Why .childAdded is not called when new data is added? Firebase - swift

Question

I am trying to read data from media when data is updated on /media node, but .observe(.childAdded is not called.
For example, I update data at /media/-LKN1j_FLQuOvnhEFfao/caption , but I never receive the event in observeNewMedia .
I can read the data with no problem the first time when ViewDidLoad completes.
The first step is to download the user data, second is to get the locality from currentUser and the last step is to attach a listener .childAdded on media.
I suspect that the event is not triggered because fetchMedia is called inside DDatabaseRReference.users(uid: uid).reference().observe(.value
media
-LKNRdP4ZsE3YrgaLB30
caption: "santa"
mediaUID: "-LKNRdP4ZsE3YrgaLB30"
locality: "barking"
users
Q6Dm3IMLNLgBH3ny3rv2CMYf47p1
media
-LKNReJCxgwtGRU6iJmV: "-LKNRdP4ZsE3YrgaLB30"
email: "john#gmail.com"
locality: "barking"
//enables the programmer to create references to different childs in Firebase
enum DDatabaseRReference {
case root
case users(uid:String)
case media //to store photos
func reference() -> DatabaseReference {
return rootRef.child(path)
}
//return root reference to our database
private var rootRef: DatabaseReference {
return Database.database().reference()
}
private var path: String {
switch self { //self is the enum DDatabaseReference
case .root:
return ""
case .users(let uid):
return "users/\(uid)"
case .media:
return "media"
}
}
}//end of enum DatabaseReference
class NewsfeedTableViewController: UITableViewController {
override func viewDidLoad() {
super.viewDidLoad()
//observe ~/users/uid
DDatabaseRReference.users(uid: uid).reference().observe(.value, with: { (snapshot) in
DispatchQueue.main.async {
if let userDict = snapshot.value as? [String : Any] {
self.currentUser = UserModel(dictionary: userDict)
self.fetchMedia()
self.tableView.reloadData()
}
}
})
}
func fetchMedia() {
Media.observeNewMedia((currentUser?.locality)!) { (newMedia) in
//check if newly downloaded media is already in media array
if !self.media.contains(newMedia) {
self.media.insert(newMedia, at: 0)
self.tableView.reloadData()
}else {
//remove old media and add the newly updated one
guard let index = self.media.index(of: newMedia) else {return}
self.media.remove(at: index)
self.media.insert(newMedia, at: 0)
self.tableView.reloadData()
}
}
}
}//end of NewsfeedTableViewController
class Media {
class func observeNewMedia(_ userLocality: String, _ completion: #escaping (Media) -> Void) {
DDatabaseRReference.media.reference().queryOrdered(byChild: "locality").queryEqual(toValue: userLocality).observe(.childAdded, with: { snapshot in
guard snapshot.exists() else {
print("no snap ")
return}
print("snap is \(snapshot)")
let media = Media(dictionary: snapshot.value as! [String : Any])
completion(media)
})
}
} //end of class Media

Answer 1

Let's first update the structure so make it more queryable
assume a users node
users
-Q6Dm3IMLNLgBH3ny3rv2CMYf47p1 //this is each users uid
email: "john#gmail.com"
locality: "barking"
and a media node that contains media for all users
media
-abcdefg12345 //node created with childByAutoId
caption: "santa"
for_uid: -Q6Dm3IMLNLgBH3ny3rv2CMYf47p1 //matches the uid in the /users node
Then our main viewController which contains a reference to Firebase and logs the user in
class ViewController: UIViewController {
var ref: DatabaseReference!
override func viewDidLoad() {
super.viewDidLoad()
self.ref = Database.database().reference()
//log user in which will populate the Auth.auth.currentUser variable
}
.
.
.
We need an object to store the media in and then an array to hold those objects
class MediaClass {
var key = ""
var caption = ""
init(k: String, c: String) {
self.key = k
self.caption = c
}
}
var mediaArray = [MediaClass]()
then set up the observers which will add, update or remove from the array when media for this user is added, changed or removed.
let thisUid = Auth.auth().currentUser?.uid
let mediaRef = self.ref.child("media")
let queryRef = mediaRef.queryOrdered(byChild: "for_uid").queryEqual(toValue: thisUid)
queryRef.observe(.childAdded, with: { snapshot in
let dict = snapshot.value as! [String: Any]
let key = snapshot.key
let caption = dict["caption"] as! String
let m = MediaClass.init(k: key, c: caption)
self.mediaArray.append(m)
self.tableView.reloadData()
})
queryRef.observe(.childChanged, with: { snapshot in
let dict = snapshot.value as! [String: Any]
let key = snapshot.key
let caption = dict["caption"] as! String
let index = self.mediaArray.index { $0.key == key } //locate this object in the array
self.mediaArray[index!].caption = caption //and update it's caption
self.tableView.reloadData()
})
//leaving this an an exercise
queryRef.observe(.childRemoved....
Note we added .childAdded, .childChanged and .childRemoved events to the media node via a query so the only events the app will receive are the ones that pertain to this user.
Also note there's no error checking so that needs to be added.