When app extensions don't launch
I had a difficult bug to root out regarding app extensions (related to my previous post). One of my apps could launch the notification service and content extensions without fail, whereas another would only show old-style iOS 9 notifications.
Looking at the device log I noticed when a notification arrived, iOS could not find an appropriate extension to launch, despite the "mutable-content" entry being set properly in the payload.
SpringBoard(UserNotificationsServer)[57] <Info>: Received incoming message on topic se.mydomain.myapp pkd(MobileCoreServices)[88] <Notice>: No plugins found to match query <private> pkd[88] <Info>: match returned 0 plug-ins
Investigating my successful case, I observed that the .app container housed a plugins folder which included two .appex binaries (for the service and content notifications). In my app which wasn't working, however, the same extensions had the file extension .app (as opposed to .appex).
Checking the Packaging series of Build Settings in Xcode for the extensions, I noticed a .xcconfig file was overriding the filename for the extensions, and accordingly causing iOS to not find any .appex that could service the notification. Correcting this build setting fixed the problem and notifications were thereafter parsed properly.