Removing value from local Dictionary when firebase child deleted swift

Hello I am learning on how to properly use firebase realtime database. I am building an app where user can change their work locations (base). in my profile settings controller user choose their base via a pickerView. The real time database gets updated properly. I have the list of all users and seperate list per base . Where is get stuck is on the main view controller.
I have collection view that display a list of user of my own location.
My goal is to see who comes to my location or depart my location.
I have 2 functions one that fetch my own user datas . which determine the current base i want to dispaly

func fetchCurrentUser() {
        
        if currentUser == nil {
            common.errorAlert("Could not retrieve user details.")
            return
        }
        
        guard let userID = Auth.auth().currentUser?.uid else { return }
        
        ref.child(userID).observeSingleEvent(of: DataEventType.value) { (snapshot) in
            if !snapshot.exists() { return }
            let userDetailsData = snapshot.value as? NSDictionary
            self.id = userDetailsData?["id"] as? String ?? ""
            self.myBase = userDetailsData?["base"] as? String ?? ""
            self.indexBasePicker = userDetailsData?["baseIndex"] as? Int ?? 0
            self.headerPosition(self.indexBasePicker)
            if self.myBase != "" {
                self.fetchAllUsers()
            }
        }
    }

The second fetchallusers at my location:

func fetchAllUsers() {
        
        ref.child(myBase).observe(.childAdded, with: { (snapshot) in
            
            if let dictionary = snapshot.value as? [String: AnyObject] {
                let user = User(dictionary: dictionary)
                if  user.isAdmin == true {
                    self.usersArray.append(user)
                    self.reloadTeamCollectionView()
                }
            }
        })
        
        ref.child(myBase).observe(.childRemoved, with: { (snapshot) in
            
            if let dictionary = snapshot.value as? [String: AnyObject] {
                let user = User(dictionary: dictionary)
                
                for (index, userArray) in self.usersArray.enumerated() {
                    if userArray.id == user.id {
                        print(index)
                        self.usersArray.remove(at: index)
                        self.reloadTeamCollectionView()
                    }
                }
            }
        })
        
    }

the problem is when someone join my current location it gets added to the list , sometimes severaltimes. and when someone departs some time the app crashes with index out of range error

