feat: Handle iOS background downloads

ios/App/App/AppDelegate.swift

@@ -6,6 +6,7 @@ import RealmSwift
 class AppDelegate: UIResponder, UIApplicationDelegate {
 
     var window: UIWindow?
+    var backgroundCompletionHandler: (() -> Void)?
     
     func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
         // Override point for customization after application launch.
@@ -72,6 +73,12 @@ class AppDelegate: UIResponder, UIApplicationDelegate {
         // tracking app url opens, make sure to keep this call
         return ApplicationDelegateProxy.shared.application(application, continue: userActivity, restorationHandler: restorationHandler)
     }
+    
+    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
+        // Stores the completion handler for background downloads
+        // The identifier of this method can be ignored at this time as we only have one background url session
+        backgroundCompletionHandler = completionHandler
+    }
 
     override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
         super.touchesBegan(touches, with: event)

ios/App/App/plugins/AbsDownloader.swift

@@ -15,9 +15,10 @@ public class AbsDownloader: CAPPlugin, URLSessionDownloadDelegate {
     static private let downloadsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
     
     private lazy var session: URLSession = {
+        let config = URLSessionConfiguration.background(withIdentifier: "AbsDownloader")
         let queue = OperationQueue()
         queue.maxConcurrentOperationCount = 5
-        return URLSession(configuration: .default, delegate: self, delegateQueue: queue)
+        return URLSession(configuration: config, delegate: self, delegateQueue: queue)
     }()
     private let progressStatusQueue = DispatchQueue(label: "progress-status-queue", attributes: .concurrent)
     private var downloadItemProgress = [String: DownloadItem]()
@@ -78,6 +79,18 @@ public class AbsDownloader: CAPPlugin, URLSessionDownloadDelegate {
         }
     }
     
+    // Called when downloads are complete on the background thread
+    public func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
+        DispatchQueue.main.async {
+            guard let appDelegate = UIApplication.shared.delegate as? AppDelegate,
+                let backgroundCompletionHandler =
+                appDelegate.backgroundCompletionHandler else {
+                    return
+            }
+            backgroundCompletionHandler()
+        }
+    }
+    
     private func handleDownloadTaskUpdate(downloadTask: URLSessionTask, progressHandler: DownloadProgressHandler) {
         do {
             guard let downloadItemPartId = downloadTask.taskDescription else { throw LibraryItemDownloadError.noTaskDescription }