/Network/BackgroundDownloadService.swift

@@ -9,32 +9,32 @@
 import Foundation
 import os
 
-enum BackgroundDownloadServiceError: Error {
+enum BackgroundDownloadError: Error {
     case missingInstructionsError
     case fileSystemError(_ underlyingError: Error)
-    case networkError(_ underlyingError: Error?)
-    case unexpectedResponseError
-    case unexpectedStatusCode
+    case clientError(_ underlyingError: Error)
+    case serverError(_ underlyingResponse: URLResponse?)
 }
 
 class BackgroundDownloadService: NSObject {
     var backgroundCompletionHandler: (() -> Void)?
 
     private var session: URLSession!
-
-    private var foregroundCompletionHandlers = [String: ((result: Result<URL, BackgroundDownloadServiceError>) -> ())]()
-
-    private let queue = DispatchQueue(label: "com.williamboles.background.download.service")
+    private let store = BackgroundDownloadStore()
 
     // MARK: - Singleton
 
     static let shared = BackgroundDownloadService()
 
     // MARK: - Init
 
-    private override init() {
+    override init() {
         super.init()
 
+        configureSession()
+    }
+
+    private func configureSession() {
         let configuration = URLSessionConfiguration.background(withIdentifier: "com.williamboles.background.download.session")
         configuration.sessionSendsLaunchEvents = true
         let session = URLSession(configuration: configuration,
@@ -47,98 +47,90 @@ class BackgroundDownloadService: NSObject {
 
     func download(from remoteURL: URL,
                   saveDownloadTo localURL: URL,
-                  completionHandler: @escaping ((_ result: Result<URL, BackgroundDownloadServiceError>) -> ())) {
-        queue.async { [weak self] in
-            os_log(.info, "Scheduling to download: %{public}@", remoteURL.absoluteString)
-
-            self?.foregroundCompletionHandlers[remoteURL.absoluteString] = completionHandler
-            UserDefaults.standard.set(localURL, forKey: remoteURL.absoluteString)
-
-            let task = self?.session.downloadTask(with: remoteURL)
-            task?.earliestBeginDate = Date().addingTimeInterval(2) // Remove this in production, the delay was added for demonstration purposes only
-            task?.resume()
-        }
+                  completionHandler: @escaping BackgroundDownloadCompletion) {
+        os_log(.info, "Scheduling to download: %{public}@", remoteURL.absoluteString)
+
+        store.storeMetadata(from: remoteURL,
+                            to: localURL,
+                            completionHandler: completionHandler)
+
+        let task = session.downloadTask(with: remoteURL)
+        task.earliestBeginDate = Date().addingTimeInterval(2) // Remove this in production, the delay was added for demonstration purposes only
+        task.resume()
     }
 }
 
 // MARK: - URLSessionDownloadDelegate
 
 extension BackgroundDownloadService: URLSessionDownloadDelegate {
-    func urlSession(_ session: URLSession, 
+    func urlSession(_ session: URLSession,
                     downloadTask: URLSessionDownloadTask,
                     didFinishDownloadingTo location: URL) {
-        queue.sync {
-            guard let originalRequestURL = downloadTask.originalRequest?.url?.absoluteString else {
-                os_log(.error, "Unexpected nil URL")
-                // Unable to call the closure here as we use originalRequestURL as the key to retrieve the closure
-
-                return
-            }
-
+        guard let fromURL = downloadTask.originalRequest?.url else {
+            os_log(.error, "Unexpected nil URL")
+            // Unable to call the closure here as we use fromURL as the key to retrieve the closure
+            return
+        }
+
+        let fromURLAsString = fromURL.absoluteString
+
+        os_log(.info, "Download request completed for: %{public}@", fromURLAsString)
+
+        let tempLocation = FileManager.default.temporaryDirectory.appendingPathComponent(location.lastPathComponent)
+        try? FileManager.default.moveItem(at: location,
+                                          to: tempLocation)
+
+        store.retrieveMetadata(for: fromURL) { [weak self] toURL, completionHandler in
             defer {
-                self.foregroundCompletionHandlers[originalRequestURL] = nil
-                UserDefaults.standard.removeObject(forKey: originalRequestURL)
+                self?.store.removeMetadata(for: fromURL)
             }
 
-            os_log(.info, "Download request completed for: %{public}@", originalRequestURL)
-
-            let foregroundCompletionHandler = self.foregroundCompletionHandlers[originalRequestURL]
-
-            guard let response = downloadTask.response as? HTTPURLResponse else {
-                os_log(.error, "Unexpected response for: %{public}@", originalRequestURL)
-                foregroundCompletionHandler?(.failure(.unexpectedResponseError))
+            guard let toURL else {
+                os_log(.error, "Unable to find existing download item for: %{public}@", fromURLAsString)
+                completionHandler?(.failure(BackgroundDownloadError.missingInstructionsError))
                 return
             }
 