any help would be tremendously appreciated

.
"users" : {
"BdxqjmhjM0O3nV8uK9iC3HICHy32" : {
"availableStatus" : false,
"base" : "FRA",
"baseIndex" : 4,
"committee" : "★",
"email" : "",
"fBMessenger" : "",
"fileNumber" : "",
"iMessage" : "",
"id" : "BdxqjmhjM0O3nV8uK9iC3HICHy32",
"isAdmin" : true,
"isOnline" : true,
"name" : "",
"phone" : "",
"position" : "",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/BdxqjmhjM0O3nV8uK9iC3HICHy32%2Fprofile_images%2FprofilePhoto?alt=media&token=e898f36f-7c3a-42a3-be4b-0a1b74cbd224",
"whatsapp" : ""
},
"DKfHhnwMgbOfGg3IpqG9QxlayxV2" : {
"availableStatus" : false,
"base" : "FRA",
"baseIndex" : 4,
"committee" : "★ Officers",
"email" : "",
"fBMessenger" : "",
"fileNumber" : "",
"iMessage" : "",
"id" : "DKfHhnwMgbOfGg3IpqG9QxlayxV2",
"isAdmin" : true,
"isOnline" : false,
"name" : "",
"phone" : "",
"position" : "",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/DKfHhnwMgbOfGg3IpqG9QxlayxV2%2Fprofile_images%2FprofilePhoto?alt=media&token=204f4187-b508-405e-9835-bfdf5250019f",
"whatsapp" : ""
},
"GMupdPogn7YUag855TnWJkuzVzo1" : {
"availableStatus" : "Online",
"base" : "FRA",
"baseIndex" : 4,
"committee" : "Eap",
"email" : "lisa@lisa.com",
"fBMessenger" : "",
"fileNumber" : "u182874",
"iMessage" : "",
"id" : "GMupdPogn7YUag855TnWJkuzVzo1",
"isAdmin" : true,
"name" : "",
"phone" : "",
"position" : "",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/GMupdPogn7YUag855TnWJkuzVzo1%2Fprofile_images%2FprofilePhoto?alt=media&token=8b3ec821-adad-47a9-943f-2cd4af50e436",
"whatsapp" : ""
},
"LS8plMZmlmSXsu9kVZxXRKsuPT13" : {
"availableStatus" : true,
"base" : "FRA",
"baseIndex" : 4,
"committee" : "Communication",
"email" : ",
"fBMessenger" : "",
"fileNumber" : "",
"iMessage" : "",
"id" : "LS8plMZmlmSXsu9kVZxXRKsuPT13",
"isAdmin" : true,
"isOnline" : true,
"name" : "",
"phone" : "",
"position" : "Chair",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/LS8plMZmlmSXsu9kVZxXRKsuPT13%2Fprofile_images%2FprofilePhoto?alt=media&token=10ec728b-db42-485a-849d-0e2bcc0b3ba3",
"whatsapp" : "+33642056110"
},
"sqfl1FmeiiOuoq8dPePULYDEsnv2" : {
"availableStatus" : true,
"base" : "FRA",
"baseIndex" : 4,
"committee" : "",
"email" : "nonadmin@test.com",
"fBMessenger" : "",
"fileNumber" : "u182222",
"iMessage" : "",
"id" : "sqfl1FmeiiOuoq8dPePULYDEsnv2",
"isAdmin" : false,
"isOnline" : false,
"name" : "Nonadmin Test",
"phone" : "",
"position" : "",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/sqfl1FmeiiOuoq8dPePULYDEsnv2%2Fprofile_images%2FprofilePhoto?alt=media&token=d4cfa840-954e-41fa-8490-23a5cd49fa56",
"whatsapp" : ""
}
},
"usersOffline" : {
"DKfHhnwMgbOfGg3IpqG9QxlayxV2" : {
"base" : "FRA",
"baseIndex" : 4,
"committee" : "★ Officers",
"email" : "merv",
"fBMessenger" : "",
"fileNumber" : "",
"iMessage" : "",
"id" : "",
"isAdmin" : true,
"isOnline" : false,
"name" : "Merv",
"phone" : "",
"position" : "Vice President",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/DKfHhnwMgbOfGg3IpqG9QxlayxV2%2Fprofile_images%2FprofilePhoto?alt=media&token=204f4187-b508-405e-9835-bfdf5250019f",
"whatsapp" : ""
},
"sqfl1FmeiiOuoq8dPePULYDEsnv2" : {
"base" : "FRA",
"baseIndex" : 4,
"committee" : "",
"email" : "nonadmin@test.com",
"fBMessenger" : "",
"fileNumber" : "u182222",
"iMessage" : "",
"id" : "sqfl1FmeiiOuoq8dPePULYDEsnv2",
"isAdmin" : false,
"isOnline" : false,
"name" : "Nonadmin Test",
"phone" : "",
"position" : "",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/sqfl1FmeiiOuoq8dPePULYDEsnv2%2Fprofile_images%2FprofilePhoto?alt=media&token=d4cfa840-954e-41fa-8490-23a5cd49fa56",
"whatsapp" : ""
}
},
"usersOnline" : {
"BdxqjmhjM0O3nV8uK9iC3HICHy32" : {
"base" : "FRA",
"baseIndex" : 4,
"committee" : "★ Officers",
"email" : "",
"fBMessenger" : "",
"fileNumber" : "",
"iMessage" : "",
"id" : "",
"isAdmin" : true,
"isOnline" : true,
"name" : "",
"phone" : "",
"position" : "",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/BdxqjmhjM0O3nV8uK9iC3HICHy32%2Fprofile_images%2FprofilePhoto?alt=media&token=e898f36f-7c3a-42a3-be4b-0a1b74cbd224",
"whatsapp" : "+31620422185"
},
"LS8plMZmlmSXsu9kVZxXRKsuPT13" : {
"base" : "FRA",
"baseIndex" : 4,
"committee" : "Communication",
"email" : "",
"fBMessenger" : "",
"fileNumber" : "",
"iMessage" : "",
"id" : "LS8plMZmlmSXsu9kVZxXRKsuPT13",
"isAdmin" : true,
"isOnline" : true,
"name" : "",
"phone" : "",
"position" : "Chair",
"profileImageUrl" : "https://firebasestorage.googleapis.com/v0/b/myunion-16f31.appspot.com/o/LS8plMZmlmSXsu9kVZxXRKsuPT13%2Fprofile_images%2FprofilePhoto?alt=media&token=10ec728b-db42-485a-849d-0e2bcc0b3ba3",
"whatsapp" : "+33642056110"
}
}
}

