added toJson and fromJson functions

lib/model/latlng.dart

@@ -4,4 +4,12 @@ class LatLng {
 
   LatLng(this.lat, this.lng);
   LatLng.empty() : lat = 0, lng = 0;
+
+  factory LatLng.fromJson(Map<String, dynamic> json) {
+    return LatLng(json['lat'] as double, json['lng'] as double);
+  }
+
+  Map<String, double> toJson() {
+    return {'lat': lat, 'lng': lng};
+  }
 }

lib/model/location.dart

@@ -11,4 +11,15 @@ class Location {
 
   String get address => _address ?? '';
   LatLng get coordinates => _coordinates ?? LatLng.empty();
+
+  factory Location.fromJson(Map<String, dynamic> json) {
+    return Location(
+      address: json['address'] as String,
+      coordinates: LatLng.fromJson(json['coordinates']),
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {'address': address, 'coordinates': coordinates};
+  }
 }

lib/model/task.dart

@@ -54,4 +54,48 @@ class Task {
       url: url ?? this.url,
     );
   }
+
+  factory Task.fromJson(Map<String, dynamic> json) {
+    return Task(
+      id: json['id'] as String,
+      title: json['title'] as String,
+      description: json['description'] as String? ?? '',
+      start: json['start'] != null
+          ? DateTime.parse(json['start'] as String)
+          : null,
+      due: json['due'] != null ? DateTime.parse(json['due'] as String) : null,
+      isCompleted: json['isCompleted'] as bool? ?? false,
+      category: json['category'] as String? ?? '',
+      subtasks:
+          (json['subtasks'] as List<dynamic>?)
+              ?.map((e) => Task.fromJson(e as Map<String, dynamic>))
+              .toList() ??
+          [],
+      alarms:
+          (json['alarms'] as List<dynamic>?)
+              ?.map((e) => DateTime.parse(e as String))
+              .toList() ??
+          [],
+      location: json['location'] != null
+          ? Location.fromJson(json['location'] as Map<String, dynamic>)
+          : null,
+      url: json['url'] as String? ?? '',
+    );
+  }
+
+  Map<String, dynamic> toJson() {
+    return {
+      'id': id,
+      'title': title,
+      'description': description,
+      'start': start?.toIso8601String(),
+      'due': due?.toIso8601String(),
+      'isCompleted': isCompleted,
+      'category': category,
+      'subtasks': subtasks.map((e) => e.toJson()).toList(),
+      'alarms': alarms.map((e) => e.toIso8601String()).toList(),
+      'location': location?.toJson(),
+      'url': url,
+    };
+  }
 }