Combine: How to clean up resources while an AnyCancellable is being cancelled? - swift

Question

Overview:
I have a async task to fetch from the database
I have created a Future for the async task (fetching from the database).
Question:
How can execute custom code when the Future is cancelled?
Purpose:
I would like the database connection to be closed when the subscription is cancelled.
For example, I would like to use Combine to rewrite this helper method:
// Similar to https://developer.apple.com/documentation/coredata/nspersistentcontainer/1640564-performbackgroundtask
func withDatabaseFTSContext(block: #escaping (FMDatabase?) -> Void) {
queue.async {
guard let database = self.database else {
block(nil)
return
}
database.open()
let simpleTokenizer = FMSimpleTokenizer(locale: nil)
FMDatabase.registerTokenizer(simpleTokenizer, withKey: "simple")
database.installTokenizerModule()
block(database)
database.close()
}
}
Could I leverage Combine to rewrite this method to return FMDatabase as a parameter of a publisher?
I was attempting to use Combine but it does not work. The database will be closed before cancel()
private func withDatabaseFTSContext() -> AnyPublisher<FMDatabase?, Never> {
return Future<FMDatabase?, Never> { promise in
self.queue.async {
guard let database = self.database else {
promise(.success(nil))
return
}
database.open()
let simpleTokenizer = FMSimpleTokenizer(locale: nil)
FMDatabase.registerTokenizer(simpleTokenizer, withKey: "simple")
database.installTokenizerModule()
promise(.success(database))
database.close() // When to close this database? Currently it will be closed before `cancel()`
}
}.eraseToAnyPublisher()
}

Answer 1

Short answer: there isn't a callback that triggers through to the underlying Future that you can use to clean things up on a subscriber cancel. In the Combine design, these functions are very intentionally separated and don't have reference links back to their publishers.
(In addition, Future is a tricky figure in the Combine world because the closure is invoked immediately upon creation time, rather than when you have a subscription (if you want that, wrap in the Future publisher in a Deferred publisher)).
All that being said, what you likely want to do to solve your underlying problem is reframe how you're treating this to separate the concerns of managing the FMDB instance and publishing data. One pattern that's been reasonably useful in this context is to the make an object that holds the lifetime of the FMDB reference, and handle cleaning up resources on it's deinit(). You can then also have a function which vends a Publisher of whatever you need from that same object, and then the cancellation of the request is changed semantically to only cancelling getting the database, not cancelling and cleaning up the database connection.