i have 2 references

  let ref = Database.database().reference(withPath: "users")
let refOnline = Database.database().reference(withPath: "usersOnline")
let refOffline = Database.database().reference(withPath: "usersOffline")

this is how the user profile gets updated whe the save button is pressed

   func updateDatasIsOnline(online: Bool) {
if currentUser == nil {
//      common.errorAlert("Error could not read user details")
print("Error could not read user details")
return
}
let filename = "profilePhoto"
Storage.storage().reference().child(self.currentUser?.uid ?? "").child("profile_images").child(filename).downloadURL { (url, error) in
guard let downloadURL = url else {
common.errorAlert(error?.legibleLocalizedDescription ?? "")
return }
let userValues = [
"id": self.currentUser?.uid ?? "",
"name": self.nameTextField.text ?? "",
"committee": self.selectedCommitteeName,
"position": self.positionTextField.text ?? "",
"base": self.myBase,
"baseIndex": self.basePickerPosition,
"isAdmin": self.isAdmin,
"isOnline": online,
"email": self.emailTextField.text ?? "",
"phone": self.reformatPhoneNumberForDataBase(self.phoneNumberTextField.text ?? ""),
"iMessage": self.iMessageTextField.text ?? "",
"whatsapp": self.reformatPhoneNumberForDataBase(self.whatsappTextField.text ?? ""),
"fBMessenger": self.fBMessengerTextField.text ?? "",
"profileImageUrl": downloadURL.absoluteString,
"fileNumber": self.fileNbr
] as [String : Any]
if online == true {
self.ref.child(self.currentUser?.uid ?? "").updateChildValues(["isOnline" : true])
self.refOnline.child(self.currentUser?.uid ?? "").setValue(userValues)
self.refOffline.child(self.currentUser?.uid ?? "").removeValue()
} else {
self.ref.child(self.currentUser?.uid ?? "").updateChildValues(["isOnline" : false])
self.refOffline.child(self.currentUser?.uid ?? "").setValue(userValues)
self.refOnline.child(self.currentUser?.uid ?? "").removeValue()
}
}
}

It seems line you are moving a lot of data around within the database. That can get expensive but also consumes a lot of bandwidth. In other words, with the current code, if a users status changes from online to offline, your code is currently removing all of the user data from one node and re-writing it to another when all that's really needed is to toggle an online flag from yes to no.

I present another option that may help in the long run and while it's not troubleshooting your existing code, it may be a better option.

The idea here is to keep the user data in one place and simply change the child nodes to reflect a different status; a different base, online/offline etc.

To do that, that if a property (child node) falls within the criteria of a query, you'll be notified. If it falls out of the criteria you'll be notified.

