iOS 13 Killing app because it never posted an incoming call to the system after receiving a PushKit VoIP callback

Question

After upgrading to iOS beta 13 I've noticed an unpleasant thing: my app crashes sometimes on incoming VoIP pushes.

In the crash report I see the following:

iOS 13 Killing app because it never posted an incoming call to the system after receiving a PushKit VoIP callback 

Fatal Exception: NSInternalInconsistencyException
0  CoreFoundation                 0x1af21b9f0 __exceptionPreprocess
1  libobjc.A.dylib                0x1af7284fc objc_exception_throw
2  CoreFoundation                 0x1af11efec + 
 [_CFXNotificationTokenRegistration keyCallbacks]
3  Foundation                     0x1aeda1330 -[NSAssertionHandler 
 handleFailureInMethod:object:file:lineNumber:description:]
4  PushKit                        0x19caa6b54 -[PKPushRegistry 
 _terminateAppIfThereAreUnhandledVoIPPushes]
5  libdispatch.dylib              0x1afa441ec _dispatch_client_callout
6  libdispatch.dylib              0x1af9f6c6c 
_dispatch_lane_barrier_sync_invoke_and_complete
7  PushKit                        0x19caa5b74 __73-[PKPushRegistry 
 voipPayloadReceived:mustPostCall:withCompletionHandler:]_block_invoke
8  libdispatch.dylib              0x1afa43678 
 _dispatch_call_block_and_release
9  libdispatch.dylib              0x1afa441ec 
  _dispatch_client_callout

10 libdispatch.dylib              0x1af9f61f8 
_dispatch_main_queue_callback_4CF$VARIANT$mp
11 CoreFoundation                 0x1af1992a0 
CFRUNLOOP_IS_SERVICING_THE_MAIN_DISPATCH_QUEUE
12 CoreFoundation                 0x1af1942a8 __CFRunLoopRun
13 CoreFoundation                 0x1af1937ac CFRunLoopRunSpecific
14 GraphicsServices               0x1ae395180 GSEventRunModal
15 UIKitCore                      0x1b6e60244 UIApplicationMain
16 VOIPProject                    0x1009822d8 main + 25 
(AppDelegate.swift:25)
17 libdyld.dylib                  0x1af6e9e7c start

I can not understand how to fix the problem. Am I obliged to post CallKit incoming call screen whenever I receive a VoIP push? It sounds crazy because I check if the notification is valid before showing the screen with incoming call. Can anybody explain what should I do?

Answer 1

On this thread from apple forums, someone from apple staff explained this:

On iOS 13.0 and later, incoming Voice over IP calls must be reported when they are received and before the didReceiceIncomingPush() method finishes execution, using the CallKit framework, or the system will terminate your app.

Repeatedly failing to report calls may prevent your app from receiving any more incoming call notifications.

Basically, you can no longer use VoIP pushes for non VoIP messaging, and will need to use regular push notifications for those.

This was announced during the WWDC session "Advances in App Background Execution" https://developer.apple.com/videos/play/wwdc2019/707/

---

I've been searching for answers on how to adapt an app for this change, and what I could gather is the following:

Voip Pushes

When your app receive this kind of push, it will need to report a new incoming call using CallKit. Therefore, this kind of push will be exclusive for calls that use CallKit.

It's recommended that you set the notification's apns-expiration to 0, so you won't receive a push and be forced to present a call screen for a call that already expired.

Push Notifications

Regular push notifications are another option. If your server has all the information you need to write the notification text, you can send notifications that won't even run your app in the background. If you need to modify the content of the notification before presenting it to the user, you can use a Notification Service app extension, and if you need your app to be woken up and execute something in background, you can send silent push notifications.

Notification Service App Extension

To use this, you must set your notification's mutable-content to 1. This way, your extension will receive the notification before it is presented to the user, allowing you to change its content, with a 30 seconds time limit.

The cons are that your app will stay in the background, only your extension will be allowed to run. This might mean that you will need to share information and code between your app and the extension, either by using user defaults, keychain, or by sharing your entire database (which might not be a simple task if your app is not prepared for that).

Silent Push Notifications

To send silent push notifications, you must set your notification's content-available to 1 and remove it's alert, badge and sound. This notification will wake up your app in the background, and call your app delegate's didReceiveRemoteNotification.

The downsides are quite annoying for this option:

• You will only get 30 seconds to run.
• These notifications must have a apns-priority of 5, which might cause them to be grouped and delivered in bursts, and even throttled or not delivered.
• If the user force closes the app, it will completely ignore all silent notifications until the user opens the app again.