extended logic of alarm interface

lib/model/alarm.dart

@@ -1,3 +1,29 @@
+import 'location_alarm.dart';
+import 'time_alarm.dart';
+
 abstract class Alarm {
   String get id;
+  Map<String, dynamic> toJson();
+
+  factory Alarm.fromJson(Map<String, dynamic> json) {
+    if (json.containsKey('triggerAt')) {
+      return TimeAlarm.fromJson(json);
+    }
+
+    if (json.containsKey('location')) {
+      return LocationAlarm.fromJson(json);
+    }
+
+    throw ArgumentError('Unknown alarm type');
+  }
+
+  @override
+  bool operator ==(Object other) {
+    if (other is! Alarm) return false;
+
+    return hashCode == other.hashCode;
+  }
+
+  @override
+  int get hashCode => id.hashCode;
 }

lib/model/location_alarm.dart

@@ -23,6 +23,7 @@ class LocationAlarm implements Alarm {
     );
   }
 
+  @override
   Map<String, dynamic> toJson() {
     return {
       'id': id,

lib/model/time_alarm.dart

@@ -15,6 +15,7 @@ class TimeAlarm implements Alarm {
     );
   }
 
+  @override
   Map<String, dynamic> toJson() {
     return {'id': id, 'triggerAt': triggerAt.toIso8601String()};
   }