Swift - Execute code after sometime on receiving a silent notification - ios

Question

In my iOS application, I want to execute a piece of code after 5 seconds of receiving a silent notification from Firebase.
The tentative workflow is something like this:
A silent notification from Firebase arrives on the device => OK
Application extracts some data and displays a local notification => OK
After 5 seconds, I want to list the notifications delivered via UNUserNotificationCenter.current().getDeliveredNotifications => Not OK
In all the above steps, there's no user intervention and the app is always in background.
The attempted code is as follows:
//Receive the silent notification
func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any],
fetchCompletionHandler completionHandler: #escaping (UIBackgroundFetchResult) -> Void) {
// Print message ID.
if let messageID = userInfo[gcmMessageIDKey] {
print("Message ID 2: \(messageID)")
}
showLocalNotification(userInfo: userInfo, fetchCompletionHandler: completionHandler)
/* This was one of the attempts
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2)) {
completionHandler(UIBackgroundFetchResult.newData)
}
*/
}
//Show Local notification with the data received in silent notification
func showLocalNotification(userInfo: [AnyHashable: Any], fetchCompletionHandler completionHandler: #escaping (UIBackgroundFetchResult) -> Void) {
let content = UNMutableNotificationContent()
content.title = NSLocalizedString("notificationTitle", comment: "")
content.body = NSLocalizedString("notificationBody", comment: "")
content.userInfo = userInfo
// Create the request
let uuidString = UUID().uuidString
let request = UNNotificationRequest(identifier: uuidString, content: content, trigger: nil)
// Schedule the request with the system.
let notificationCenter = UNUserNotificationCenter.current()
notificationCenter.add(request) { (error) in
if error != nil {
// Handle any errors.
print(error!)
} else {
self.startTimer()
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(2)) {
completionHandler(UIBackgroundFetchResult.newData)
}
//completionHandler(UIBackgroundFetchResult.newData)
/* This was another approach instead of using timer
DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(5)) {
self.deliveredNotifications()
}
*/
}
}
}
// Start timer to wait for 5 seconds
func startTimer() {
print("starting timer")
if timer == nil {
timer = DispatchSource.makeTimerSource(queue: DispatchQueue.global())
timer!.schedule(deadline: .now() + .seconds(5))
timer!.setEventHandler {
print("in event handler")
self.deliveredNotifications()
self.timer = nil
}
timer!.resume()
}
print("timer started")
}
//List notifications
func deliveredNotifications() {
let notificationCenter = UNUserNotificationCenter.current()
notificationCenter.getDeliveredNotifications(completionHandler: { notifications -> () in
print("in dispatch")
print(notifications.count)
for request in notifications {
print(request.request.identifier)
}
})
}
In the above code, print("in event handler") never gets printed on the console and hence deliveredNotifications() does not execute. The other two print messages in startTimer() are printed successfully.
However, if I directly call deliveredNotifications() after displaying the local notification, it executes fine.
Also, if I launch the app or send another silent notification, the timer callback gets executed but that is not the desired behavior.
So, is there anything that I am missing or I can do to get this working?
I saw some similar questions, but could not achieve anything concrete:
Silent push only work properly when second push arrive (app in background)
How to clear a remote pushed notification for iOS?

Answer 1

Assuming your App support background task, can you try something like this:
func startTimer() {
print("starting timer")
if timer == nil {
let task = UIApplication.shared.beginBackgroundTask(expirationHandler: nil)
timer = DispatchSource.makeTimerSource(queue: DispatchQueue.global())
timer!.schedule(deadline: .now() + .seconds(5))
timer!.setEventHandler {
print("in event handler")
self.deliveredNotifications()
self.timer = nil
UIApplication.shared.endBackgroundTask(task)
}
timer!.resume()
}
print("timer started")
}
I think the main reason is that you are not starting background task, so as soon as your App enters background, execution stops.
NOTE: You probably need to a expirationHandler in case you exceed the allow background time, and do some error handling.