properly handle listener

Author: racheldosh

firestore/FirestoreSwiftUIExample/ViewModels/RestaurantListViewModel.swift

@@ -10,22 +10,37 @@ class RestaurantListViewModel: ObservableObject {
 
   @Published var restaurants = [Restaurant]()
   private var db = Firestore.firestore()
+  private var listener: ListenerRegistration?
 
-  func fetchData() {
-    db.collection("restaurants").addSnapshotListener { (querySnapshot, error) in
-      if let error = error {
-        print("Error getting restaurants: \(error.localizedDescription)")
-        return
-      }
+  deinit {
+    unsubscribe()
+  }
+
+  func unsubscribe() {
+    if listener != nil {
+      listener?.remove()
+      listener = nil
+    }
+  }
+
+  func subscribe() {
+    if listener == nil {
+      listener = db.collection("restaurants").addSnapshotListener { [weak self] (querySnapshot, error) in
+        guard let documents = querySnapshot?.documents else {
+          print("Error fetching documents: \(error!)")
+          return
+        }
 
-      self.restaurants = querySnapshot?.documents.compactMap { document in
-        do {
-          return try document.data(as: Restaurant.self)
-        } catch let error {
-          print(error)
-          return nil
+        guard let self = self else { return }
+        self.restaurants = documents.compactMap { document in
+          do {
+            return try document.data(as: Restaurant.self)
+          } catch let error {
+            print(error)
+            return nil
+          }
         }
-      } ?? []
+      }
     }
   }
 

firestore/FirestoreSwiftUIExample/Views/ContentView.swift

@@ -18,7 +18,10 @@ struct ContentView: View {
       }
       .navigationBarTitle("Friendly Eats", displayMode: .inline)
       .onAppear() {
-        self.restaurantListViewModel.fetchData()
+        restaurantListViewModel.subscribe()
+      }
+      .onDisappear() {
+        restaurantListViewModel.unsubscribe()
       }
       .toolbar {
         ToolbarItem(placement: .navigationBarLeading) {