Using Firebase Realtime Database with SwiftUI List - Clearing a list - swift

Question

I'm wondering what's the most effective way to clear a list's data that's presented in a View via an observable object? At the moment, everytime I reload the view the data gets duplicated (as we are checking the query for updates and parsing the responses). This is not the expected behaviour. The expected behaviour would be to only update the #Published property "if" the database indicates that a new notification has been received.
I know the culprit is the code within the .onAppear block - I'm just not sure architecturally how I might solve this. How could I use this listener while only parsing new data, not data that was previously parsed?
I attempted to clear the list .onAppear, but resulted in a crash indicated that I tried to delete a section while there were already 0 sections so that didn't work.
I've thought of possibly providing the Message object with a Static Unique ID to upload with the Message object when sending to Firebase (Or using the firebase key itself). That way I could use a set of dictionary objects using the unique ID to identify the object in the dictionary. This may help me avoid duplicate entries.
struct Updates: View {
#ObservedObject var dataController = DataController()
var body: some View {
ZStack {
VStack {
List {
ForEach(self.dataController.messages, id: \.id) { message in
Text(message.message)
}
}
}.onAppear {
self.dataController.query.observe(.childAdded) { snapshot in
let data = JSON(snapshot.value as Any)
if let message = Message.parseFirebaseQuery(dataJSON: data){
self.dataController.messages.append(message)
}
}
}
}
}
}
class DataController: ObservableObject {
#Published var query = ChatPathsAndReferences.refs.databaseMessages.queryLimited(toLast:100).queryOrdered(byChild: "date")
#Published var messages = [Message]()
}

Answer 1

I resolved this by adding an init block to my ObservableObject
class DataController: ObservableObject {
#Published var query = ChatPathsAndReferences.refs.databaseMessages.queryLimited(toLast:100).queryOrdered(byChild: "date")
#Published var messages = [Message]()
init() {
self.query.observe(.childAdded) { snapshot in
let data = JSON(snapshot.value as Any)
if let chatMessage = ChatMessage.parseFirebaseQuery(dataJSON: data){
self.messages.append(chatMessage)
}
}
}
}
So that now, when I create the view, the object initialises the observer in the data controller rather than the view.
struct Updates: View {
// As you can see, we initialise the data controller which inits the observer.
// Since the messages in the data controller are #Published,
// we don't need to worry about checking for updates again in the view
#ObservedObject var chatDataController: DataController = DataController()
var body: some View {
GeometryReader { geometry in
ZStack {
Color("Background")
VStack {
// messages
List {
ForEach(self.chatDataController.messages, id: \.id) { message in
Text(message.message)
}
}
}
}
}
}