Firebase Sorting - ios

Question

Basically, I have an app on Firebase. The thing is, when Firebase sorts the data, instead of a chronological order, it muddles the data.
When I went online and search why, I found that it was because I was using the snapshot.value instead of snapshot.children.
However, I'm not completely sure how to change the code accordingly, could someone help?
Here is the code:
func retrieveChatLog() {
FIRDatabase.database().reference().child("chats").child(chatID).observe(.value, with: {(snapshot) in
let chats = snapshot.value as! [String : AnyObject]
self.messages.removeAll()
for (_, value) in chats {
if let sender = value["sender"], let message = value["message"], let senderID = value["senderUID"], let date = value["date"] {
let messageToShow = Message()
messageToShow.message = message as! String
messageToShow.sender = sender as! String
messageToShow.senderUID = senderID as! String
let formatter = DateFormatter()
formatter.dateFormat = "dd.MM.yyyy"
let curDate = formatter.date(from: date as! String)
messageToShow.date = curDate as! Date
if messageToShow.senderUID != "" {
self.messages.append(messageToShow)
}
}
}
self.tableView.reloadData()
})
FIRDatabase.database().reference().removeAllObservers()
}

Answer 1

The problem isn't because you use snapshot.value. You can use .value or .childAdded. The problem is, if you use .value, you don't want to cast the snapshots to a dictionary because dictionaries don't preserve order. Instead, you're going to want to cast to an array to preserve order. Here's one way you could resolve this:
FIRDatabase.database().reference().child("chats").child(chatID).observe(.value, with: {(snapshot) in
let chats = snapshot.children.allObjects as! [FIRDataSnapshot]
self.messages.removeAll()
for chat in chats {
if let value = chat.value as? [String: AnyObject] {
if let sender = value["sender"], let message = value["message"], let senderID = value["senderUID"], let date = value["date"] {
let messageToShow = Message()
messageToShow.message = message as! String
messageToShow.sender = sender as! String
messageToShow.senderUID = senderID as! String
let formatter = DateFormatter()
formatter.dateFormat = "dd.MM.yyyy"
let curDate = formatter.date(from: date as! String)
messageToShow.date = curDate as! Date
if messageToShow.senderUID != "" {
self.messages.append(messageToShow)
}
}
}
}
self.tableView.reloadData()
})
FIRDatabase.database().reference().removeAllObservers()
}

Answer 2

Change this line
FIRDatabase.database().reference().child("chats").child(chatID).observe(.value, with: {(snapshot) in...
To this
FIRDatabase.database().reference().child("chats").child(chatID).observe(.childAdded, with: {(snapshot) in

Answer 3

There is an accepted answer but I wanted to provide another solution that may be a bit tighter and more expandable.
It starts with a class to hold the messages. In this case I am keeping just the firebase key and timestamp but other vars could easily be added. You'll note the timestamp var that is read from Firebase in the same format: dd.mm.yy. This would be useful for sorting if needed. Also note that if you want to display a nicely formatted mm/dd/yyyy format, it's available though the formattedDate computed property.
The messagesArray is an array of MessageClass objects which can be used as a datasource for a tableView for example.
Finally, the loadMessages function to load in all of the messages. As mentioned in the other answer, casting the Snapshot to a dictionary loses ordering guarantee. However, if we iterate over the snapshot directly, the ordering stays intact.
class MessageClass {
var timestamp: String
var fbKey: String
var formattedDate: String {
let formatter = DateFormatter()
formatter.dateFormat = "dd.MM.yyyy"
let d = formatter.date(from: timestamp)
let outputFormattter = DateFormatter()
outputFormattter.dateFormat = "MM/dd/yyyy"
let finalDate = outputFormattter.string(from: d!)
return finalDate
}
init(withSnap: DataSnapshot ) {
let snapDict = withSnap.value as! [String: AnyObject]
self.timestamp = snapDict["timestamp"] as! String
self.fbKey = withSnap.key
}
}
var messagesArray = [MessageClass]()
func doButton0Action() {
let messagesRef = self.ref.child("messages")
messagesRef.observe(.value, with: { snapshot in
for child in snapshot.children {
let snap = child as! DataSnapshot
let m = MessageClass(withSnap: snap)
self.messagesArray.append(m)
}
for msg in self.messagesArray { //test code to print the array once loaded
print("key: \(msg.fbKey) date: \(msg.formattedDate)")
}
})
}
This code is very verbose and could be condensed considerably but it's very readable. (it needs guards and error checking as well).
I would strongly encourage you to store your timestamps in a
yyyymmddhhmmss
format in firebase. It lends itself to sorting / querying
Also, as a side-note, instead of relying on the date the node was created (by key) to keep your ordering, consider leveraging the timestamp when reading in the nodes using .order(by: timestamp). That will guarantee they are always in the correct order even if messages are changed around or the keys are modified.