Missing WCSessionDelegate didFinish callback transferring file from WatchOS to iOS(iPhone) using transferFile(_:metadata:) -> WCSessionFileTransfer

I am using WCSession.transferFile(_:metadata:) -> WCSessionFileTransfer for transferring some files from WatchOS (10.5) to iPhone iOS 17.5.1

Watch Connectivity WCSession.transferFile(_:metadata:)

let session: WCSession = WCSession.default
session.delegate = self
session.activate()
...
...
...
fileTransfer = session.transferFile(file.url, metadata: ["alternateId" : file.alternateId, "type" : file.type])

The following delegate handles the file transferring result

func session(_ session: WCSession, didFinish fileTransfer: WCSessionFileTransfer, error: Error?) { 
...
...
...
}

The problem is that the didFinish response from WCSessionDelegate does not fire at all.

Attaching the debug console I was able to retrieve the following error:

[WCFileStorage persistOutgoingFileTransfer:] error serializing file transfer <WCSessionFileTransfer: 0x300155d60, session file: <WCSessionFile: 0x3001575c0, identifier: 0C8857EC-7D74-4E78-BA28-6C5526DE8949, file: FILE.EXT, hasMetadata: YES>, transferring: YES> due to Error Domain=NSCocoaErrorDomain Code=4866 "Caught exception during archival: This object may only be encoded by an NSXPCCoder. ( 0 CoreFoundation 0x000000019b064f2c 00E76A98-210C-3CB5-930B-F236807FF24C + 540460 1 libobjc.A.dylib 0x0000000192ef6018 objc_exception_throw + 60 2 Foundation 0x0000000199fe7778 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 1419128 3 Foundation 0x0000000199ea0e14 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 81428 4 WatchConnectivity 0x000000021d055f60 1AB4DDD6-9238-3965-B744-819F2916C8CC + 126816 5 Foundation 0x0000000199ea0e14 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 81428 6 WatchConnectivity 0x000000021d0567f0 1AB4DDD6-9238-3965-B744-819F2916C8CC + 129008 7 Foundation 0x0000000199ea0e14 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 81428 8 Foundation 0x0000000199f30628 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 669224 9 WatchConnectivity 0x000000021d0583ac 1AB4DDD6-9238-3965-B744-819F2916C8CC + 136108 10 WatchConnectivity 0x000000021d04390c 1AB4DDD6-9238-3965-B744-819F2916C8CC + 51468 11 WatchConnectivity 0x000000021d046640 1AB4DDD6-9238-3965-B744-819F2916C8CC + 63040 12 Foundation 0x0000000199ea9be0 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 117728 13 Foundation 0x0000000199ea9aa0 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 117408 14 Foundation 0x0000000199ea98a0 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 116896 15 Foundation 0x0000000199ea7b40 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 109376 16 Foundation 0x0000000199f2c558 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 652632 17 Foundation 0x0000000199f2c1a4 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 651684 18 libdispatch.dylib 0x0000000105ed7764 _dispatch_block_async_invoke2 + 148 19 libdispatch.dylib 0x0000000105ec67bc _dispatch_client_callout + 20 20 libdispatch.dylib 0x0000000105ec98e0 _dispatch_continuation_pop + 676 21 libdispatch.dylib 0x0000000105ec8bb8 _dispatch_async_redirect_invoke + 680 22 libdispatch.dylib 0x0000000105edaae4 _dispatch_root_queue_drain + 404 23 libdispatch.dylib 0x0000000105edb4d8 _dispatch_worker_thread2 + 188 24 libsystem_pthread.dylib 0x00000001f7ebb8f8 _pthread_wqthread + 228 25 libsystem_pthread.dylib 0x00000001f7eb80cc start_wqthread + 8 )" UserInfo={NSDebugDescription=Caught exception during archival: This object may only be encoded by an NSXPCCoder. ( 0 CoreFoundation 0x000000019b064f2c 00E76A98-210C-3CB5-930B-F236807FF24C + 540460 1 libobjc.A.dylib 0x0000000192ef6018 objc_exception_throw + 60 2 Foundation 0x0000000199fe7778 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 1419128 3 Foundation 0x0000000199ea0e14 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 81428 4 WatchConnectivity 0x000000021d055f60 1AB4DDD6-9238-3965-B744-819F2916C8CC + 126816 5 Foundation 0x0000000199ea0e14 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 81428 6 WatchConnectivity 0x000000021d0567f0 1AB4DDD6-9238-3965-B744-819F2916C8CC + 129008 7 Foundation 0x0000000199ea0e14 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 81428 8 Foundation 0x0000000199f30628 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 669224 9 WatchConnectivity 0x000000021d0583ac 1AB4DDD6-9238-3965-B744-819F2916C8CC + 136108 10 WatchConnectivity 0x000000021d04390c 1AB4DDD6-9238-3965-B744-819F2916C8CC + 51468 11 WatchConnectivity 0x000000021d046640 1AB4DDD6-9238-3965-B744-819F2916C8CC + 63040 12 Foundation 0x0000000199ea9be0 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 117728 13 Foundation 0x0000000199ea9aa0 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 117408 14 Foundation 0x0000000199ea98a0 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 116896 15 Foundation 0x0000000199ea7b40 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 109376 16 Foundation 0x0000000199f2c558 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 652632 17 Foundation 0x0000000199f2c1a4 3D3A12E3-F5E9-361F-B00A-4A5E8861AA55 + 651684 18 libdispatch.dylib 0x0000000105ed7764 _dispatch_block_async_invoke2 + 148 19 libdispatch.dylib 0x0000000105ec67bc _dispatch_client_callout + 20 20 libdispatch.dylib 0x0000000105ec98e0 _dispatch_continuation_pop + 676 21 libdispatch.dylib 0x0000000105ec8bb8 _dispatch_async_redirect_invoke + 680 22 libdispatch.dylib 0x0000000105edaae4 _dispatch_root_queue_drain + 404 23 libdispatch.dylib 0x0000000105edb4d8 _dispatch_worker_thread2 + 188 24 libsystem_pthread.dylib 0x00000001f7ebb8f8 _pthread_wqthread + 228 25 libsystem_pthread.dylib 0x00000001f7eb80cc start_wqthread + 8 )}

