Combine publisher for observing data - ios

Question

Goal
Create a function that listens to changes in Firestore and publishes the result or the error
The code
func observe<T: Codable>(document: String, inCollection collection: String) -> AnyPublisher<T, Error> {
let docRef = self.db.collection(collection).document(document)
return Future<T, Error> { promise in
let docRef = self.db.collection(collection).document(document)
let listener = docRef.addSnapshotListener { (snapshot, error) in
guard let object = T(dictionary: snapshot?.data()), error == nil else {
promise(.failure(error ?? CRUDServiceError.encodingError))
return
}
promise(.success(object))
}
// Cancel the listener when the publisher is deallocated
let cancellable = AnyCancellable {
listener.remove()
}
}.eraseToAnyPublisher()
}
Future by define produces only a single value, not suitable for subscribing. PassThroughSubject inside the function failed also.
Error leads to publisher completion. We want to keep listening to changes even after the error is received, I found multiple approaches to achieve this, but they all require specific code on subscribing. I want to handle this problem one time inside the observe function. You can read some solutions here

Answer 1

Check out this gist: https://gist.github.com/IanKeen/934258d2d5193160391e14d25e54b084
With the above gist you can then do:
func observe<T: Codable>(document: String, inCollection collection: String) -> AnyPublisher<T, Error> {
let docRef = self.db.collection(collection).document(document)
return AnyPublisher { subscriber in
let docRef = self.db.collection(collection).document(document)
let listener = docRef.addSnapshotListener { (snapshot, error) in
guard let object = T(dictionary: snapshot?.data()), error == nil else {
subscriber.send(completion: .failure(error ?? CRUDServiceError.encodingError))
return
}
subscriber.send(object)
}
return AnyCancellable { listener.remove() }
}
}
As for the error... A Publisher cannot emit any more values after it emits an error. This is a fundamental part of the contract. Then best you can do is convert the error into some other type and emit as a next event instead of a completion event.
Something like this would do it:
func observe<T: Codable>(document: String, inCollection collection: String) -> AnyPublisher<Result<T, Error>, Never> {
let docRef = self.db.collection(collection).document(document)
return AnyPublisher { subscriber in
let docRef = self.db.collection(collection).document(document)
let listener = docRef.addSnapshotListener { (snapshot, error) in
guard let object = T(dictionary: snapshot?.data()), error == nil else {
subscriber.send(.failure(error ?? CRUDServiceError.encodingError))
return
}
subscriber.send(.success(object))
}
return AnyCancellable { listener.remove() }
}
}
But I suspect that once Firestore send an error to the callback, it too will stop calling your callback with new snapshots... So I don't think this is actually useful.