Update Firebase local data with observeSingleEvent - ios

Question

so I have a function to retrieve the user information from a given user id:
func getUserDataFrom(_ userID: String, completion: #escaping (_ userData: DBUser) -> Void) {
ref.child(usersTable).child(userID).observeSingleEvent(of: .value) { (snapshot) in
if let userDic = snapshot.value as? NSDictionary {
let userData = DBUser(with: userDic)
completion(userData)
}
}
}
The problem is that this returns the local data instead of reading from Firebase. I'd like to retrieve the data from the server (as long as there's internet connection) and only read from disk if it's not available.
I know that the easiest way to accomplish this would be using a listener, but I'm making a Today Extension and they use way too much memory increasing the chances of a crash.
I've also researched about keepSynced feature but since the database reference to the users table will have a lot of children I don't know if this will affect the memory of my extension.
Long story short: I'd like to read data from Firebase once, and only read from disk if there isn't internet connection with the minimum memory usage possible.
Thank you in advance.

Answer 1

I retrieve some explanation, I think it might help you in your case :
ObserveSingleEventType with keepSycned will not work if the Firebase
connection cannot be established on time. This is especially true
during appLaunch or in the appDelegate where there is a delay in the
Firebase connection and the cached result is given instead. It will
also not work at times if persistence is enabled and
observeSingleEvent might give the cached data first. In situations
like these, a continuous ObserveEventType is preferred and should be
used if you absolutely need fresh data.
I think you don't have the choice to use a continuous listener. But to avoid performance issues why you don't remove yourself your listeners when you don't it anymore.

Answer 2

In the fresh project I created and added your code, it retrieves data from Firebase when there's a connection and when not, from local storage. Because of that, we conclude the above code is correctly fetching Firebase data from their server.
However, in my experience observeSingleEvent and offline persistence has been a tad intermittent (perhaps a 'feature'?). To fix it, force the data at the reference to stay sync'd
let usersTableRef = Database.database().reference(withPath: usersTable)
let thisUsersTableRef = usersTableRef.child(userId)
thisUsersTableRef.keepSynced(true)
//optional: thisUsersTableRef.child("temp").setValue(true)
thisUsersTableRef.observeSingleEvent(of: .value)
See Offline Capabilities for a bit more info and further examples.
Also see this post from 2015 for some insight on observers/listeners.