Here's what a proposed user node would look like. Note there are no offline or online nodes, just the users node

users
uid_0 //a users uid
base: "FRA"
name: "Frank"
online: "no"
uid_1
base: "ENG"
name: "Henry"
online: "yes"
uid_0 
base: "FRA"
name: "Leroy"
online: "yes"

From this you know that Frank is stationed in France but not online, Henry is in England and online and Leroy is also in France and online.

Here's my users class and an array to store them in

class UserAndBaseClass {
var uid = ""
var name = ""
var base = ""
var online = ""
init(withSnap: DataSnapshot) {
self.uid = withSnap.key
self.name = withSnap.childSnapshot(forPath: "name").value as? String ?? "No Name"
self.base = withSnap.childSnapshot(forPath: "base").value as? String ?? "No Base"
self.online = withSnap.childSnapshot(forPath: "online").value as? String ?? "Offline"
}
}
var usersArray = [UserAndBaseClass]()

In the first section of code, addedQuery, queries all of the users that belong to the current users base, FRA and stores them in an array. This leaves an observer on the users node so if any users base becomes FRA, it will then fall within that query's parameters and is added to the query. (hence firing the .childAdded event)

The second section, removedQuery, establishes a query on all users that match the query and the event is fired if the base becomes something other than FRA - e.g. the child is removed from the query parameters and the .childRemoved event fires.

Note we are not using .childChanged at all since the first two queries cover it.

func observeUserBases() {
let currentUserBase = "FRA"
//get everyone in the current users base, FRA. This also fires if a user changes to this base
let usersRef = self.ref.child("users")
let addedQuery = usersRef.queryOrdered(byChild: "base").queryEqual(toValue: currentUserBase)
addedQuery.observe(.childAdded, with: { snapshot in
let user = UserAndBaseClass(withSnap: snapshot)
self.usersArray.append(user)
print("\(user.name) has been added to your base")
})
//this will catch any users that were removed as well as users who left your base
let removedQuery = usersRef.queryOrdered(byChild: "base").queryEqual(toValue: "FRA")
removedQuery.observe(.childRemoved, with: { snapshot in
let keyToRemove = snapshot.key
if let index = self.usersArray.firstIndex(where: { $0.uid == keyToRemove } ) {
let name = self.usersArray[index].name
print("\(name) has left your base")
self.usersArray.remove(at: index)
}
})
let onlineQuery = usersRef.queryOrdered(byChild: "online").queryEqual(toValue: "yes")
onlineQuery.observe(.childAdded, with: { snapshot in
let keyOfChanged = snapshot.key
let base = snapshot.childSnapshot(forPath: "base").value as? String ?? "No Base"
if base == currentUserBase {
if let index = self.usersArray.firstIndex(where: { $0.uid == keyOfChanged } ) {
let name = self.usersArray[index].name
print("\(name) is now online")
self.usersArray[index].online = "yes"
}
}
})
let offlineQuery = usersRef.queryOrdered(byChild: "online").queryEqual(toValue: "no")
offlineQuery.observe(.childAdded, with: { snapshot in
let keyOfChanged = snapshot.key
let base = snapshot.childSnapshot(forPath: "base").value as? String ?? "No Base"
if base == currentUserBase {
if let index = self.usersArray.firstIndex(where: { $0.uid == keyOfChanged } ) {
let name = self.usersArray[index].name
print("\(name) has gone offline")
self.usersArray[index].online = "no"
}
}
})
}

The last two sections, online and offline query, are structured a little different; each is looking for nodes that match the query parameters. One is looking for nodes that match online = yes, the offline one matches nodes that match offline = no. Noting that the code is actually watching ALL users but ignoring users that are not in the current users base.

An alternative to this is to add individual observers to each users/online node that's part of this base. You would do that in the addedQuery closure and add an observer to each users node as it's added, and then in the removeQuery, remove the observer as the user leaves the base. I will leave that to an exercise.

Hope this helps by presenting an alternative to moving all of that data around.