iOS Setup

import FirebaseFirestore

let db = Firestore.firestore()

struct User: Codable {
    @DocumentID var id: String?
    var firstName: String
    var lastName: String
    var born: Int
}

let user = User(firstName: "Ada", lastName: "Lovelace", born: 1815)

do {
    // Add a new document with a generated ID using Codable
    let ref = try db.collection("users").addDocument(from: user)
    print("Document added with ID: \(ref.documentID)")
} catch {
    print("Error adding document: \(error)")
}

do {
    let querySnapshot = try await db.collection("users").getDocuments()
    
    // Map documents to the User struct automatically
    let users = querySnapshot.documents.compactMap { document in
        try? document.data(as: User.self)
    }
    
    for user in users {
        print("Found user: \(user.firstName) \(user.lastName)")
    }
} catch {
    print("Error getting documents: \(error)")
}

import SwiftUI
import FirebaseFirestore

@MainActor
@Observable 
final class DataManager {
    private var listenerHandle: ListenerRegistration?
    var data: [String] = []
    
    func startListening(for userId: String) {
        // 1. Clean up any existing listener to prevent duplicates if the ID changes
        stopListening()
        
        // 2. Start the regular listener and capture the handle
        listenerHandle = Firestore.firestore().collection("users").document(userId).addSnapshotListener { snapshot, error in
            // Handle updates
        }
    }
    
    func stopListening() {
        listenerHandle?.remove()
        listenerHandle = nil
    }
    
    // 3. Guarantee cleanup when the View is destroyed and this object is deallocated
    isolated deinit {
        stopListening()
    }
}

struct MyView: View {
    @State private var manager = DataManager()
    @Environment(AuthManager.self) var authManager
    
    var body: some View {
        List(manager.data, id: \.self) { item in
            Text(item)
        }
        // .task(id:) automatically re-runs if the userId changes.
        // The view model handles stopping the old listener and starting the new one.
        .task(id: authManager.userId) {
            if let userId = authManager.userId {
                manager.startListening(for: userId)
            } else {
                manager.stopListening()
            }
        }
    }
}