Handling Push Notifications when App is Terminated

The app does not process push notification in the background, what it really does the OS is wake up the app once you press in the notification. You can catch this moment in the following way:

    - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
    { 

        if ([launchOptions objectForKey:UIApplicationLaunchOptionsRemoteNotificationKey]) {
      // Your app has been awoken by a notification...
        }
   }

You can retrieve notifications delivered to your app by using the getDeliveredNotifications(completionHandler:) method. Note that this only returns notifications currently displayed in the Notification Center and not the ones that have been manually cleared by the user.

UNUserNotificationCenter.current().getDeliveredNotifications { notifications in

    // notifications: An array of UNNotification objects representing the local
    // and remote notifications of your app that have been delivered and are still
    // visible in Notification Center. If none of your app’s notifications are
    // visible in Notification Center, the array is empty.

    // As said in the documentation, this closure may be executed in a background
    // thread, so if you want to update your UI you'll need to do the following:
    DispatchQueue.main.sync { /* or .async {} */ 
        // update UI
    }
}

1) When application is running in background and When application is running in foreground application:didReceiveRemoteNotification: method will called as below.

- (void)application:(UIApplication *)application didReceiveRemoteNotification:(NSDictionary *)userInfo
{
    if (application.applicationState == UIApplicationStateInactive)
    {
        // opened from a push notification when the app was on background
        NSLog(@"userInfo->%@", [userInfo objectForKey:@"aps"]);
    }
    else if(application.applicationState == UIApplicationStateActive)
    {
        // a push notification when the app is running. So that you can display an alert and push in any view
        NSLog(@"userInfo->%@", [userInfo objectForKey:@"aps"]);
    }
}

2) When application is not launched (close) then application:didFinishedLaunchingWithOptions method will called.

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    if (launchOptions != nil)
    {
        // opened from a push notification when the app is closed
        NSDictionary* userInfo = [launchOptions objectForKey:UIApplicationLaunchOptionsRemoteNotificationKey];
        if (userInfo != nil)
        {
             NSLog(@"userInfo->%@", [userInfo objectForKey:@"aps"]);
        }
    }
    else
    {
        // opened app without a push notification.
    }
}

3) There is no way to remove a specific notification as of. The way to remove all the notifications from your app so they don't show in the Notification Center when the user opens the app from one of them, is to set the app badge to 0.

As per your question, there is no way to hold all the notification when you open the app, better you call an api to get all notification as per time stamp from your back end/server that's how Facebook does.