You’re asking about threads when you should be asking about queues. NSXPCConnection replies are always delivered on a specific serial queue internal to the connection. There’s a comment in
<Foundation/NSXPCConnection.h>that says as much:
Each NSXPCConnection instance has a private serial queue. This queue is used when sending messages to reply handlers, interruption handlers, and invalidation handlers.
Which thread runs blocks posted to that queue is not specified.
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