A lot of different issues have been raised in this thread, so I've loosely summarized them to the following 4 questions issues:
1) Reporting a call immediately does not work with the SIP register-invite flow
On iOS 13, there are cases where you will need to initiate a CallKit call that you previously would have silently ignored. However, in practice, this should not be the common case, particularly for truly “non-existent" calls, since that would mean you had been notifying the user of calls that did not exist.
The more common cases here are calls that have either already ended (because the caller hung up) or can't be completed (because network conditions prevent the call from connecting). While calls will be created for both of these cases, there are few different techniques that you can use to mitigate any disruptions:
While you must report an incoming call immediately, you are still free to decide the call has failed and tell the system later, asynchronously. To do this, call reportCallWithUUID:endedAtDate:reason:. This will tear down the incoming call UI even if the user has not answered the call.
If the user is particularly fast at tapping the accept call button, or if the network conditions are poor or there is otherwise a lot of latency, then you should simply wait until the necessary handshakes are complete before calling fulfill on the CXAnswerCallAction. To the user, it simply appears that the call is taking some time to connect, which is a common experience even with standard phone calls.
Note that the system takes a few seconds for the incoming call UI to animate in, during which the app has the opportunity to complete this handshake, so this will only have a user-visible impact if it takes a significant time for the handshake to complete.
At any time, you can asynchronously update the UI with the reportCallWithUUID:updated: API. That means that if you cannot put the caller ID info in the push payload, you can simply choose to present dummy information (like "Connecting Call..." for the caller name) and update it asynchronously once they get the real information from your server.
2) Sending a push to cancel an incoming call
While your app currently has an active call (ringing or answered), your app is not required to create additional calls for VoIP pushes received during this call. This is intended to be used to support advanced functionality like dynamic call priority, but it could also be used to cancel an incoming call.
Having said that, this is not an approach I would recommend or rely upon. The reality is that as soon as your app receives it's PushKit notification it should be connecting to your server, so unless network conditions are very poor, you should be able to communicate to your client through that connection faster than PushKit. More to the point, if network conditions mean that you can't connect to your server, then trying to handle this with another push isn't a great idea either, since poor connectivity (and timing generally) opens the door to edge cases you'd want to avoid - for example, a client receiving ONLY the cancelation and not the original call notification.
As a side note here, keep in mind that as part of adapting to the new requirements you'll want to make sure the VoIP notification has a short or zero "apns-expiration" to prevent newly available devices from being notified of out-of-date calls. This will also minimize the cancelation issue, since a “newly available device" (for example, a phone that was just powered on) will ONLY receive notifications about calls that are occurring at that particular instant.
3) Block-lists/Do Not Disturb
CallKit respects the system Do Not Disturb setting, so most apps will not have to worry about system-level Do Not Disturb functionality. If your app has it's own blocking/do not disturb system built in, you can also maintain that list server-side and not send pushes to the blocked devices. If you absolutely need to do "local" call blocking, then you can report a call and then end it. The call will be briefly visible to the user, but you can also configure the source of the call to communicate what's going on ("Blocked Call...").
4) Using VoIP pushes to trigger syncs or other non-VoIP use cases
VoIP pushes were always intended to specifically support call notifications and nothing else. The good news here is that using Notification Service Extension is the best substitute for most of the functionality that you previously handled with PushKit:
A few examples:
- For general messaging, you can connect to your server in the extension delegate, download any missed messages, and then update the notification content as appropriate.
- The same approach can be used to tell the user about calls missed while the device was offline and/or pending voicemails.
- For non-user facing functionality, like data synchronization or other app maintenance, I would recommend checking out the new BackgroundTasks framework:
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