-            guard response.statusCode == 200 else {
-                os_log(.error, "Unexpected status code of: %{public}d, for: %{public}@", response.statusCode, originalRequestURL)
-                foregroundCompletionHandler?(.failure(.unexpectedStatusCode))
+            guard let response = downloadTask.response as? HTTPURLResponse, 
+                        response.statusCode == 200 else {
+                os_log(.error, "Unexpected response for: %{public}@", fromURLAsString)
+                completionHandler?(.failure(BackgroundDownloadError.serverError(downloadTask.response)))
                 return
             }
 
-            os_log(.info, "Download successful for: %{public}@", originalRequestURL)
-
-            guard let saveDownloadToURL = UserDefaults.standard.url(forKey: originalRequestURL) else {
-                os_log(.error, "Unable to find existing download item for: %{public}@", originalRequestURL)
-                foregroundCompletionHandler?(.failure(.missingInstructionsError))
-
-                return
-            }
+            os_log(.info, "Download successful for: %{public}@", fromURLAsString)
 
             do {
-                try FileManager.default.moveItem(at: location,
-                                                 to: saveDownloadToURL)
+                try FileManager.default.moveItem(at: tempLocation,
+                                                 to: toURL)
 
-                foregroundCompletionHandler?(.success(saveDownloadToURL))
+                completionHandler?(.success(toURL))
             } catch {
-                foregroundCompletionHandler?(.failure(.fileSystemError(error)))
+                completionHandler?(.failure(BackgroundDownloadError.fileSystemError(error)))
             }
         }
     }
 
     func urlSession(_ session: URLSession,
                     task: URLSessionTask,
                     didCompleteWithError error: Error?) {
-        queue.async { [weak self] in
-            guard let error = error else {
-                return
-            }
-
-            guard let originalRequestURL = task.originalRequest?.url?.absoluteString else {
-                os_log(.error, "Unexpected nil URL")
-
-                return
-            }
-
-            defer {
-                self?.foregroundCompletionHandlers[originalRequestURL] = nil
-                UserDefaults.standard.removeObject(forKey: originalRequestURL)
-            }
-
-            os_log(.info, "Download failed for: %{public}@", originalRequestURL)
+        guard let error = error else {
+            return
+        }
+
+        guard let fromURL = task.originalRequest?.url else {
+            os_log(.error, "Unexpected nil URL")
+            return
+        }
+
+        let fromURLAsString = fromURL.absoluteString
+
+        os_log(.info, "Download failed for: %{public}@", fromURLAsString)
+
+        store.retrieveMetadata(for: fromURL) { [weak self] _, completionHandler in
+            completionHandler?(.failure(BackgroundDownloadError.clientError(error)))
 
-            let foregroundCompletionHandler = self?.foregroundCompletionHandlers[originalRequestURL]
-            foregroundCompletionHandler?(.failure(.networkError(error)))
+            self?.store.removeMetadata(for: fromURL)
         }
     }
 

BackgroundTransfer-Example/Network/BackgroundDownloadStore.swift

@@ -0,0 +1,56 @@
+//
+//  BackgroundDownloadStore.swift
+//  BackgroundTransfer-Example
+//
+//  Created by William Boles on 02/05/2018.
+//  Copyright © 2018 William Boles. All rights reserved.
+//
+
+import Foundation
+
+typealias BackgroundDownloadCompletion = (_ result: Result<URL, Error>) -> ()
+
+class BackgroundDownloadStore {
+    private var inMemoryStore = [String: BackgroundDownloadCompletion]()
+    private let persistentStore = UserDefaults.standard
+
+    private let queue = DispatchQueue(label: "com.williamboles.background.download.service",
+                                      qos: .userInitiated,
+                                      attributes: .concurrent)
+
+    // MARK: - Store
+
+    func storeMetadata(from fromURL: URL,
+                       to toURL: URL,
+                       completionHandler: @escaping BackgroundDownloadCompletion) {
+        queue.async(flags: .barrier) { [weak self] in
+            self?.inMemoryStore[fromURL.absoluteString] = completionHandler
+            self?.persistentStore.set(toURL, forKey: fromURL.absoluteString)
+        }
+    }
+
+    // MARK: - Retrieve
+
+    func retrieveMetadata(for forURL: URL, 
+                          completionHandler: @escaping ((URL?, BackgroundDownloadCompletion?) -> ())) {
+        return queue.async { [weak self] in
+            let key = forURL.absoluteString
+
+            let toURL = self?.persistentStore.url(forKey: key)
+            let metaDataCompletionHandler = self?.inMemoryStore[key]
+
+            completionHandler(toURL, metaDataCompletionHandler)
+        }
+    }
+
+    // MARK: - Remove
+
+    func removeMetadata(for forURL: URL) {
+        queue.async(flags: .barrier) { [weak self] in
+            let key = forURL.absoluteString
+
+            self?.inMemoryStore[key] = nil
+            self?.persistentStore.removeObject(forKey: key)
+        }
+    }
+}