watchOS 2.0 - Can't cancel WCSessionFileTransfer - ios

Question

Our app lets a user select records on iPhone that they want to be displayed in the watch app.
It works like this:
The user taps "Add to watch" on a record from their iPhone
A new version of the watch database is generated and sent to the watch
The watch app receives and saves the file and updates its interface
A new database file is sent to the watch and processed for each change. This is fine if the watch is awake since it will give the user live updates, but if the watch is asleep while the user makes 7 changes, it means the watch is accepting and processing 7 new files as soon as it wakes up.
We really only care about the most recent version of the watch database, so I'm trying to cancel all old outstanding file transfers.
Code:
On iPhone, each time a record is added/removed from watch database, we attempt (unsuccessfully) to cancel pending file transfers and then queue the latest database file:
// create watch database and store it at self.urlToDatabase
[self generateNewWatchDatabase];
if ([WCSession isSupported])
{
WCSession *session = [WCSession defaultSession];
session.delegate = self;
[session activateSession];
// this is the problem - cancel doesn't seem to do anything
for (WCSessionFileTransfer *fileTransfer in session.outstandingFileTransfers)
[fileTransfer cancel];
[session transferFile:self.urlToDatabase metadata:nil];
}
In the above code, calling [fileTransfer cancel] successfully removes the WCSessionFileTransfer object from session.outstandingFileTransfers, but didReceiveFile is still being called multiple times below.
Accepting the file on the watch:
- (void)session:(WCSession *)session didReceiveFile:(WCSessionFile *)file
{
// this method gets called once for every time -transferFile:metadata: is called above,
// even after cancelling outstanding file transfers
[self replaceDatabaseWithFile:file];
[self refreshItemsTable];
}
How do we cancel outstanding file transfers?
Edit
As per #ccjensen's recommendation, I tried the following in the method that fires when the user adds/removes a record to/from the watch:
// store a reference to the file transfer and immediately cancel it
WCSessionFileTransfer *transfer = [session transferFile:self.urlToDatabase metadata:nil];
[transfer cancel];
This still results in the file being sent to the watch, instead of cancelling it as one would expect.
I also tried the following:
Kill watch app (by holding the side button until 'Power Off' appears, and then holding it again)
Add/remove records from iPhone
Relaunch watch app
Even in this scenario the watch receives all 'cancelled' file transfers.

Answer 1

The documentation for the cancel method says:
Use this method to cancel a file transfer before it completes. If the file has already been transferred, calling this method has no effect.
So it sounds like the cancels are "best effort" and might not end up being able to cancel them in all cases, especially if the file has already been transferred.
Are you seeing it never work, even if you call cancel immediately (try testing without the watch app running as that seems to expedite the transfers)?

Answer 2

turned out the reason this worked was that the file url did not match the transfer's url I was checking 🙈
I recently found that keeping hold of the WCSessionFileTransfer in my own array and canceling them proved more reliable than using [WCSession defaultSession].outstandingFileTransfers.
NSMutableArray<WCSessionFileTransfer*>* inProgressTransfers = [NSMutableArray array];
So each time you call TransfeFile: metaData:
WCSessionFileTransfer* transfer = [[WCSession defaultSession] transferFile:url metadata:metadata];
if (transfer)
{
[self.inProgressTransfers addObject:transfer];
}
then at an appropriate time
for (WCSessionFileTransfer* ourTransfer in self.inProgressTransfers)
{
[ourTransfer cancel];
[self.inProgressTransfers removeObject:ourTransfer];
}
For some reason, keeping hold of the transfers ourself makes calling cancel work much more reliably.. hope that helps someone