IOS Swift Nested DispatchGroup for Nested Network requests handling arrays

Question

I'm experiencing crashes and I'm not too sure how to handle the situation with nested dispatchgroup inside a dispatchgroup. I know I'm doing something wrong and getting crashes and would like some help with how to handle below situation:

I am using IOS Swift and Firebase and basically grabbing relevant mutual friends by first grabbing a friendList, and then grabbing the friends of each of the friends on my friendList (as those are my mutual friends), if I have not grabbed them earlier (I use a list to track ids of friends Ive already grabbed), I send another network request to fb to grab the number of mutual friends between current user and mutual friend and check if they are relevant enough to be added.

However I have another request after that grabs school friends from firebase and I need to make sure there arent duplicate entries because there are school friends that are also mutual friends. I'm using Dispatch groups like so:

// Iterates through friendList to grab mutual friends
        for user in currUser.friendList {
            // Grabs user friend list
            let userFriendListRef = Database.database().reference().child("friend-list").child(user.userID)
            userFriendListRef.observeSingleEvent(of: .value, with: { (snapshot) in
                guard snapshot.exists(),
                    let userFriendList = snapshot.value as? [String: Any] else {
                        logger.info("No mutual friends grabbed from friend")
                        return
                }

                // Mutual friends dispatchGroup
                self.mutualFriendsDispatchGroup.enter()

                // If exists friends, then see if matches user's interest
                self.filterMutualFriendsToMatchUserInterest(using: userFriendList)
            })
        }

        self.mutualFriendsDispatchGroup.notify(queue: .main) {
            logger.info("Done mutual friends")
        }

// Checks if mutual friend matches interest and then adds it into collectionView
fileprivate func filterMutualFriends(using userFriendList: [String: Any]) {
// Maintains a counter
var searchedMutualFriendCounter = 0

// Iterates through userFriendList
for (userID, _) in userFriendList {
    searchedMutualFriendCounter += 1 // Increments counter

    // Ensures not repeating a mutual friend
    guard usersAddedToHomeScroll[userID] == nil,
        searchedUsers[userID] == nil,
        !blockedUsers.contains(userID) else {

            // Handles mutual friend dispatch group leave condition
            if searchedMutualFriendCounter == userFriendList.count {
                self.mutualFriendsDispatchGroup.leave()
                return
            }

            continue
    }

    searchedUsers[userID] = true
    grabFriendsDispatchGroup.enter()

    // Checks if has enough mutual friends, if yes, grab mutual friend data, else skip
    checkIfFriendHasEnoughMutualFriends(userID) { (result) -> Void in
        // Makes sure that has enough mutual friends
        guard result else {
            logger.info("Not enough mutual friends to show in userFriendScroll for \(userID)")
            self.grabFriendsDispatchGroup.leave()

            // Handles mutual friend dispatch group leave condition
            if searchedMutualFriendCounter == userFriendList.count {
                self.mutualFriendsDispatchGroup.leave()
            }

            return
        }
        logger.info("Mutual friend ID grabbed for \(userID)")
        self.grabMutualFriendData(userID, index: searchedMutualFriendCounter, total: userFriendList.count)
    }

}
}

  fileprivate func getAllFriends() {

    // Grabs mutual friends
    getMutualFriends()

    // Gets school friends
    getSchoolFriends()

// Reloads data after grabbing it all
grabFriendsDispatchGroup.notify(queue: .main) {
    self.collectionView.reloadData()
}
}

I also call mutualFriendsDispatchGroup.leave() in grabMutualFriendData(...) method.

I apologize for the large amount of code, I was trying to figure out basically how to put in sync lots of network requests nest in a network request to grab mutual friends and before my grab school friends so that I dont get duplicate entries on my collectionView presenting the grabbed users.

Note: The counter thing in filterMutualFriends(...) is a hack I was attempting that would exit out of the outer dispatchgroup once you've iterated through the friendlist of a friend. The outer mutual friends dispatchGroup is the one crashing.

Answer 1

Could not figure out a proper long-term solution to fix the issue, so I had to hack around it and use a bad workaround which just removes duplicate users everytime a new user is grabbed and then reloads the collectionView. However, note that this will and can cause problems in the code.