IOS Swift Nested DispatchGroup for Nested Network requests handling arrays - ios

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.