Firebase Query snapshot nil?

Question

I\'m creating a simple chat app to learn Swift and Firebase. I have a query that checks for a chat room\'s messages to load into a tableView. The query returns a snapshot bu

Answer 1

Your dictionary look like :

▿ 1 element
  ▿ 0 : 2 elements
    - key : "-KotqLUUucaRagTRt967"
    ▿ value : 3 elements
      ▿ 0 : 2 elements
        - key : sender
        - value : eGTYRSo81JefgasYLRHUFHUTnEC3
      ▿ 1 : 2 elements
        - key : text
        - value : test
      ▿ 2 : 2 elements
        - key : timestamp
        - value : 1499914135546

But in fact it should look like (If you want to access to dictionary["sender"]) :

▿ 2 elements
  ▿ 0 : 2 elements
    - key : sender
    - value : eGTYRSo81JefgasYLRHUFHUTnEC3
  ▿ 1 : 2 elements
    - key : text
    - value : test
  ▿ 2 : 2 elements
    - key : timestamp
    - value : 1499914135546

First solution :

You need to add .child("-KotqLUUucaRagTRt967") in your query.

OR

Second solution :

You need to do something like that :

query.observe(.childAdded, with: { snapshot in
    for child in snapshot.children {
        guard let value = child.value as? NSDictionary else {
            return
        }

        guard let sender = value["sender"] as? String else {
            return
        }

        // You can user the sender
    }
})

UPDATE :

query.observe(.childAdded, with: { snapshot in
    for child in snapshot.children.allObjects as! [FIRDataSnapshot] {
        if let value = child.value as? [String:Any], let sender = value["sender"] as? String {  
            // You can user the sender
        }
    }
})

NOTE

I replaced observeSingleEvent with observe, as Frank van Puffelen said, it's an uncommon combination for .childAdded.

Answer 2

I do not think you want a ChildAdded handler, so I go with a observeSingleEvent example since you want to query data from the database at that time without triggers. When you use a observeSingleEvent, it is important to keep the database in sync. I would recommend using the code below:

query.keepSynched(true) //keeps data in sync with database, if you have data persistince on in your appDelegate
query.observeSingleEvent(of: .value, with: { (snapshot) in //notice the changed here
            print(snapshot)
            //Since you want to loop again because there could be multiple
            //messages in that chatroom which all have a unique ID, do this loop:
            let enumerator = snapshot.children
            while let rest = enumerator.nextObject() as? FIRDataSnapshot {
               //this is 1 single message here
               let values = rest.value as? NSDictionary
               for (key, value) in values{
               print("Key: \(key), value: \(value)") 
               }
               //lets say you want to check if there is a value with a key named "text":
            let textUser = values?["text"] as? String ?? "No text found" 
               //providing a default value if there is no text. You could leave it empty, than it is nil (not recommend)
               //Not only is this a lot more readable, I do not use force unwrapping so your app 
               //cannot cause an exception.

               //Get more values from the dictionary as I did with textUser
               //Initialize message than here. You will see you do not need to force unwrap anything :D
               }

            }

        })

Answer 3

As Pipiks explained, your issue is that you are attempting to access individual message details at one level higher than the returned data.

The snapshot value is giving you a dictionary of chat messages, where the top level's keys are the chat message keys. To map your returned data to an array of messages, I would use the following code:

query.observeSingleEvent(of: .childAdded, with: { snapshot in

    guard let messagesDict = snapshot.value as? [String: AnyObject] else { return }

    self.messages = messagesDict.flatMap({ (messageId: String, messageData: Any) -> Message? in
        guard
            let sender = messageData["sender"] as? String,
            let text = messageData["text"] as? String,
            let timestamp = messageData["timestamp"] as? Int,
            let message = Message(key: messageKey, sender: sender, text: text, timestamp: timestamp) else {
                return nil
        }
        return message
    })

    self.tableView.reloadData()
})

What this does is map your dictionary of messages to an array of Message objects.

I have used a flatMap to filter out any messages which do not have sender, text or timestamp values (so the flatMap returns a [Message] object).

Does that solve the issue?