Sample implementation to receive unencrypted ably messages via native Apple Push Notification Service (APNS) delivered by ably in foreground and background.

AppDelegate.swift

//
//  AppDelegate.swift
//  swift-ios
//

import Foundation
import UIKit
import Ably

class AppDelegate: NSObject, UIApplicationDelegate, ARTPushRegistererDelegate, UNUserNotificationCenterDelegate {
    var realtime: ARTRealtime!
    
    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey : Any]? = nil) -> Bool {
        // assign ably instance
        realtime = getAblyRealtime()
        
        UNUserNotificationCenter.current().delegate = self
        // probably this is not necessary if only background notifications are used
        UNUserNotificationCenter.current().requestAuthorization(options: [.provisional, .badge, .alert]) { granted, _ in
            print("Permission granted: \(granted)")
        }
        
        // registers device using server authentication to ably with client id
        realtime.push.activate()
        
        // starts APNS registration process
        UIApplication.shared.registerForRemoteNotifications()
        
        // Do I have to call realtime.push.deactivate()? When, why, how?
        
        return true
    }
    
    // MARK: APNS Push Message Delegate
    
    func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable : Any], fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
        print("APNS: ", userInfo)
        completionHandler(.newData)
    }

    // MARK: APNS Setup Delegate
    
    func application(_ application: UIApplication, didRegisterForRemoteNotificationsWithDeviceToken deviceToken: Data) {
        print("registering for remote notifications with device token...")
        ARTPush.didRegisterForRemoteNotifications(withDeviceToken: deviceToken, realtime: realtime)
    }

    func application(_ application: UIApplication, didFailToRegisterForRemoteNotificationsWithError error: Error) {
        print("ERROR: didFailToRegisterForRemoteNotificationsWithError")
        ARTPush.didFailToRegisterForRemoteNotificationsWithError(error, realtime: realtime)
    }
    
    // MARK: ARTPushRegistererDelegate
    
    func didActivateAblyPush(_ error: ARTErrorInfo?) {
        print("SUCCESS: didActivateAblyPush")
        if let error = error {
            print("ERROR: didActivateAblyPush, err=\(String(describing: error))")
            return
        }
    }
    
    func didDeactivateAblyPush(_ error: ARTErrorInfo?) {
        if let error = error {
            print("ERROR: didDeactivateAblyPush", error)
            return
        }
    }
    
    // MARK: Ably Realtime Client (Singleton)
    
    func getAblyRealtime() -> ARTRealtime {
        if let realtime = realtime {
            return realtime
        }
        realtime = ARTRealtime(options: getOptions())
        return realtime
    }
    
    func getOptions() -> ARTClientOptions {
        let options = ARTClientOptions()
        // this is dependent on auth method (we use server side authentication)
        // https://ably.com/tutorials/token-authentication
        options.authUrl = URL(string: "https://my-server-endpoint/ably_token_requests")!
        options.authMethod = "POST"
        options.authHeaders = ["X-API-KEY": "secret"]
        options.pushRegistererDelegate = self
        return options
    }
}

In general what is your recommendation?

App in Background = Use APNS for Message Delivery
App in Foreground = Use Ably Realtime steady connection