Dispatch Queue Async Call

Question

I am firing off a network request inside a for loop that is within another network request. I'm using Core Data but I am fairly certain this is not a Core Data issue, and is an async issue.

The 2 print statements inside the the Firebase request print the data properly, but without the DispatchQueue the array returns as empty (before the network request completes).

Here's a picture of the crash:

Here is the code itself:

var userReps = [UserRepresentation]()

// Fetch all Friends -> update Core Data accordingly
func fetchFriendsFromServer(completion: @escaping (Error?) -> Void = { _ in}){
    let backgroundContext = CoreDataStack.shared.container.newBackgroundContext()

    // 1. Fetch all friends from Firebase
    FirebaseDatabase.UserDatabaseReference.child(CoreUserController.shared.userPhoneNumber).child(UserKeys.UserFriends).child(UserKeys.UserAcceptedFriends).observe(.value) { (data) in
        if let dictionary = data.value as? [String: String] {
            var userReps = [UserRepresentation]()
            let group = DispatchGroup()

            group.enter()
            for friend in dictionary {
                let friendName = friend.value
                let friendId = friend.key

                FirebaseDatabase.UserDatabaseReference.child(friendId).observe(.value, with: { (data) in
                    if let dictionary = data.value as? [String: Any] {
                        guard let gender = dictionary[UserKeys.UserGender] as? String else {return}
                        guard let bio = dictionary[UserKeys.UserBio] as? String else {return}
                        guard let status = dictionary[UserKeys.UserStatus] as? String else {return}
                        guard let avatarUrl = dictionary[UserKeys.UserAvatarUrlKey] as? String else {return}

                        let friendRepresentation = UserRepresentation(avatarUrl: avatarUrl, name: friendName, number: friendId, gender: gender, status: status, bio: bio)
                        userReps.append(friendRepresentation)
                        print("HERE, friends fetched: ", friendRepresentation)
                        print("HERE, reps fetched: ", userReps)
                        group.leave()
                    }
                })
            }

            group.notify(queue: .main) {
                // 2. Update Core Data value with Firebase values
                self.updateFriends(with: userReps, in: backgroundContext)

                // 3. Save Core Data background context
                do {
                    try  CoreDataStack.shared.save(context: backgroundContext)
                } catch let error {
                    print("HERE. Error saving changes to core data: \(error.localizedDescription)")
                }
            }
        }
    }
}

Any help would go a long way

Shehata Gamal · Accepted Answer · 2019-07-12 00:43:58Z

1

Since

let group = DispatchGroup()

is a local variable and you use observe here

FirebaseDatabase.UserDatabaseReference.child(friendId).observe(.value, with: { (data) in

it will re-call it after function deallocation either make it an instance variable or make this single observe

 FirebaseDatabase.UserDatabaseReference.child(friendId).observeSingleEvent(of:.value) { (data) in

Also make enter inside the for loop

for friend in dictionary {  
  group.enter()

edited Jul 12, 2019 at 0:43

answered Jul 12, 2019 at 0:07

Shehata Gamal

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1 Comment

Ryan Over a year ago

It would have taken a long time to notice observe was causing the issue. Thank you so much

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