`thread-id` field has no noticeable effect

Originator:jon
Number:rdar://29670386 Date Originated:2016-12-14
Status:Open Resolved:
Product:ct:iOS + SDK Product Version:iOS 10.1.1 (14B150)
Classification: Reproducible:Always
 
Area:
APNS (Notifications)

Summary:
The `thread-id` APNs payload field was recently introduced. As described in the APNs docs (https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/PayloadKeyReference.html#//apple_ref/doc/uid/TP40008194-CH17-SW1), users should:

> Provide this key with a string value that represents the app-specific identifier for grouping notifications. The system groups notifications with the same thread identifier together in Notification Center and other system interfaces.

In practice, though, I can't find any noticeable behavior change with or without a thread ID included in notifications.

Steps to Reproduce:
Send a notifications to the APNs server with a variety of thread IDs (some with the same ID, some with different IDs, and some without thread IDs at all). Notifications with an ID will include the `thread-id` field as a property of the `aps` object, as shown in this example:

```
{"aps":{"alert":"Thread ID example","thread-id":"example-thread-id"}}
```

Expected Results:
Notifications with the same thread ID will appear in proximity to each other either on the receiving device's home screen or in Notification Center.

Actual Results:
Notifications appear in the order in which they were received, regardless of their thread ID.

Version:
iOS 10.1.1 (14B150)

Notes:
This has come up a couple of times on the APNs forums:

- https://forums.developer.apple.com/message/188955
- https://forums.developer.apple.com/thread/69520

Additional discussion is also available at https://github.com/relayrides/pushy/issues/382.

Configuration:
HTTP/2-based APNs API with token authentication

Comments

Additional discussion and reading suggests that iOS itself won't do anything with a thread ID, but will give the receiving application an opportunity to take action. From the didReceiveNotification: documentation (https://developer.apple.com/reference/usernotificationsui/unnotificationcontentextension/1648525-didreceivenotification?language=objc):

The system calls this method to deliver notifications to your Notification Content app extension. Use this method to configure the contents of your view controller or to incorporate content from a new notification. This method may be called multiple times while your view controller is visible. Specifically, it is called again when a new notification arrives whose threadIdentifier value matches the thread identifier of the notification already being displayed. The method is called on the main thread of your extension.

While I no longer believe anything is wrong from a technical perspective, I would suggest that there's an opportunity to clarify the thread-id documentation. The limited discussion in the APNs payload key reference suggests to me that the OS itself will act on a thread ID, and does not provide a link to a more detailed description of the expected behavior.

Thanks for your consideration!

By jon at Dec. 15, 2016, 11:11 p.m. (reply...)

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