overhaul using a trip struct that notifies its ui dependencies

frontend/lib/structs/trip.dart

@@ -5,35 +5,56 @@ import 'dart:collection';
 import 'dart:convert';
 
 import 'package:anyway/structs/landmark.dart';
+import 'package:flutter/foundation.dart';
+import 'package:geocoding/geocoding.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
-class Trip {
-  final String uuid;
-  final String cityName;
-  // TODO: cityName should be inferred from coordinates of the Landmarks
-  final int totalTime;
-  final LinkedList<Landmark> landmarks;
+class Trip with ChangeNotifier {
+  String uuid;
+  int totalTime;
+  LinkedList<Landmark> landmarks;
   // could be empty as well
 
+  Future<String> get cityName async {
+    if (GeocodingPlatform.instance == null) {
+      return '${landmarks.first.location[0]}, ${landmarks.first.location[1]}';
+    }
+    List<Placemark> placemarks = await placemarkFromCoordinates(landmarks.first.location[0], landmarks.first.location[1]);
+    return placemarks.first.locality ?? 'Unknown';
+  }
+
 
   Trip({
-    required this.uuid,
-    required this.cityName,
-    required this.landmarks,
-    this.totalTime = 0
-  });
-
+    this.uuid = 'pending',
+    this.totalTime = 0,
+    LinkedList<Landmark>? landmarks
+    // a trip can be created with no landmarks, but the list should be initialized anyway
+  }) : landmarks = landmarks ?? LinkedList<Landmark>();
+  
 
   factory Trip.fromJson(Map<String, dynamic> json) {
     Trip trip = Trip(
       uuid: json['uuid'],
-      cityName: json['city_name'] ?? 'Not communicated',
-      landmarks: LinkedList()
+      totalTime: json['total_time'],
     );
 
     return trip;
   }
 
+  void loadFromJson(Map<String, dynamic> json) {
+    uuid = json['uuid'];
+    totalTime = json['total_time'];
+    notifyListeners();
+  }
+  void addLandmark(Landmark landmark) {
+    landmarks.add(landmark);
+    notifyListeners();
+  }
+
+  void updateUUID(String newUUID) {
+    uuid = newUUID;
+    notifyListeners();
+  }
 
   factory Trip.fromPrefs(SharedPreferences prefs, String uuid) {
     String? content = prefs.getString('trip_$uuid');
@@ -47,7 +68,7 @@ class Trip {
 
   Map<String, dynamic> toJson() => {
     'uuid': uuid,
-    'city_name': cityName,
+    'total_time': totalTime,
     'first_landmark_uuid': landmarks.first.uuid
   };