Sometimes the implementation does not work when the application is disconnected - ios

Question

I want to delete some data in google firesore when the app disconnects.
However, the following code does not work correctly.
Also, it works correctly in the simulator, but not on the actual machine.
import Firebase
import FirebaseFirestore
class SceneDelegate: UIResponder, UIWindowSceneDelegate {
let db = Firestore.firestore()
var window: UIWindow?
func sceneDidDisconnect(_ scene: UIScene) {
db.collection("parentCol").document("doc").collection("col_1").document("doc_1").delete()
db.collection("parentCol").document("doc").collection("col_2").document("doc_2").delete()
db.collection("parentCol").document("doc").delete()
}
}
What I expect
(I am producing a game application.)
The host determines the roomId and produces a room.
Guests connect to the room via that roomId.
When the game ends or the player disconnects from the application, server data that is no longer needed is deleted.
//data structure
col: rooms
|
|- doc: 1234 (roomId)
| |
| |- col: members
| |
| |- doc: name ( here I store an array of String)
| |- 1: Tom
| |- 2: Ares
| |- 3: Michael
|
|- doc: abc (roomId)
　
What I tried
It worked fine in the simulator.
However, when I tried it on the actual device, it did not work correctly.
Only doc_1 was deleted in the following code.↓
func sceneDidDisconnect(_ scene: UIScene) {
db.collection("parentCol").document("doc").collection("col_1").document("doc_1").delete()
db.collection("parentCol").document("doc").delete()
}
Only doc was deleted in the following code.↓
func sceneDidDisconnect(_ scene: UIScene) {
db.collection("parentCol").document("doc").delete()
}
Nothing is deleted when the following code.↓
func sceneDidDisconnect(_ scene: UIScene) {
db.collection("parentCol").document("doc").collection("col_1").document("doc_1").delete()
db.collection("parentCol").document("doc").collection("col_2").document("doc_2").delete()
db.collection("parentCol").document("doc").delete()
}
　
I know that after an app is disconnected, it will only run for a few seconds. But that should be more than enough time to delete data.
If you have any solutions, please let me know.
My native language is not English, so sorry if it is hard to read.

Answer 1

I think what happens on simulator (i.e. sceneDidDisconnect is a few seconds delayed) is rather an anomaly, while the behavior you see on device is correct (i.e. sceneDidDisconnect is almost instantaneous).
I recommend that you re-read Managing your app’s life cycle. It states that scene disconnection is for cleanup, while any data operations should be done prior to that when the app is about to leave the foreground-active state:
Upon leaving the foreground-active state, save data and quiet your app’s behavior. See Preparing your UI to run in the background.
Upon entering the background state, finish crucial tasks, free up as much memory as possible, and prepare for your app snapshot. See Preparing your UI to run in the background.
At scene disconnection, clean up any shared resources associated with the scene.
So use sceneWillResignActive(_:) to perform the deletion instead. Furthermore: as Apple states -
Don’t rely on specific app transitions to save all of your app's critical data.
So if you can do these deletions on some other app events, do that, and only use app events as a backup. And also make sure your app handles a situation where the data may still exist on next invocation of the app (because if the app is killed in some more brutal ways may never allow any of the shutdown functions to run).