Chaining calls when using Future in Swift similar to PromiseKit - swift

Question

Below there are three functions. The first one is the function that I need to refactor. Basically what I'm hoping for is something similar what can be achieved using Promise Kit but in this case using Swifts combine framework.
The second function loginWithFacebook() returns a AuthCredential.
This AuthCredential needs to be passed on to the last functions which returns a type Future<UserProfileCompact, Error> which is a similar return type to the main function (1st function).
My question is is there a way to achieve this in a Swifty way, similar to Promise Kit doing this operation: return loginWithFacebook().then {loginWithFirebase(:_)}
// Call site is a View Model
// Main Function that needs to be refactored
func loginwithFacebook() -> Future<UserProfileCompact, Error> {
//This returs a Future Firebase Credential
loginWithFacebook()
//The above credential needs to be passed to this method and this returns a type Future<UserProfileCompact, Error>
loginWithFirebase(<#T##credentials: AuthCredential##AuthCredential#>)
}
private func loginWithFacebook() -> Future<AuthCredential,Error> {
return Future { [weak self] promise in
self?.loginManager.logIn(permissions: ["public_profile","email"], from: UIViewController()) { (loginResult, error) in
if let error = error {
promise(.failure(error))
} else if loginResult?.isCancelled ?? false {
//fatalError()
}
else if let authToken = loginResult?.token?.tokenString {
let credentials = FacebookAuthProvider.credential(withAccessToken: authToken)
promise(.success(credentials))
}
else{
fatalError()
}
}
}
}
private func loginWithFirebase(_ credentials: AuthCredential) -> Future<UserProfileCompact, Error> {
return Future { promise in
Auth.auth().signIn(with: credentials) { (result, error) in
if let error = error {
//Crashlytics.crashlytics().record(error: error)
promise(.failure(error))
}
else if let user = result?.user {
//Crashlytics.crashlytics().setUserID(user.uid)
let profile = UserProfileCompactMapper.map(firebaseUser: user)
promise(.success(profile))
}
else {
fatalError()
}
}
}
}

Answer 1

You can use a .flatMap operator, which takes a value from upstream and produces a publisher. This would look something like below.
Note, that it's also better to return a type-erased AnyPublisher at the function boundary, instead of the specific publisher used inside the function
func loginwithFacebook() -> AnyPublisher<UserProfileCompact, Error> {
loginWithFacebook().flatMap { authCredential in
loginWithFirebase(authCredential)
}
.eraseToAnyPublisher()
}