Unfortunately this error is not forwarded to WCSessionDelegate

The error looks related to NSXPCCoder but I do not find any specific documentation about it.

NSCocoaErrorDomain Code=4866 "Caught exception during archival: This object may only be encoded by an NSXPCCoder.

I've also tried calling WCSession.outstandingFileTransfers to retrieve the files that are queued for delivery but have not yet been delivered to the counterpart and I noticed that the WCSession keeps all oustandingFileTransfers.

Am I doing something wrong? How do I handle this type of error? Is there a specific limit to the file that can be transferred using WCSessionFileTransfer?

UPDATE: I've found a discussion thread on the Apple Developer Forum. It looks like a bug that affects the latest version of watchOS/iOS.

https://developer.apple.com/forums/thread/751623

Unfortunately I cannot perform extensive test about this issue since the simulator app doesn’t support the transferFile(_:metadata:) method.

I hope that it will be fixed soon but in the meantime I am looking for a possible workaround. Any advice?

Solution

While waiting for a proper fix from Apple, I implemented the following workaround...

Since the file is effectively sent to the phone, the only thing missing is the notification callback on the sending device (watchOS), the idea is to send a custom message from the iPhone to the watchOS to notify the watch that the file was properly transferred. It will be send as soon as the didFinish callback has received on the iPhone side (receiving device).

WCSession.sendMessage(message, replyHandler: nil)

The content of the message will be the id of the successfully transferred file.

This way, the watchOS can trigger the sending of the next file and unlock the file transfer. Obviously I had to move the triggering of the next file sending out of the WCSessionDelegate didFinish callback. It is moved to the WCSessionDelegate didReceiveMessage

Another side effect of this issue is unusual battery drain caused by the zombies file transfers for the session. This can be avoided by programmatically cleaning up the completed transfers.

        session.outstandingFileTransfers
            .filter({$0.progress.isFinished})
            .forEach { fileTransfer in
                fileTransfer.cancel()
            }

Another option (suggested by Apple guys) is to look at WCSessionFileTransfer.progress still reflects the progress. So for the issue of session(_:didFinish:error:) not being triggered, you might be able to work around it by checking if the file transfer progress is 100% completed.